Standard ECMA-262

)

表示这个非终结符 WhileStatement 代表 while token，其后跟左括号 token，其后跟 Expression，其后跟右括号 token，其后跟 Statement。这里出现的 Expression 和 Statement 本身是非终结符。另一个例子，语法定义：

表示这个 ArgumentList 可以代表一个 AssignmentExpression，或 ArgumentList，其后跟一个逗号，其后跟一个 AssignmentExpression。这个 ArgumentList 的定义是递归的，也就是说，它定义它自身。其结果是，一个 ArgumentList 可能包含用逗号隔开的任意正数个参数，每个参数表达式是一个 AssignmentExpression。这样，非终结符共用了递归的定义。

终结符或非终结符可能会出现后缀下标 “_opt”，表示它是可选符号。包含可选符号实际指定了包含两个右值，一个是省略可选元素的，另一个是包含可选元素的。这意味着：

opt

是以下的一种简便的缩写：

以及:

for

(

opt

;

opt

)

是以下的一种简便的缩写:

for

(

;

opt

)

for

(

;

opt

)

也是下面的一种缩写:

for

(

;

)

for

(

;

)

for

(

;

)

for

(

;

)

所以在这个例子中，非终结符 IterationStatement 实际上有四个可供选择的右值。

产生式可以通过形如 “_[parameters]” 这样的下标注释被参数化，会以后缀的形式出现在产生式定义的非终结符中。“_parameters” 可以是单独的名称，也可以是一个逗号分隔的名称列表。被参数化的产生式是一系列定义参数名称以下划线开头，追加到参数化的非终结符后面的所有组合的产生式的缩写。这就意味着：

[Return]

是下面的缩写:

以及:

[Return, In]

是下面的缩写:

StatementList_Return_In

多个参数产生数个产生式的组合，在一个完整语法中不必引用所有。

对生产式的非终结符右值的引用也可以被参数化，例如：

[+In]

是下面的缩写:

ExpressionStatement_In

又如:

[~In]

是下面的缩写:

非终结符引用可能同时具有参数列表和一个 “_opt” 后缀。例如：

[+In]

opt

是下面的缩写:

Initializer_In

在右侧非终结符引用参数名称前置以 “_?”，使该参数值取决于当前产生式的左侧符号引用的参数名称是否出现。例如：

[In]

[?In]

是下面的缩写:

VariableDeclaration_In

Initializer_In

如果右值选择是以 “[+parameter]” 作为前缀，则表示选择只有命名参数用于引用产生式的非终结符时可用。如果右值选择是以 “[~parameter]” 作为前缀，则表示选择只有命名参数不是用于引用产生式的非终结符时可用。这意味着:

[Return]

[+Return]

是下面的缩写:

另外一个例子:

[Return]

[~Return]

是下面的缩写:

在文法定义中当单词 “one of” 跟随一个或多个分号后的时候，表示随后的一行或多行的终结符是一个可选的定义。例如，ECMAScript的词法文法包含下列的产生式：

one of

它仅仅是下面的缩写:

如果 “[empty]” 出现在一个产生式的右值，它暗示着这个产生式的右值不包含终结符或者非终结符。

如果 “[lookahead ∉ set]” 出现在一个产生式的右值，它暗示着如果随后立即的输入的token序列是给出的集合（set）的成员，那么这个产生式也许不会被使用。这个集合可以用一个逗号分割的由一到两个被花括号包裹的元素终结序列的列表表示。为了方便，这个集合也可以用非终结符表示，在这种情况下，它代表所有能由非终结符展开得到的终结符的集合。如果这个集合包含一个单独的终结符，那么短语 “[lookahead ≠ terminal]” 也许会被使用。

例如，给出定义:

one of

则定义:

[lookahead ∉ { 1, 3, 5, 7, 9 }]

DecimalDigit

[lookahead ∉ DecimalDigit]

能匹配字母 n 后跟随由偶数起始的一个或多个十进制数字，或一个十进制数字后面跟随一个非十进制数字。

类似的，如果产生式的右值出现 “[lookahead ∈ set]”，则表示产生式只有在随后立即的输入的token序列是给出的集合（set）的成员才能使用。如果集合只包含一个单一的终结符，则可以使用 “[lookahead == terminal]”。

如果产生式的右侧出现 “[no LineTerminator here]”，那么它表示此产生式是个受限的产生式：如果 LineTerminator 在输入流的指定位置出现，那么此产生式将不会被适用。例如，产生式：

throw

[no LineTerminator here]

;

如果一个 LineTerminator 出现在脚本的 throw token 和 Expression 之间的时候，暗示着这个产生式也许不会被使用。

除非通过一个受限的产生式禁止一个 LineTerminator 的出现，否则任意数量 LineTerminator 的出现也许会发生在输入元素流中任意两个连续的 token 之间，并且不影响脚本的语义可接受性。

当一个词法文法或者数字字符串文法的产生式中可选部分出现多码点的 token 时，意味着码点序列将会组成一个 token。

一个产生式的右值也许会通过 “but not” 指定某些不被允许的扩展，然后暗示这个扩展将是被排除在外的。例如，产生式：

but not ReservedWord

意味着非终结符 Identifier 也许会被任意的码点序列替换，这些码点能够替换提供的 IdentifierName，但是相同序列的码点不能替换 ReservedWord。

最后，一些非终结符通过 sans-serif 字体书写的描述性的短语来描述，在这样的案例中，列出所有的可选部分是不切实际的。

any Unicode code point

5.2 算法约定

此规范通常使用带编号的列表来指定算法的步骤。这些算法是用来精确地指定 ECMAScript 语言结构所需的语义。该算法无意暗示任何具体实现使用的技术。在实践中，也许可用更有效的算法实现一个给定功能。

算法可能被显式参数化，在这种情况下，参数名和用法必须作为算法定义的一部分提供。

算法步骤可以细分为多个连续的子步骤。子步骤需要用缩进表示，可以进一步被划分成缩进的子步骤。大纲编号约定用作标识子步骤，第一级子步骤使用小写字母字符标记，第二级小步骤用小写罗马数字标记。如果步骤超过三级，那么这些规则从第四级开始从数字重复进行标记。例如：

Top-level step
1. Substep.
2. Substep.
  1. Subsubstep.
    1. Subsubsubstep
      1. Subsubsubsubstep
        Subsubsubsubsubstep

一个步骤或者子步骤也许会使用 “if” 去断言它的子步骤的条件。在这样的案例中，这些子步骤只能适用于 “if” 断言为真的情况。如果一个步骤或者子步骤以 “else” 开始，那么表示这是之前的 “if” 断言的同一级的否定断言。

一个步骤也许会指定它的子步骤的迭代程序。

一个以 “Assert:” 开头的步骤断言一个它的算法里不变的条件。这样的断言被用作使算法不变量显式地展示，否则它将是隐式的。这样的断言不增加额外的语义需求，因此实现不必检查。它们被使用只是为了使算法更清晰。

对于任意形如 “Let x be someValue” 的值，算法步骤也许会声明命名别名。这些别名都是类似类型的，即 x 和 someValue 指向同样的基础数据，对它们的修改是互相可见的。那些想避免这样的类似引用的算法步骤需要显式地复制一份右值：即 “Let_x_ be a copy of someValue” 创建了一个 someValue 的浅复制。

一旦声明完毕，别名可以在任何后续步骤中引用，而不能在别名声明前的步骤中使用。别名也许会通过形如 “Set x to someOtherValue” 这样的形式被修改。

5.2.1 抽象操作

为了便于它们在本规范的多个地方使用，一些算法，我们称之为抽象操作，以参数化函数形式命名和书写，以便它们能够在其它算法中通过名称来引用。抽象操作通常使用函数应用风格被引用，如 OperationName(arg1, arg2)。一些抽象操作被当作像类规范里抽象的多态发送一样。这样的类似类的方法抽象操作通常通过一个方法应用风格被引用，如 someValue.OperationName(arg1, arg2)。

5.2.2 语法导向操作

一个语法导向操作是一个具名操作，它的定义包含了一系列算法，每个算法与一个或者多个 ECMAScript 文法的产生式相关联。一个有多个可选定义的产生式通常对于每个可选部分都有一个独立的算法。当一个算法与一个文法产生式相关联的时候，它也许会引用这个产生式可选部分的终结符或者非终结符，就好像它们是这个算法的参数一样。当出现这种情况时，非终结符指向实际的源代码中与之匹配的可选部分的定义。

当一个算法与一个产生式的可选部分关联的时候，这个可选部分通常不会带上 “[ ]” 文法注释。这样的注释只应该影响可选部分的语法识别，不应该对相关的可选部分的语义有影响。

语法导向操作同一个解析节点一起被调用，还可以在以下算法步骤 1，3，4 中使用这个约定的其它参数:

Let status be the result of performing SyntaxDirectedOperation of SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode passing "value" as the argument.

除非另有明确说明，否则，所有相关联的产生式对于每个也许会被应用到这个产生式左值的非终结符的操作都有一个隐式的定义。如果存在的话，这个隐式的定义简单地再次对同样的参数运用这个相同的定义，对于这些相关联的产生式唯一的右值非终结符，随后返回处理结果。例如，假设一些算法有如下形式的步骤： “Return the result of evaluating Block”，并且这里有一个产生式：

{

}

但是解析操作并没有关联这个产生式的算法。那么在这样的案例中，解析操作隐式地包含了下面形式的关联：

运行时语义: 解析

{

StrUnsignedDecimalLiteral

}

Return the result of evaluating StatementList.

5.2.3 运行时语义

必须在运行时被调用的指定语义的算法就叫运行时语义。运行时语义通过抽象操作或者指定语法操作来定义。这样的算法必须返回一个完成记录。

5.2.3.1 隐式完成值

本规范的算法常常隐式地返回一个 [[Type]] 值为 normal 的 Completion Records。除非一个返回一个值的算法语句在上下文明显指定返回一个不是 Completion Record 的值。例如：

Return "Infinity".

与下面的是同一个含义：

Return NormalCompletion("Infinity").

然而，如果一个 “return” 语句的值表达式是一个 Completion Record 构造的字面量，那么 Completion Record 的结果将被返回。如果这个值表达式是一个对抽象操作的调用，那么 “return” 语句简单地返回这个抽象操作产生的 Completion Record。

抽象操作 Completion(completionRecord) 被用作强调之前计算的 Completion Record 被返回。Completion 这个抽象操作接收一个参数，即 completionRecord，然后执行下面的步骤：

Assert: completionRecord is a Completion Record.
Return completionRecord as the Completion Record of this abstract operation.

一个在算法步骤中没有值的 “return” 语句与下面的是同样的意思：

Return NormalCompletion(undefined).

对于任意的在一个上下文中没有显示地需要一个完整的 Completion Record 值的引用，等价于一个显式地对 Completion Record 的 [[Value]] 字段的引用，除非 Completion Record 是一个中断完成。

5.2.3.2 抛出一个异常

描述抛出一个异常的算法步骤，例如：

Throw a TypeError exception.

与下面的是同一个含义：

Return ThrowCompletion(a newly created TypeError object).

5.2.3.3 ReturnIfAbrupt

算法步骤说明或者等同于：

ReturnIfAbrupt(argument).

意思就是:

If argument is an abrupt completion, return argument.
Else if argument is a Completion Record, let argument be argument.[[Value]].

算法步骤说明或者等同于：

ReturnIfAbrupt(AbstractOperation()).

与下面的是同一个含义：

Let hygienicTemp be AbstractOperation().
If hygienicTemp is an abrupt completion, return hygienicTemp.
Else if hygienicTemp is a Completion Record, let hygienicTemp be hygienicTemp.[[Value]].

这里的 hygienicTemp 是临时的，并且只在 ReturnIfAbrupt 有关的步骤中中可见。

算法步骤说明或者等同于：

Let result be AbstractOperation(ReturnIfAbrupt(argument)).

与下面的是同一个含义：

If argument is an abrupt completion, return argument.
If argument is a Completion Record, let argument be argument.[[Value]].
Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt 缩写

对抽象操作和以 ? 为前缀的指定语法操作的调用暗示着 ReturnIfAbrupt 需要被应用到结果的 Completion Record 中。例如，步骤：

? OperationName().

等价于下面的步骤：

ReturnIfAbrupt(OperationName()).

相似的，对于程序方法的风格，步骤：

? someValue.OperationName().

等价于:

ReturnIfAbrupt(someValue.OperationName()).

相似的，前缀 ! 被用作暗示下列的对于抽象或者指定语法操作的调用将不会返回一个打断的完成值，并且作为结果的 Completion Record 的 [[Value]] 字段应该被用作替换操作的返回值。例如，步骤：

Let val be ! OperationName().

等价于下面的步骤：

Let val be OperationName().
Assert: val is never an abrupt completion.
If val is a Completion Record, set val to val.[[Value]].

对于运行时语义的指定语法的操作通过在调用操作前替换 ! 或者 ? 来利用这个缩写。

Perform ! SyntaxDirectedOperation of NonTerminal.

5.2.4 静态语义

上下文无关文法不能足够好的去表达所有定义的规则，不管是一个输入元素的流形成的一个合法的将被解析的 ECMAScript Script 还是 Module。在一些情况中，需要额外的规则通过使用 ECMAScript 算法约定或者散文要求来展示。这样的规则总是与一个文法的产生式相关联，被称作产生式的静态语义。

静态语义规则拥有名称，并且通常用一个算法来定义。具有名称的静态语义规则与文法产生式相关联，对于每个可采用的具有名称的静态语义规则，一个包含多个可选部分定义的产生式通常会对每个可选部分定义一个独有的算法。

除非在本规范中其它指定的文法产生式可选部分隐式地包含一个叫做 Contains 的接收一个值为包含相关产生式的文法的终结符或者非终结符的 symbol 参数静态语义规则的定义。默认的包含定义为：

For each child node child of this Parse Node, do
1. If child is an instance of symbol, return true.
2. If child is an instance of a nonterminal, then
  1. Let contained be the result of child Contains symbol.
  2. If contained is true, return true.
Return false.

上面的定义是显式地重写了规范的产生式。

一个特殊的静态语义规则是早期（提前）错误规则。早期错误规则定了早期错误条件（见 16），它们与具体的文法产生式相关联。大多数早期错误规则的解析不是在本规范的算法中显式地被调用。一个符合规范的实现必须在第一次解析 Script 或者 Module 之前，验证所有被用作解析 Script 或者 Module 的产生式的早期错误规则。如果违反了任意的早期错误规则，那么 Script 或者 Module 就是不合法的，并且不能被解析执行。

5.2.5 数学操作

除非其它地方特别注明不包括无限并且不包括负 0（为了与正 0 区分），数学操作，例如加，减，否定（逻辑非），乘，除以及随后在条款中定义的数学函数应总是被当作是对于数学中所有真实数字计算出来的准确的结果。本规范算法中的浮点运算包含显式地的步骤，它们对于处理无穷，有符号的0，以及舍入是必要的。如果一个数学操作或者函数被应用在一个浮点数上，那么它必须被当作应用到用浮点数表示的具体的数学值；这样的浮点数必须是有限的，并且如果它是 +0 或者 -0 的话，那么相应的数学值就是简简单单的 0。

数学函数 abs(x) 用来计算 x 的绝对值，如果 x 是负数，那么结果就是 -x，否则结果就是 x 本身。

数学函数 min(x1, x2, ..., xN) 计算从 x1 到 xN 的最小值。数学函数 max(x1, x2, ..., xN) 计算从 x1 到 xN 的最大值。这些数学函数的域以及范围包括 +∞ 和 -∞。

符号 “x modulo y” （ y 必须是有限的且是非 0 的）计算出一个值 k，它的符号与 y 一致（或者是 0），这样的 k 满足 abs(k) < abs(y)，以及能找到某个整数y，使得 x-k = q * y。

��学函数 floor(x) 产生一个不大于 x 的最大整数（最接近正无穷）。

Note

floor(x) = x-(x modulo 1).

6 ECMAScript 数据类型和值

本规范中算法控制每个具有相关类型的值。可能的值类型直接定义在本章节中。这些类型进一步分为 ECMAScript 语言类型与规范类型。

在本规范中，“Type(x)”表示法是“the type of x”的简写，其中“type”指的是在本章节中定义的 ECMAScript 语言类型与规范类型。当一个值使用术语“empty”命名值时，它相当于说“没有任何类型的值”。

6.1 ECMAScript 语言类型

一个 ECMAScript 语言类型对应那些 ECMAScript 程序员在 ECMAScript 语言中直接使用的值。 ECMAScript 语言类型有 Undefined, Null, Boolean, String,Symbol, Number, Object。一个ECMAScript 语言值是指拥有一个 ECMAScript 语言类型特点的值。

6.1.1 Undefined 类型

Undefined 类型仅仅只有一个值，undefined。任何尚未分配值的变量值都为 undefined。

6.1.2 Null 类型

Null 类型只有一个值, null.

6.1.3 Boolean 类型

Boolean 类型表示具有 true 和 false 两个值的一个逻辑实体。

6.1.4 String 类型

String 类型是零个或多个 16 位无符号整数值（“元素”）的所有有序序列的集合，最大长度为 2⁵³-1 个元素。字符串类型通常用于表示正在运行的 ECMAScript 程序中的文本数据，在这种情况下，String中的每个元素都被视为一个 UTF-16 码元。每个元素被认为占据该序列内的一个位置。这些位置用非负整数索引。第一个元素（如果有）在索引 0 处，下一个元素（如果有的话）在索引 1 处，以此类推。 String 的长度是序列中的元素个数（例如，十六位值）。空字符串的长度为零，因此不包含元素。

ECMAScript 操作符不应用不同的语义解释字符串内容。操作符把字符串的每个元素视为单个 UTF-16 代码单元进行解释。但是，ECMAScript 并不约束这些代码单元的值或这些代码单元之间的关系，因此进一步将字符串内容解释为 UTF-16 中编码的 Unicode 代码点序列的操作必须考虑格式不对的子序列。此类操作使用如下规则对每个代码单元特殊处理，使其数值范围在 0xD800 到 0xDBFF（Unicode 标准定义为首代理，或者更正式的高代理码元）或范围 0xDC00 到 0xDFFF（定义为尾代理，或者更正式的低代理码元）：

不是首代理也不是尾代理的码元被解释为具有相同值的码点。
两个码元序列，其第一个码元 c1 是一个首代理，第二个码元 c2 是一个尾代理，这样的两个码元，被称为代理对，被解释为一个值为 (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000 的码点。(见 10.1.2)
一个首代理或者尾代理，但不是一个代理对的一部分的码元被解释为具有相同值的码点。

函数 String.prototype.normalize (see 21.1.3.12) 可以显式地标准化一个字符串值. String.prototype.localeCompare (see 21.1.3.10) 在内部规范化字符串值，但没有其他操作隐式规范化它们运行的字符串。只有明确指定为语言或区域设置敏感的操作才会生成对语言敏感的结果。

Note

这种设计背后的理由是尽可能的保持 String 的实现简单和高效。如果 ECMAScript 源文本符合 Normalized Form C，只要它们不包含任何 Unicode 转义序列，则字符串字面量保证也是标准化的。

在本规范中，短语"the string-concatenation of A,B,..."表示字符串代码单元的序列是每个参数的代码单元的连接。

6.1.5 Symbol 类型

Symbol 类型是所有可以用于作为 Object 属性 key 值的非字符串值的集合。(6.1.7)。

每个可能的 Symbol 值都是唯一的和不可变的。

每个 Symbol 值永远地保持一个与之关联的叫做 [[Description]] 的值（它要么是 undefined 要么是一个 String 值）。

6.1.5.1 总所周知的 Symbol

总所周知的 Symbol 是那些被本规范中算法明确引用的内置 Symbol 值。它们通常被用作属性的 key，这些 key 的值作为规范的算法扩展点。除非另有说明，总所周知的 Symbol 的值被所有域(8.2)共享。

在本规范内，一个总所周知的 Symbol 值通过形如 @@name 的形式被引用，这里的 “name” 是表 Table 1 列出的值之一。

Table 1: Well-known Symbols

规范名称	[[Description]]	值和目的
@@asyncIterator	`"Symbol.asyncIterator"`	返回对象的默认 AsyncIterator 的方法。由 `for`-`await`-`of` 语句的语义调用。
@@hasInstance	`"Symbol.hasInstance"`	确定构造函数对象是否识别对象为构造函数实例之一的方法。由 `instanceof` 运算符语义来调用。
@@isConcatSpreadable	`"Symbol.isConcatSpreadable"`	一个 Boolean 值，如果为 true，表示一个对象应该被 `Array.prototype.concat` 扁平化的加入到数组元素中。
@@iterator	`"Symbol.iterator"`	返回一个对象的默认 Iterator 的方法。由 for-of 声明的语义来调用。
@@match	`"Symbol.match"`	将字符串与正则表达式匹配的正则表达式方法。由 `String.prototype.match` 方法调用。
@@replace	`"Symbol.replace"`	替换匹配字符串子串的正则表达式方法。由 `String.prototype.replace` 方法调用。
@@search	`"Symbol.search"`	一个正则表达式方法，它返回正则表达式匹配到的内容在字符串中的索引。由`String.prototype.search`方法调用。
@@species	`"Symbol.species"`	作为构造函数的函数值属性，用来创建派生对象。
@@split	`"Symbol.split"`	一个正则表达式方法，它在正则表达式匹配的索引位置拆分字符串。由 `String.prototype.split` 方法调用。
@@toPrimitive	`"Symbol.toPrimitive"`	将一个对象转换为相应原始值的方法。由抽象操作 ToPrimitive 调用。
@@toStringTag	`"Symbol.toStringTag"`	用于创建对象的默认字符串描述的 String 值属性。通过内置方法 `Object.prototype.toString` 访问。
@@unscopables	`"Symbol.unscopables"`	一个对象值属性，其自身和继承的属性名是从相关对象的 with 环境绑定中排除的属性名。

6.1.6 Number 类型

Number 类型有 18437736874454810627 (即，2⁶⁴-2⁵³+3)个值，表示双精度 64 位格式的 IEEE 754-2008 值由 IEEE 二进制浮点数标准规定，除了 9007199254740990 (即，2⁵³-2)这个在 IEEE 标准中独特的 “Not-a-Number” 值，其在 ECMAScript 中表示为一个特殊的 NaN 值。(注意 NaN 值在程序中由 NaN 表达式产生)在某些实现中，外部代码可能能够检测各种 Not-a-Number 值之间的差异，但这种行为依赖于实现; 对于 ECMAScript 代码，所有的 NaN 值彼此是不可区分。

Note

在一个 Number 数值被存入之后，在一个 ArrayBuffer(见24.1) 或一个 SharedArrayBuffer(见24.2) 中可能观察到的位模式不一定和 ECMAScript 实现所使用的那个 Number 数值的内部表示相同。

还有另外两个特殊的值，叫做正无穷和负无穷。为了简洁起见，这些值分别由符号 +∞ 和 -∞ 表示。（请注意，这两个无穷数值由程序表达式 +Infinity（或 Infinity ）和 -Infinity 产生。）

其他 18437736874454810624（即，2⁶⁴-2⁵³）个值称为有限值。其中一半为正数，一般为负数；每个有限正数存在相同大小的对应负数。

注意其中有一个正零，一个负零。为了简洁，他们也分别用 +0 和 -0 来说明。（注意这两个不同的 0，通过程序表达式 +0(或 0) 和 -0 产生。）

18437736874454810622（即 2⁶⁴-2⁵³-2）个有限非 0 值有两种：

其中 18428729675200069632 (即 2⁶⁴-2⁵⁴) 个值是标准的, 具有如下格式

s \times m \times 2 e

其中 s 为 +1 或 -1，m 是一个小于 2⁵³ 但大于等于 2⁵² 的整数，e 是从 -1074 到 971 的整数。

其余 9007199254740990 (即 2⁵³-2) 个值是非标准的, 有如下格式

s \times m \times 2 e

其中 s 是 +1 或 -1 ，m 是小于 2 的 52 次方的正整数，e 是 -1074。

注意，大小不大于 2⁵³ 的所有正整数和负整数可以在 Number 类型中表示（实际上，整数 0 具有两个表示，+0 和 -0）。

如果一个有限的数值非零且用来表达它（上文两种形式之一）的整数 m 是奇数，则该数值有奇数意义。否则，它有偶数意义。

在本规范中，当 x 表示一个精确的非零实数数学量（甚至可以是无理数，比如π）时，“the Number value for x” 意为，以下面的方式选择一个 Number 值。考虑 Number 类型中除了 -0 和两个被加入在数值类型中但不可呈现的值，2¹⁰²⁴ (+1 × 2⁵³ × 2⁹⁷¹) 以及 -2¹⁰²⁴(-1 × 2⁵³ × 2⁹⁷¹)的所有有限值的集合。选择此集合中值最接近 x 的一员。若集合中的两值近似相等，那么选择有偶数意义的那个；为此，2¹⁰²⁴和-2¹⁰²⁴这两个超额值被认为有偶数意义。最终，若选择 2¹⁰²⁴ ，用 +∞ 替换它；若选择 -2¹⁰²⁴，用 -∞ 替换它；若选择 +0，有且只有 x 小于零时，用 -0 替换它；其它任何被选取的值都不用改变。结果就是 x 的 Number 值。（此过程正是IEEE 754-2008“round to nearest, ties to even”模式对应的行为。）

某些 ECMAScript 运算符仅处理闭区间 -2³¹ 到 2³¹-1，或者 0 到 2¹⁶-1 的整数。这些操作符接收任意 Number 类型的值，首先会将它们转换成像上面提到的这些范围内的值。参考7.1的数字转换操作。

6.1.7 Object 类型

对象在逻辑上是属性的集合。每个属性都是一个数据属性或一个访问器属性：

数据属性关联 key 值和一个 ECMAScript 语言值以及一组值为 Boolean 类型的特性。
访问器属性关联 key 值和一个或两个访问器函数以及一组值为 Boolean 类型的特性。访问器函数用于存储或检索与该属性相关联的 ECMAScript 语言值。

属性使用 key 值来标识。一个属性的 key 值是 ECMAScript String 值或 Symbol 值。所有 String 值（包括空字符串）和 Symbol 值作为属性的 key 都是有效的。属性名称是指属性的 key，其值为字符串。

一个值为 +0 或者一个 ≤ 2⁵³-1 的整数作为属性 key 字符串值的数字索引是一个以典型的数字 String（见 7.1.16）。一个数组索引是范围在 +0 ≤ i < 2³²-1 的整数。

属性的 key 用来访问属性以及其值。属性有两种访问方式：get 和 set ，分别对应值的检索和赋值。通过 get 和 set 可访问的属性既包括直接属于的自身属性，也包括通过继承关系由其它相关对象提供的继承属性。继承属性也可能是另一个对象的自身属性或者继承属性。对象的每个自身属性必须有一个与对象的其他自身属性 key 值不同 key 值。

所有的对象都是逻辑上的属性集合，但是在用于访问和操纵它们的属性的语义中存在多种形式的对象。普通对象是对象的最常见形式，拥有默认的对象语义。奇异对象是指其属性语义与默认语义的各个方面都有区别的对象。

6.1.7.1 属性特性

在本规范中特性用来定义和解释对象属性的状态。数据属性 key 值与表Table 2中列出的特性关联。

Table 2: Attributes of a Data Property

特性名称	值域	描述
[[Value]]	任何 ECMAScript 语言类型	get 访问检索到的属性值
[[Writable]]	Boolean	如果为 false，尝试通过 [[Set]] 去改变属性的 [[Value]] 特性的 ECMAScript 代码将不会成功。
[[Enumerable]]	Boolean	如果为 true, 这个属性被 for-in 枚举时将会是可枚举的(见13.7.5)。否则，这个属性就是不可枚举的。
[[Configurable]]	Boolean	如果为 false，尝试删除这个属性，把这个属性改为访问器属性，或者改变它的的特性(除了修改 [[Value]]，或修改 [[Writable]] 为 false 之外)都会失败。

访问器属性关联表Table 3中的列出一个值。

Table 3: Attributes of an Accessor Property

特性名称	值域	描述
[[Get]]	Object \| Undefined	如果这个值是一个对象，那么它必须是一个函数对象。在每次取值(get 访问该属性被执行)时，这个函数的内部 [[Call]] 方法(Table 6)将会以一个空 arguments 列表调用获取属性值。
[[Set]]	Object \| Undefined	如果这个值是一个对象，那么它必须是一个函数对象。在每次赋值(set 访问该属性被执行)时，这个函数的内部 [[Call]] 方法(Table 6)将会以一个包含分配的值为参数的 arguments 调用。内部方法 [[Set]] 应该会对后续的内部方法 [[Get]] 调用产生的返回值产生影响，但不是必须的。
[[Enumerable]]	Boolean	如果为 true,这个属性被 for-in 枚举时将会是可枚举的(见13.7.5)。否则，这个属性就是不可枚举的。
[[Configurable]]	Boolean	如果为 false，任何尝试删除这个属性，把这个属性改为数据属性，或者改变它的其它特性的操作都会失败。

如果一个属性的特性的初始值没有被本规范显式地指定的，那么使用在表Table 4中定义的默认值。

Table 4: Default Attribute Values

特性名	默认值
[[Value]]	undefined
[[Get]]	undefined
[[Set]]	undefined
[[Writable]]	false
[[Enumerable]]	false
[[Configurable]]	false

6.1.7.2 对象的内部方法以及内部槽

在 ECMAScript 中，对象的实际语义是通过算法来调用内部方法指定的。在 ECMAScript 引擎中的每个对象都与一系列的定义它的运行时行为的内部方法相关联。这些内部方法不属于 ECMAScript 语言的一部分。本规范定义它们纯粹是为了说明目的。但是，在 ECMAScript 具体实现中的每个对象必须表现得与其相关关联的内部方法一致。其中确切的方式由实现者来决定。

内部方法名称是多态的。这意味着当一个通用内部方法被调用的时候，不同的对象值可能会执行不同的算法。调用内部方法实际的对象是这个调用 “target”。如果在运行时，实现的算法尝试使用一个对象不支持的内部方法，抛出一个TypeError异常。

内部槽对应于相关对象的内部状态，用于各种 ECMAScript 规范算法。内部槽不是对象的属性，也不会被继承。根据具体的内部槽规范，内部状态可能由任意 ECMAScript 语言类型的值或者指定的 ECMAScript 规范类型的值组成。除非另有指定，否则分配内部槽应作为创建一个对象的过程的部分，并且不应该动态地添加到对象上。除非另有指定，否则一个内部槽的初始值是 undefined。本规范内很多算法都会创建有内部槽的对象。然而，ECMAScript 语言不提供直接去访问一个对象内部槽的方法。

本规范内的内部方法和内部槽使用闭合的双方括号 [[]] 来标识。

表 Table 5 总结了本规范中适用于所有通过 ECMAScript 代码创建或者操作的对象的基本内部方法。每个对象必须有所有基本内部方法的算法。不过，对于那些方法不必所有的对象都用一样的算法。

表 Table 5 中的 “Signature” 列以及其它相似的表格描述了每个内部方法的调用模式。调用模式总是包含一个括起来的描述参数名的列表。如果一个参数名与 ECMAScript 类型的名字一样，则该名称描述了参数值的所需类型。如果一个内部方法显式地返回了一个值，则它的参数列表接着是一个 “→” 符号，以及返回值的类型名称。在 “Signature” 列中使用的类型名称指向在6中定义的类型，以及一些附加的名称。 “any” 意味着这个值可以是任意的 ECMAScript 语言值。一个内部方法会隐式地返回一个 Completion Record。除了它的参数，一个内部方法还总会访问调用这个方法的目标对象。

Table 5: Essential Internal Methods

Internal Method	Signature	Description
[[GetPrototypeOf]]	( ) → Object \| Null	确定为对象提供继承属性的对象。null 值表示没有继承属性。
[[SetPrototypeOf]]	(Object \| Null) → Boolean	关联这个对象与提供继承属性的对象。传递 null 表示没有继承属性。返回 true 表示操作成功完成，返回 false 表示操作失败。
[[IsExtensible]]	( ) → Boolean	决定是否允许添加额外的属性到这个对象上。
[[PreventExtensions]]	( ) → Boolean	控制一个新的属性是否能加到这个对象上。返回 true 表示操作成功，返回 false 表示操作失败。
[[GetOwnProperty]]	(`propertyKey`) → Undefined \| Property Descriptor	返回对象自身属性中 key 值为 `propertyKey` 的属性的属性描述符，如果没有这个属性返回`undefined`。
[[DefineOwnProperty]]	(`propertyKey`, `PropertyDescriptor`) → Boolean	创建或者改变 key 值为 `propertyKey` 的自身属性状态为 `PropertyDescriptor`。返回 true 表示属性被成功创建/更新，返回 false 表示属性不能被创建/更新。
[[HasProperty]]	(`propertyKey`) → Boolean	返回一个 Boolean 值，代表这个对象自身或者继承了一个 key 值为 `propertyKey` 的属性。
[[Get]]	(`propertyKey`, `Receiver`) → any	返回这个对象里 key 值为 `propertyKey` 的属性的值。如果必须运行一些的 ECMAScript 代码来检索这个属性值，`Receiver` 就会作为解析代码时的 this 值。
[[Set]]	(`propertyKey`, `value`, `Receiver`) → Boolean	设置这个对象 key 值为 `propertyKey` 的属性的值为 `value`。如果必须运行一些的 ECMAScript 代码来检索这个属性值，`Receiver` 就会作为解析代码时的 this 值。返回 true 表示这个属性值已经设置，返回 false 表示不能被设置。
[[Delete]]	(`propertyKey`) → Boolean	移除这个对象 key 值为 `propertyKey` 的自身属性。如果这个属性没有被移除，仍然存在，则返回 false。如果属性被移除，或不存在，则返回 true。
[[OwnPropertyKeys]]	( ) → List of propertyKey	返回一个包含所有自身属性 key 值的 List。

表 Table 6 总结了用来支持对象被作为函数调用的其他基础内部方法。函数对象是支持内部方法 [[Call]] 的对象。构造器是一个支持 [[Construct]] 内部方法的对象。支持 [[Construct]] 的每个对象都必须支持 [[Call]]; 也就是说，每个构造器都必须是一个函数对象。因此，构造器也可以称为构造函数或构造函数对象。

Table 6: Additional Essential Internal Methods of Function Objects

Internal Method	Signature	Description
[[Call]]	(any, a List of any) → any	执行这个对象相关联的代码。通过一个函数表达式来调用。通过调用表达式，传递给内部函数的 arguments 为一个 this 值，和一个包含传递给函数的参数列表，实现该内部方法的对象是可调用的
[[Construct]]	(a List of any, Object) → Object	创建一个对象。通过 `new` 或者 `super` 操作符调用。这个内部方法的第一个参数是一个包含操作符的参数的列表。第二个参数是 `new` 操作符初始化时应用的对象。实现这个内部方法的对象被称为构造函数。一个函数对象不一定是构造函数，这样的非构造函数的函数对象没有 [[Construct]] 内部方法。

这些普通对象和标准的怪异对象的基础内部方法语义在9指出。如果怪异对象的任何特定内部方法不被实现支持，那么在尝试使用时，必须抛出一个 TypeError 异常。

6.1.7.3 基本内部方法的不变量

ECMAScript 引擎中的内部方法必须符合以下指定的不变量列表。普通 ECMAScript 对象以及该规范中的标准怪异对象应保持这些不变量。ECMAScript Proxy 对象通过运行时检查 [[ProxyHandler]] 对象的陷阱调用结果的手段来保持这些不变量。

任何实现提供的怪异对象必须也保持这些不变量。违反这些不变量可能会导致 ECMAScript 代码出现不可预料的行为以及安全问题。然而，对于造成内存安全的违反不变量行为的实现决不能妥协。

实现不允许通过任何方式规避这些不变量。比如，通过提供替代接口来实现基本内部方法的功能而不强制执行他们的不变量。

定义:

内部方法的 target 是调用这个内部方法的对象。
如果 target 的 [[IsExtensible]] 内部方法返回值是 false 或者 [[PreventExtensions]] 内部方法返回值是 true，那么这个 target 是不可扩展的
一个不存在的属性是指一个不可扩展的 target 的自身属性中不存在的属性。
所有对 SameValue 的引用都是根据 SameValue 算法定义的。

[[GetPrototypeOf]] ( )

返回的类型必须是 Object 或者 Null。
如果 target 是不可扩展的，并且 [[GetPrototypeOf]] 返回了一个值 v，那么任何将来对 [[GetPrototypeOf]] 的调用都应该返回与 v 相同的 SameValue。

Note 1

对象的原型链的长度应该是有限的（也就是说，从任意对象开始，递归地对结果应用 [[GetPrototypeOf]] 内置方法最终应该得到 null）。但是，如果原型链包含任何不使用 [[GetPrototypeOf]] 的普通对象定义的奇异对象，则此要求不强制作为对象级不变量。这样的环形原型链可能会在访问对象属性时产生无限循环。

[[SetPrototypeOf]] (`V`)

返回的类型必须是 Boolean。
如果 target 是不可扩展的，[[SetPrototypeOf]] 必须返回 false，除非 V 与 target 执行 [[GetPrototypeOf]] 内置方法的 SameValue 相同。

[[IsExtensible]] ( )

返回的类型必须是 Boolean。
如果 [[IsExtensible]] 返回 false，所有将来对 target 的 [[IsExtensible]] 内置方法调用都必须返回 false。

[[PreventExtensions]] ( )

返回的类型必须是 Boolean。
如果 [[PreventExtensions]] 返回 false，所有将来对 target 的 [[IsExtensible]] 内置方法的的调用都必须返回 false，并且此刻开始 target 被认为是不可扩展的。

[[GetOwnProperty]] (`P`)

返回的类型必须是 Property Descriptor 或者 Undefined。
如果返回的类型是 Property Descriptor，那么返回值必须是一个完整的 property descriptor (见6.2.5.6)。
一个属性 P 被数据属性 Desc 描述，如果 Desc.[[Value]] 等于 v，且 Desc.[[Writable]] 以及 Desc.[[Configurable]] 都为 false，那么所有将来调用 [[GetOwnProperty]](P) 内置方法时，返回的 SameValue 必须是属性的 Desc.[[Value]]。
如果 P 属性除了 [[Writable]] 特性，其他特性在将来可能变化，或者属性可能消失，那么 P 的 [[Configurable]] 特性必须为 true。
如果 P 属性的 [[Writable]] 特性可能从 false 变为 true，那么 [[Configurable]] 特性必须为 true。
如果对象是不可扩展的且 P 属性不存在，那么将来所有对 target 的 [[GetOwnProperty]] (P) 内置方法的调用必须描述 P 属性不存在（即 [[GetOwnProperty]] (P) 必须返回 undefined）。

Note 2

作为第三个不变量的推论，如果一个属性被描述为一个数据属性并且随着时间它可能返回不同的值，那么 Desc.[[Writable]] 和 Desc.[[Configurable]] 特性必须是 true，虽然没有机制通过其它内置方法去改变暴露出来的值。

[[DefineOwnProperty]] (`P`, `Desc`)

返回的类型必须是 Boolean。
如果 P 预先已经作为 target 的一个不可配置的自身属性被观察，那么 [[DefineOwnProperty]] 必须返回 false，除非满足一下条件之一：
1. P 是一个不可配置的可写数据属性。一个不可配置的可写数据属性可以被转换为一个不可配置不可写的数据属性。
2. Desc 中的所有特性都是作为 P 属性特性的 SameValue。
如果 target 是不可配置的且 P 属性不存在，那么 [[DefineOwnProperty]] (P, Desc) 必须返回 false。也就是说，一个不可配置的 target 对象不能扩展新属性。

[[HasProperty]] ( `P` )

返回的类型必须是 Boolean。
如果 target 自身属性 P 预先已经作为一个不可配置的数据属性或者访问器属性被观察，[[HasProperty]] 必须返回 true。

[[Get]] (`P`, `Receiver`)

如果 P，其值为 V，预先作为一个不可配置，不可写的 target 自身属性被观察，那么 [[GET]] 必须返回 SameValue。
如果 P，其 [[Get]] 特性为 undefined，预先作为一个不可配置的 target 自身访问器属性被观察，那么 [[Get]] 操作必须返回 undefined。

[[Set]] ( `P`, `V`, `Receiver`)

返回的类型必须是 Boolean。
如果 P 预先作为一个不可配置，不可写的 target 自身数据属性被观察，那么 [[Set]] 必须返回 false。除非 V 是 P 的 [[Value]] 特性的 SameValue。
如果 P，其 [[Set]] 特性为 undefined，预先作为一个不可配置的 target 自身访问器属性被观察，那么 [[Set]] 操作必须返回 false。

[[Delete]] ( `P` )

返回的类型必须是 Boolean。
如果 P 预先作为一个不可配置的 target 自身数据或访问器属性被观察，那么 [[Delete]] 必须返回 false。

[[OwnPropertyKeys]] ( )

返回值必须是一个 List。
返回的列表不能包含任何重复的元素。
返回的 List 的每个元素类型为 String 或者 Symbol。
返回的 List 必须包含至少预先已经被观察的自身属性的所有不可配置的 key。
如果对象是不可扩展的，那么返回的 List 必须只包含使用 [[GetOwnProperty]] 观察的对象所有属性的 key。

[[Construct]] ( )

返回值类型必须是 Object。

6.1.7.4 总所周知的内部对象

总所周知的内联函数是在该规范中通过算法明确引用的内置对象，这些内置对象通常有特定域的不变量。除非特别指出，每个内部对象在每个领域通常对应一系列相似对象的集合。

在本规范中像 %name% 这样的引用意味着关联到相应的 name 域的内部对象。决定当前域以及他的内联函数在8.3描述。总所周知的内联函数在表Table 7列出。

Table 7: Well-known Intrinsic Objects

内在名称	全局名称	ECMAScript 语言关联
%Array%	`Array`	`Array` 构造函数 (22.1.1)
%ArrayBuffer%	`ArrayBuffer`	`ArrayBuffer` 构造函数 (24.1.2)
%ArrayBufferPrototype%	`ArrayBuffer.prototype`	%ArrayBuffer% `prototype` 数据属性的初始值
%ArrayIteratorPrototype%		数组迭代器对象的原型 (22.1.5)
%ArrayPrototype%	`Array.prototype`	%Array% `prototype` 数据属性的初始值 (22.1.3)
%ArrayProto_entries%	`Array.prototype.entries`	%ArrayPrototype% `entries` 数据属性的初始值 (22.1.3.4)
%ArrayProto_forEach%	`Array.prototype.forEach`	%ArrayPrototype% `forEach` 数据属性的初始值 (22.1.3.10)
%ArrayProto_keys%	`Array.prototype.keys`	%ArrayPrototype% `keys` 数据属性的初始值 (22.1.3.14)
%ArrayProto_values%	`Array.prototype.values`	%ArrayPrototype% `values` 数据属性的初始值 (22.1.3.30)
%AsyncFromSyncIteratorPrototype%		The prototype of async-from-sync iterator objects (25.1.4)
%AsyncFunction%		async 函数对象的构造器 (25.7.1)
%AsyncFunctionPrototype%		%AsyncFunction% `prototype` 数据属性的初始值
%AsyncGenerator%		The initial value of the `prototype` property of %AsyncGeneratorFunction%
%AsyncGeneratorFunction%		The constructor of async iterator objects (25.3.1)
%AsyncGeneratorPrototype%		The initial value of the `prototype` property of %AsyncGenerator%
%AsyncIteratorPrototype%		An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%	`Atomics`	`Atomics` 对象 (24.4)
%Boolean%	`Boolean`	`Boolean` 构造函数 (19.3.1)
%BooleanPrototype%	`Boolean.prototype`	%Boolean% `prototype` 数据属性的初始值 (19.3.3)
%DataView%	`DataView`	`DataView` 构造函数 (24.3.2)
%DataViewPrototype%	`DataView.prototype`	%DataView% `prototype` 数据属性的初始值
%Date%	`Date`	`Date` 构造函数 (20.3.2)
%DatePrototype%	`Date.prototype`	%Date% `prototype` 数据属性的初始值
%decodeURI%	`decodeURI`	`decodeURI` 函数 (18.2.6.2)
%decodeURIComponent%	`decodeURIComponent`	`decodeURIComponent` 函数 (18.2.6.3)
%encodeURI%	`encodeURI`	`encodeURI` 函数 (18.2.6.4)
%encodeURIComponent%	`encodeURIComponent`	`encodeURIComponent` 函数 (18.2.6.5)
%Error%	`Error`	`Error` 构造函数 (19.5.1)
%ErrorPrototype%	`Error.prototype`	%Error% `prototype` 数据属性的初始值
%eval%	`eval`	`eval` 函数 (18.2.1)
%EvalError%	`EvalError`	`EvalError` 构造函数 (19.5.5.1)
%EvalErrorPrototype%	`EvalError.prototype`	%EvalError% `prototype` 属性的初始值
%Float32Array%	`Float32Array`	`Float32Array` 构造函数 (22.2)
%Float32ArrayPrototype%	`Float32Array.prototype`	%Float32Array% `prototype` 数据属性的初始值
%Float64Array%	`Float64Array`	`Float64Array` 构造函数 (22.2)
%Float64ArrayPrototype%	`Float64Array.prototype`	%Float64Array% `prototype` 数据属性的初始值
%Function%	`Function`	`Function` 构造函数 (19.2.1)
%FunctionPrototype%	`Function.prototype`	%Function% `prototype` 数据属性的初始值
%Generator%		%GeneratorFunction% `prototype` 属性的初始值
%GeneratorFunction%		generator 对象的构造函数 (25.2.1)
%GeneratorPrototype%		%Generator% `prototype` 属性的初始值
%Int8Array%	`Int8Array`	`Int8Array` 构造函数 (22.2)
%Int8ArrayPrototype%	`Int8Array.prototype`	%Int8Array% `prototype` 数据属性的初始值
%Int16Array%	`Int16Array`	`Int16Array` 构造函数 (22.2)
%Int16ArrayPrototype%	`Int16Array.prototype`	%Int16Array% `prototype` 数据属性的初始值
%Int32Array%	`Int32Array`	`Int32Array` 构造函数 (22.2)
%Int32ArrayPrototype%	`Int32Array.prototype`	%Int32Array% `prototype` 数据属性的初始值
%isFinite%	`isFinite`	`isFinite` 函数 (18.2.2)
%isNaN%	`isNaN`	`isNaN` 函数 (18.2.3)
%IteratorPrototype%		所有标准内置迭代对象间接继承的对象
%JSON%	`JSON`	`JSON` 对象 (24.5)
%JSONParse%	`JSON.parse`	%JSON% `parse` 数据属性的初始值
%Map%	`Map`	`Map` 构造函数 (23.1.1)
%MapIteratorPrototype%		Map 迭代器对象的原型 (23.1.5)
%MapPrototype%	`Map.prototype`	%Map% `prototype` 数据属性的初始值
%Math%	`Math`	`Math` 对象 (20.2)
%Number%	`Number`	`Number` 构造函数 (20.1.1)
%NumberPrototype%	`Number.prototype`	%Number% `prototype` 属性的初始值
%Object%	`Object`	`Object` 构造函数 (19.1.1)
%ObjectPrototype%	`Object.prototype`	%Object%. (19.1.3) `prototype` 数据属性的初始值
%ObjProto_toString%	`Object.prototype.toString`	%ObjectPrototype% (19.1.3.6) `toString` 数据属性的初始值
%ObjProto_valueOf%	`Object.prototype.valueOf`	%ObjectPrototype% (19.1.3.7) `valueOf` 数据属性的初始值
%parseFloat%	`parseFloat`	`parseFloat` 函数 (18.2.4)
%parseInt%	`parseInt`	`parseInt` 函数 (18.2.5)
%Promise%	`Promise`	`Promise` 构造函数 (25.6.3)
%PromisePrototype%	`Promise.prototype`	%Promise% `prototype` 数据属性的初始值
%PromiseProto_then%	`Promise.prototype.then`	The initial value of the `then` data property of %PromisePrototype% (25.6.5.4)
%Promise_all%	`Promise.all`	The initial value of the `all` data property of %Promise% (25.6.4.1)
%Promise_reject%	`Promise.reject`	The initial value of the `reject` data property of %Promise% (25.6.4.4)
%Promise_resolve%	`Promise.resolve`	The initial value of the `resolve` data property of %Promise% (25.6.4.5)
%Proxy%	`Proxy`	`Proxy` 构造函数 (26.2.1)
%RangeError%	`RangeError`	`RangeError` 构造函数 (19.5.5.2)
%RangeErrorPrototype%	`RangeError.prototype`	%RangeError% `prototype` 属性的初始值
%ReferenceError%	`ReferenceError`	`ReferenceError` 构造函数 (19.5.5.3)
%ReferenceErrorPrototype%	`ReferenceError.prototype`	%ReferenceError% `prototype` 属性的初始值
%Reflect%	`Reflect`	`Reflect` 对象 (26.1)
%RegExp%	`RegExp`	`RegExp` 构造函数 (21.2.3)
%RegExpPrototype%	`RegExp.prototype`	%RegExp% `prototype` 数据属性的初始值
%Set%	`Set`	`Set` 构造函数 (23.2.1)
%SetIteratorPrototype%		Set 迭代器对象的原型 (23.2.5)
%SetPrototype%	`Set.prototype`	%Set% `prototype` 数据属性的初始值
%SharedArrayBuffer%	`SharedArrayBuffer`	`SharedArrayBuffer` 构造函数 (24.2.2)
%SharedArrayBufferPrototype%	`SharedArrayBuffer.prototype`	%SharedArrayBuffer% `prototype` 数据属性的初始值
%String%	`String`	`String` 构造函数 (21.1.1)
%StringIteratorPrototype%		String 迭代器对象的原型 (21.1.5)
%StringPrototype%	`String.prototype`	%String% `prototype` 数据属性的初始值
%Symbol%	`Symbol`	`Symbol` 构造函数 (19.4.1)
%SymbolPrototype%	`Symbol.prototype`	%Symbol% `prototype` 数据属性的初始值 (19.4.3)
%SyntaxError%	`SyntaxError`	`SyntaxError` 构造函数 (19.5.5.4)
%SyntaxErrorPrototype%	`SyntaxError.prototype`	%SyntaxError% `prototype` 属性的初始值
%ThrowTypeError%		一个函数对象，无条件的抛出一个新 %TypeError% 实例
%TypedArray%		所有数组类型构造器的超类 (22.2.1)
%TypedArrayPrototype%		%TypedArray% `prototype 属性的初始值
%TypeError%	`TypeError`	`TypeError` 构造函数 (19.5.5.5)
%TypeErrorPrototype%	`TypeError.prototype`	%TypeError% `prototype` 属性的初始值
%Uint8Array%	`Uint8Array`	`Uint8Array` 构造函数 (22.2)
%Uint8ArrayPrototype%	`Uint8Array.prototype`	%Uint8Array% `prototype` 数据属性的初始值
%Uint8ClampedArray%	`Uint8ClampedArray`	`Uint8ClampedArray` 构造函数 (22.2)
%Uint8ClampedArrayPrototype%	`Uint8ClampedArray.prototype`	%Uint8ClampedArray% `prototype` 数据属性的初始值
%Uint16Array%	`Uint16Array`	`Uint16Array` 构造函数 (22.2)
%Uint16ArrayPrototype%	`Uint16Array.prototype`	%Uint16Array% `prototype` 数据属性的初始值
%Uint32Array%	`Uint32Array`	`Uint32Array` 构造函数 (22.2)
%Uint32ArrayPrototype%	`Uint32Array.prototype`	%Uint32Array% `prototype` 数据属性的初始值
%URIError%	`URIError`	`URIError` 构造函数 (19.5.5.6)
%URIErrorPrototype%	`URIError.prototype`	%URIError% `prototype` 属性的初始值
%WeakMap%	`WeakMap`	`WeakMap` 构造函数 (23.3.1)
%WeakMapPrototype%	`WeakMap.prototype`	%WeakMap% `prototype` 数据属性的初始值
%WeakSet%	`WeakSet`	`WeakSet` 构造函数 (23.4.1)
%WeakSetPrototype%	`WeakSet.prototype`	%WeakSet% `prototype` 数据属性的初始值

6.2 ECMAScript规范类型

规范类型对应那些在算法中用来描述 ECMAScript 语言结构以及 ECMAScript 语言类型语义的元数据。规范类型包括 Reference，List，Completion，Property Descriptor，Lexical Environment，Environment Record，以及 Data Block。规范类型值是规范中才有的，在一个 ECMAScript 实现中不必对应任何特定的实体。规范类型值可以用来描述 ECMAScript 表达式解析后的中间结果，但是这些值不应该作为 ECMAScript 变量的对象属性或者值储存。

6.2.1 List 以及 Record 规范类型

List 类型用来解释在 new 表达式，函数调用，以及其它需要一个简单的有序的值的 list 的算法中的参数列表(见12.3.6)的求值，List 类型的值是包含单个值的列表元素的简单有序序列。这些序列可能是任意长度。列表的元素可以通过使用从 0 开始的索引随机访问。为了方便起见，可以使用类似数组的语法来访问 List 元素。例如，arguments[2] 简短的表示 arguments List 的第三个元素。

在规范中为了方便，一个字面量语法被用来表达一个新 List 值。如 « 1, 2 » 定义一个新 List 值，它拥有两个元素，每个元素被初始化为一个特定的值。一个空的 List 值可以用 « » 表示。

Record 类型用来描述在该规范中的算法内的数据聚合。一个 Record 类型值包含一个或多个命名字段。每个字段的值是一个 ECMAScript 值或者通过由与 Record 类型相关联的名称代表的抽象值。字段名称始终用双括号括起来，例如 [[Value]]。

在规范中为了方便，使用类似对象字面量的语法来表示一个 Record 值。如 { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } 定义一个拥有3个字段的 Record 值。每个字段被初始化为一个特定的值。字段名称顺序不重要。任何未明确列出的字段都被视为不存在。

在规范文本和算法中，点记号可以用于指代记录值的特定字段。例如，如果 R 是上一段所示的记录，则 R.[[Field2]] 是 “R 中名称为 [[Field2]] 的字段 ” 的缩写。

应该命名常用记录字段组合的模式，并且该名称可以用作Record字面量的前缀，以标识特定类型聚合的描述。例如：PropertyDescriptor { [[Value]]: 42，[[Writable]]: false，[[Configurable]]: true }。

6.2.2 Set 和 Relation 规范类型

Set 类型被用来解释在内存模型中无序元素的集合。Set 类型的值是简单的元素的集合，其中没有任何元素出现超过一次。元素可以从 Set 中添加或者删除。不同的 Set 可以合并，相交或者相减。

Relation 类型被用来解释在 Set 上的约束。一个关系类型的值是它的的值域中有序对的集合。例如，一个在 event 上的关系是一个有序的的 event 对的集合。对于一个关系 R 以及在 R 的值域中的两个值 a 和 b，a R b 是有序对 (a, b) 是 R 的成员的简写。当 Relation 满足这些条件的最小 Relation 时，Relation 对于某些条件是最小的。

严格偏序是指一个 Relation 值 R 满足如下条件。

对于R值域中的 a，b，c：
- a R a 不成立，且
- 如果 a R b 并且 b R c 成立，那么 a R c 成立。

Note 1

上述两种关系的特性被称为自反性和传递性。

严格全序是指一个 Relation 值 R 满足如下条件。

R 值域中的 a，b，c：
- a 全等于 b 或 a R b 或 b R a，且
- a R a 不成立，且
- 如果 a R b 同时 b R c，那么 a R c。

Note 2

上述三种关系的特性，按顺序被称为完全性，自反性，传递性

6.2.3 Completion Record 规范类型

Completion 类型是一个用来解释运行时数值和控制流传播的 Record，如执行非局部转移控制语句(break，continue，return 和 throw)的行为。

Completion 类型值是字段名为表Table 8中的Record值。这些值被称为Completion Record。

Table 8: Completion Record Fields

字段名	值	意义
[[Type]]	normal，break，continue，return，throw 之一	结束发生的完成类型
[[Value]]	任何 ECMAScript 语言值或 empty	产生的值
[[Target]]	任何 ECMAScript 字符串或 empty	用于定向控制传输的目标标签。

术语 “异常结束” 是指 [[Type]] 值为除 normal 之外的值。

6.2.3.1 Await

Algorithm steps that say

Let completion be Await(promise).

mean the same thing as:

Let asyncContext be the running execution context.
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Perform ! Call(promiseCapability.[[Resolve]], undefined, « promise »).
Let stepsFulfilled be the algorithm steps defined in Await Fulfilled Functions.
Let onFulfilled be CreateBuiltinFunction(stepsFulfilled, « [[AsyncContext]] »).
Set onFulfilled.[[AsyncContext]] to asyncContext.
Let stepsRejected be the algorithm steps defined in Await Rejected Functions.
Let onRejected be CreateBuiltinFunction(stepsRejected, « [[AsyncContext]] »).
Set onRejected.[[AsyncContext]] to asyncContext.
Let throwawayCapability be ! NewPromiseCapability(%Promise%).
Set throwawayCapability.[[Promise]].[[PromiseIsHandled]] to true.
Perform ! PerformPromiseThen(promiseCapability.[[Promise]], onFulfilled, onRejected, throwawayCapability).
Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
Set the code evaluation state of asyncContext such that when evaluation is resumed with a Completion completion, the following steps of the algorithm that invoked Await will be performed, with completion available.

where all variables in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.

Note

Await can be combined with the ? and ! prefixes, so that for example

Let value be ? Await(promise).

means the same thing as:

Let value be Await(promise).
ReturnIfAbrupt(value).

6.2.3.1.1 Await Fulfilled Functions

An Await fulfilled function is an anonymous built-in function that is used as part of the Await specification device to deliver the promise fulfillment value to the caller as a normal completion. Each Await fulfilled function has an [[AsyncContext]] internal slot.

When an Await fulfilled function F is called with argument value, the following steps are taken:

Let asyncContext be F.[[AsyncContext]].
Let prevContext be the running execution context.
Suspend prevContext.
Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
Resume the suspended evaluation of asyncContext using NormalCompletion(value) as the result of the operation that suspended it.
Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
Return undefined.

The length property of an Await fulfilled function is 1.

6.2.3.1.2 Await Rejected Functions

An Await rejected function is an anonymous built-in function that is used as part of the Await specification device to deliver the promise rejection reason to the caller as an abrupt throw completion. Each Await rejected function has an [[AsyncContext]] internal slot.

When an Await rejected function F is called with argument reason, the following steps are taken:

Let asyncContext be F.[[AsyncContext]].
Let prevContext be the running execution context.
Suspend prevContext.
Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
Resume the suspended evaluation of asyncContext using ThrowCompletion(reason) as the result of the operation that suspended it.
Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
Return undefined.

The length property of an Await rejected function is 1.

6.2.3.2 NormalCompletion

抽象操作 NormalCompletion 具有单一 argument，如:

Return NormalCompletion(argument).

Is a shorthand that is defined as follows:

Return Completion { [[Type]]: normal, [[Value]]: argument, [[Target]]: empty }.

6.2.3.3 ThrowCompletion

The abstract operation ThrowCompletion with a single argument, such as:

Return ThrowCompletion(argument).

是下面定义的简写:

Return Completion{[[Type]]: throw, [[Value]]: argument, [[Target]]: empty}.

6.2.3.4 UpdateEmpty ( `completionRecord`, `value` )

抽象操作 UpdateEmpty 有参数 completionRecord 和value，他的执行步骤如下:

Assert: If completionRecord.[[Type]] is either return or throw, then completionRecord.[[Value]] is not empty.
If completionRecord.[[Value]] is not empty, return Completion(completionRecord).
Return Completion { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.4 Reference 规范类型

Note

Reference 类型用于解释如 delete，typeof，分配操作，super 关键字以及其他语言功能的操作的行为。例如，分配一个左操作数预计将产生一个引用。

一个 Reference 是一个解析后的名称或者属性组合。一个 Reference 包含3个组件，基值组件，引用名组件，以及一个 Boolean 值的严格引用标志。一个基值组件可以是 undefined，Object， Boolean，String，Symbol，Number, 或 Environment Record。一个基值组件为 undefined 表示 Reference 解析为一个组合。引用名组件是一个 String，或 Symbol。

一个 Super Reference 是指，用于表示使用 super 关键字表示一个名称组合的 Reference。一个 Super Reference 有一个额外的 thisValue 组件，且他的基值组件永远不会是一个 Environment Record。

以下抽象操作在本规范中用于操作引用：

6.2.4.1 GetBase ( `V` )

Assert: Type(V) is Reference.
Return the base value component of V.

6.2.4.2 GetReferencedName ( `V` )

Assert: Type(V) is Reference.
Return the referenced name component of V.

6.2.4.3 IsStrictReference ( `V` )

Assert: Type(V) is Reference.
Return the strict reference flag of V.

6.2.4.4 HasPrimitiveBase ( `V` )

Assert: Type(V) is Reference.
If Type(V's base value component) is Boolean, String, Symbol, or Number, return true; otherwise return false.

6.2.4.5 IsPropertyReference ( `V` )

Assert: Type(V) is Reference.
If either the base value component of V is an Object or HasPrimitiveBase(V) is true, return true; otherwise return false.

6.2.4.6 IsUnresolvableReference ( `V` )

Assert: Type(V) is Reference.
If the base value component of V is undefined, return true; otherwise return false.

6.2.4.7 IsSuperReference ( `V` )

Assert: Type(V) is Reference.
If V has a thisValue component, return true; otherwise return false.

6.2.4.8 GetValue ( `V` )

ReturnIfAbrupt(V).
If Type(V) is not Reference, return V.
Let base be GetBase(V).
If IsUnresolvableReference(V) is true, throw a ReferenceError exception.
If IsPropertyReference(V) is true, then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be undefined or null.
  2. Set base to ! ToObject(base).
2. Return ? base.[[Get]](GetReferencedName(V), GetThisValue(V)).
Else base must be an Environment Record,
1. Return ? base.GetBindingValue(GetReferencedName(V), IsStrictReference(V)) (see 8.1.1).

Note

在步骤 5.a.ii 中创建的对象在上面抽象方法和普通对象 [[GET]] 内部方法之外是不可访问的。实现可以选择避免实际创建对象。

6.2.4.9 PutValue ( `V`, `W` )

ReturnIfAbrupt(V).
ReturnIfAbrupt(W).
If Type(V) is not Reference, throw a ReferenceError exception.
Let base be GetBase(V).
If IsUnresolvableReference(V) is true, then
1. If IsStrictReference(V) is true, then
  1. Throw a ReferenceError exception.
2. Let globalObj be GetGlobalObject().
3. Return ? Set(globalObj, GetReferencedName(V), W, false).
Else if IsPropertyReference(V) is true, then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be undefined or null.
  2. Set base to ! ToObject(base).
2. Let succeeded be ? base.[[Set]](GetReferencedName(V), W, GetThisValue(V)).
3. If succeeded is false and IsStrictReference(V) is true, throw a TypeError exception.
4. Return.
Else base must be an Environment Record,
1. Return ? base.SetMutableBinding(GetReferencedName(V), W, IsStrictReference(V)) (see 8.1.1).

Note

在步骤 6.a.ii 中创建的对象在上面抽象方法和普通对象 [[SET]] 内部方法之外是不可访问的。实现可以选择避免实际创建对象。

6.2.4.10 GetThisValue ( `V` )

Assert: IsPropertyReference(V) is true.
If IsSuperReference(V) is true, then
1. Return the value of the thisValue component of the reference V.
Return GetBase(V).

6.2.4.11 InitializeReferencedBinding ( `V`, `W` )

ReturnIfAbrupt(V).
ReturnIfAbrupt(W).
Assert: Type(V) is Reference.
Assert: IsUnresolvableReference(V) is false.
Let base be GetBase(V).
Assert: base is an Environment Record.
Return base.InitializeBinding(GetReferencedName(V), W).

6.2.5 Property Descriptor 规范类型

Property Descriptor 类型用来解释对象属性的特性的操作和具体化。Property Descriptor 的值为 Record 类型。每个字段的名称都是一个特性名称，其值是 6.1.7.1 相应的特性值。另外，任何字段都可以存在或不存在。在本规范中用于标记 Property Descriptor 记录的字面量描述是 “PropertyDescriptor”。

Property Descriptor 值可以根据某些字段的存在或使用进一步分类为数据 Property Descriptor 和访问器 Property Descriptor。包含字段 [[Value]] 或 [[Writable]] 的是数据 Property Descriptor。包含字段 [[Get]] 或 [[Set]] 的是访问器 Property Descriptor。所有的 Property Descriptor 都可能包含 [[Enumerable]]，[[Configurable]] 字段名。一个 Property Descriptor 值可能不会同时为一个数据 Property Descriptor 和一个访问器 Property Descriptor。然而，也不一定，一个通用 Property Descriptor 是一个既不是数据 Property Descriptor 也不是访问器 Property Descriptor 的 Property Descriptor值。一个完全填充的 Property Descriptor 既是一个数据 Property Descriptor 也是一个访问器 Property Descriptor，他有所有符合表 Table 2 和 Table 3 定义的属性特性字段。

在本规范中使用以下抽象操作用来操作 Property Descriptor值：

6.2.5.1 IsAccessorDescriptor ( `Desc` )

当用 Property Descriptor Desc 作为参数调用抽象操作 IsAccessorDescriptor 时，采取以下步骤：

If Desc is undefined, return false.
If both Desc.[[Get]] and Desc.[[Set]] are absent, return false.
Return true.

6.2.5.2 IsDataDescriptor ( `Desc` )

当用 Property Descriptor Desc 作为参数调用抽象操作 IsDataDescriptor 时，采取以下步骤：

If Desc is undefined, return false.
If both Desc.[[Value]] and Desc.[[Writable]] are absent, return false.
Return true.

6.2.5.3 IsGenericDescriptor ( `Desc` )

当用 Property Descriptor Desc 作为参数调用抽象操作 IsGenericDescriptor 时，采取以下步骤：

If Desc is undefined, return false.
If IsAccessorDescriptor(Desc) and IsDataDescriptor(Desc) are both false, return true.
Return false.

6.2.5.4 FromPropertyDescriptor ( `Desc` )

当用Property Descriptor Desc作为参数调用抽象操作 FromPropertyDescriptor 时，采取以下步骤：

If Desc is undefined, return undefined.
Let obj be ObjectCreate(%ObjectPrototype%).
Assert: obj is an extensible ordinary object with no own properties.
If Desc has a [[Value]] field, then
1. Perform CreateDataProperty(obj, "value", Desc.[[Value]]).
If Desc has a [[Writable]] field, then
1. Perform CreateDataProperty(obj, "writable", Desc.[[Writable]]).
If Desc has a [[Get]] field, then
1. Perform CreateDataProperty(obj, "get", Desc.[[Get]]).
If Desc has a [[Set]] field, then
1. Perform CreateDataProperty(obj, "set", Desc.[[Set]]).
If Desc has an [[Enumerable]] field, then
1. Perform CreateDataProperty(obj, "enumerable", Desc.[[Enumerable]]).
If Desc has a [[Configurable]] field, then
1. Perform CreateDataProperty(obj, "configurable", Desc.[[Configurable]]).
Assert: All of the above CreateDataProperty operations return true.
Return obj.

6.2.5.5 ToPropertyDescriptor ( `Obj` )

当用一个对象 Obj 作为参数调用抽象操作 ToPropertyDescriptor 时，采取以下步骤：

If Type(Obj) is not Object, throw a TypeError exception.
Let desc be a new Property Descriptor that initially has no fields.
Let hasEnumerable be ? HasProperty(Obj, "enumerable").
If hasEnumerable is true, then
1. Let enum be ToBoolean(? Get(Obj, "enumerable")).
2. Set desc.[[Enumerable]] to enum.
Let hasConfigurable be ? HasProperty(Obj, "configurable").
If hasConfigurable is true, then
1. Let conf be ToBoolean(? Get(Obj, "configurable")).
2. Set desc.[[Configurable]] to conf.
Let hasValue be ? HasProperty(Obj, "value").
If hasValue is true, then
1. Let value be ? Get(Obj, "value").
2. Set desc.[[Value]] to value.
Let hasWritable be ? HasProperty(Obj, "writable").
If hasWritable is true, then
1. Let writable be ToBoolean(? Get(Obj, "writable")).
2. Set desc.[[Writable]] to writable.
Let hasGet be ? HasProperty(Obj, "get").
If hasGet is true, then
1. Let getter be ? Get(Obj, "get").
2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
3. Set desc.[[Get]] to getter.
Let hasSet be ? HasProperty(Obj, "set").
If hasSet is true, then
1. Let setter be ? Get(Obj, "set").
2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
3. Set desc.[[Set]] to setter.
If desc.[[Get]] is present or desc.[[Set]] is present, then
1. If desc.[[Value]] is present or desc.[[Writable]] is present, throw a TypeError exception.
Return desc.

6.2.5.6 CompletePropertyDescriptor ( `Desc` )

当用 Property Descriptor Desc 作为参数调用抽象操作 CompletePropertyDescriptor 时，采取以下步骤：

Assert: Desc is a Property Descriptor.
Let like be Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
2. If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return Desc.

6.2.6 Lexical Environment 和 Environment Record 规范类型

Lexical Environment and Environment Record 是用来解释在嵌套的函数和块中的名称解析行为。这些类型以及他们的操作在 8.1 中定义。

6.2.7 Data Blocks

Data Block 规范类型用来描述不同且可变的字节(8 bit)大小的数字值序列。一个 Data Block 值由初始值为 0，固定数量的字节构成。

在本规范中为了方便，可以使用类似数组的语法来访问一个 Data Block 值的各个字节。这些符号将 Data Block 值的这些字节表示为一个从 0 开始的索引队列。例如，如果 db 是一个 5 字节的 Data Block 值，那么 db[2] 可以用来访问第三字节。

驻留在内存中，可以被多个代理并发引用的数据块被称为 Shared Data Block。一个 Shared Data Block 有一个 空地址 的标识（目的是平等检测 Shared Data Block 值）：它不依赖于所述块在任何进程中映射到的虚拟地址，而是用该块来表示内存中的位置集合。只有当两个块包含的位置集合相等时他们才相等。否则，他们地址集合的交集为空，他们是不相等的。最后，可以从 Data Block 区分 Shared Data Blocks。

Shared Data Block 的语义由内存模型中的 Shared Data Block 事件来定义。下方的抽象操作介绍了 Shared Data Block 事件，以及在内存模型上解析语义和事件语义的接口的行为。事件形成了一个候选执行，内存模型扮演了一个过滤器的角色。请查看内存模型来了解完整的语义。

Shared Data Block 事件由 Record 建模, 通过内存模型定义。

下列抽象操作在本规范中用于对 Data Block 值进行操作：

6.2.7.1 CreateByteDataBlock ( `size` )

当用整数参数 size 调用抽象操作 CreateByteDataBlock 时，采取以下步骤：

Assert: size≥0.
Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, throw a RangeError exception.
Set all of the bytes of db to 0.
Return db.

6.2.7.2 CreateSharedByteDataBlock ( `size` )

当用整数参数 size 调用抽象操作 CreateByteDataBlock 时，采取以下步骤：

Assert: size≥0.
Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
Let eventList be the [[EventList]] field of the element in execution.[[EventLists]] whose [[AgentSignifier]] is AgentSignifier().
Let zero be « 0 ».
For each index i of db, do
1. Append WriteSharedMemory { [[Order]]: "Init", [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventList.
Return db.

6.2.7.3 CopyDataBlockBytes ( `toBlock`, `toIndex`, `fromBlock`, `fromIndex`, `count` )

当抽象操作 CreateByteDataBlock 被调用时，采取以下步骤：

Assert: fromBlock and toBlock are distinct Data Block or Shared Data Block values.
Assert: fromIndex, toIndex, and count are integer values ≥ 0.
Let fromSize be the number of bytes in fromBlock.
Assert: fromIndex+count ≤ fromSize.
Let toSize be the number of bytes in toBlock.
Assert: toIndex+count ≤ toSize.
Repeat, while count>0
1. If fromBlock is a Shared Data Block, then
  1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  2. Let eventList be the [[EventList]] field of the element in execution.[[EventLists]] whose [[AgentSignifier]] is AgentSignifier().
  3. Let bytes be a List of length 1 that contains a nondeterministically chosen byte value.
  4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
  5. Let readEvent be ReadSharedMemory { [[Order]]: "Unordered", [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
  6. Append readEvent to eventList.
  7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
  8. If toBlock is a Shared Data Block, then
    1. Append WriteSharedMemory { [[Order]]: "Unordered", [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventList.
  9. Else,
    1. Set toBlock[toIndex] to bytes[0].
2. Else,
  1. Assert: toBlock is not a Shared Data Block.
  2. Set toBlock[toIndex] to fromBlock[fromIndex].
3. Increment toIndex and fromIndex each by 1.
4. Decrement count by 1.
Return NormalCompletion(empty).

7 抽象操作

这些操作不是 ECMAScript 语言的一部分；在这里定义他们只是为了规范 ECMAScript 语言的语义。另外，本规范中还定义了其他特定的抽象操作。

7.1 类型转换

ECMAScript 语言根据需要会自动执行类型转换。为了解释清楚某些结构的语义，定义一组转换抽象操作是很有用的。转换抽象操作是多态的；他们可以接受一个任意 ECMAScript 类型的值。但是这些操作中没有使用其他相应的规范类型。

7.1.1 ToPrimitive ( `input` [ , `PreferredType` ] )

抽象操作 ToPrimitive 接受一个 input 参数和一个可选的 PreferredType 参数。ToPrimitive 负责把 input 参数转换为一个非对象类型。如果一个对象可以转换为多个原始类型，则应该根据可选提示 PreferredType 来判断转换的类型。根据以下算法进行转换：

Assert: input is an ECMAScript language value.
If Type(input) is Object, then
1. If PreferredType is not present, let hint be "default".
2. Else if PreferredType is hint String, let hint be "string".
3. Else PreferredType is hint Number, let hint be "number".
4. Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
5. If exoticToPrim is not undefined, then
  1. Let result be ? Call(exoticToPrim, input, « hint »).
  2. If Type(result) is not Object, return result.
  3. Throw a TypeError exception.
6. If hint is "default", set hint to "number".
7. Return ? OrdinaryToPrimitive(input, hint).
Return input.

Note

当调用 ToPrimitive 没有提示参数时，通常表现和提示为 Number 类型一样。然而，对象可能通过定义一个 @@toPrimitive 方法，来覆盖这一行为。在本规范中的对象只有 Date 对象 (见 20.3.4.45) 和 Symbol(见 19.4.3.4) 会覆盖默认的 ToPrimitive 行为。Date 对象处理没有提示的调用就像提示为 String 类型一样。

7.1.1.1 OrdinaryToPrimitive ( `O`, `hint` )

当使用参数 O 和 hint 调用抽象操作 OrdinaryToPrimitive 时，执行以下步骤：

Assert: Type(O) is Object.
Assert: Type(hint) is String and its value is either "string" or "number".
If hint is "string", then
1. Let methodNames be « "toString", "valueOf" ».
Else,
1. Let methodNames be « "valueOf", "toString" ».
For each name in methodNames in List order, do
1. Let method be ? Get(O, name).
2. If IsCallable(method) is true, then
  1. Let result be ? Call(method, O).
  2. If Type(result) is not Object, return result.
Throw a TypeError exception.

7.1.2 ToBoolean ( `argument` )

抽象操作 ToBoolean 把参数 argument 转换为一个 Boolean 类型的值，见表 Table 9：

Table 9: ToBoolean Conversions

参数类型	结果
Undefined	返回 false。
Null	返回 false。
Boolean	返回 `argument`。
Number	如果 `argument` 为 +0，-0，or NaN，返回 false; 否则返回 true。
String	如果 `argument` 是一个空字符串 (长度为0的字符串)，返回 false; 否则返回 true。
Symbol	返回 true。
Object	返回 true。

7.1.3 ToNumber ( `argument` )

抽象操作把 argument 转换为一个 Number 类型的值，转换结果见表 Table 10：

Table 10: ToNumber Conversions

参数类型	结果
Undefined	返回 NaN。
Null	返回 +0。
Boolean	如果 `argument` 是 true，返回 1。如果 `argument` 是 false，返回 +0。
Number	返回 `argument` (不转换)。
String	见下面语法和转换算法。
Symbol	抛出一个 TypeError 异常。
Object	应用以下步骤： Let `primValue` be ? ToPrimitive(`argument`, hint Number). Return ? ToNumber(`primValue`).

7.1.3.1 应用于 String 类型的 ToNumber

应用 ToNumber 的字符串使用如下语法把字符串解释为一个 UTF-16 的编码码点 (6.1.4)。如果语法不能解释字符串为一个 StringNumericLiteral 终结符的扩展，那么 ToNumber 的结果是 NaN。

Note 1

这个语法的终结符都由 Unicode BMP(Basic Multilingual Plane) 码点组成，所以，如果字符串包含任何首代理或尾代理码元，不管是配对还是未配对，那么 ToNumber 的结果就会是 NaN。

Syntax

:::

opt

opt

opt

:::

opt

:::

:::

:::

:::

Infinity

opt

opt

opt

opt

以上所有未明确定义的语法符号都在数字字面量词法定义(11.8.3)

Note 2

应该注意 StringNumericLiteral 和 NumericLiteral 语法之间的一些区别：

StringNumericLiteral 可能包含前导和(或)结尾的空白和(或)行终止符。
十进制的 StringNumericLiteral 可能包含多位前导 0 的数字。
十进制的 StringNumericLiteral 可能包含一个 + 或 - 以指示其符号。
空的或者只有空格的 StringNumericLiteral 会被转换为 +0。
Infinity 和 -Infinity 公认是 StringNumericLiteral 而不是 NumericLiteral。

7.1.3.1.1 运行时语义: MV

字符串到 Number 值的转换和一个的数字字面量 Number 值的确定总体上来说是相似的(见 11.8.3)，但是也有一些细微的差别，因此在这里给出了转换一个 String 类型的数字字面量为一个 Number 类型的值的过程。这个数字由以下两个步骤确定：首先，从 String 类型的数字字面量导出数值(MV)由数学值(MV，以下简称数值)；然后，这个数字像下面描述的那样舍入。很多语法符号的数值不在下面提供，在 11.8.3.1 定义。

StringNumericLiteral:::[empty] 的数值 0.
StringNumericLiteral:::StrWhiteSpace 的数值是 0.
StringNumericLiteral:::StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt 的数值是 StrNumericLiteral 的数值, 无论是否存在空格.
StrNumericLiteral:::StrDecimalLiteral 的数值是 StrDecimalLiteral 的数值.
StrNumericLiteral:::BinaryIntegerLiteral 的数值是 BinaryIntegerLiteral 的数值.
StrNumericLiteral:::OctalIntegerLiteral 的数值是 OctalIntegerLiteral 的数值.
StrNumericLiteral:::HexIntegerLiteral 的数值是 HexIntegerLiteral 的数值.
StrDecimalLiteral:::StrUnsignedDecimalLiteral 的数值是 StrUnsignedDecimalLiteral 的数值.
StrDecimalLiteral:::+StrUnsignedDecimalLiteral 的数值是 StrUnsignedDecimalLiteral 的数值.
StrDecimalLiteral:::-StrUnsignedDecimalLiteral 的数值是 StrUnsignedDecimalLiteral 的数值的负数.(注意，如果 StrUnsignedDecimalLiteral 的数值是 0，那么这个数值的负数也是 0。下面的舍入规则描述了合理的处理转换无符号数字 0 为一个浮点数 +0 或 -0。)
StrUnsignedDecimalLiteral:::Infinity 的数值是 10¹⁰⁰⁰⁰ (一个这么大的数字将舍入为 +∞).
StrUnsignedDecimalLiteral:::DecimalDigits. 的数值是 DecimalDigits 的数值.
StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigits 的数值是第一个 DecimalDigits 的数值加上(第二个 DecimalDigits 的数值乘上 10^-n), 这里的 n 是第二个 DecimalDigits 的码点数.
StrUnsignedDecimalLiteral:::DecimalDigits.ExponentPart 的数值是 DecimalDigits 的数值乘上 10^e, 这里的 e 是 ExponentPart 的数值.
StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigitsExponentPart 的数值是(第一个 DecimalDigits 的数值加上(第二个 DecimalDigits 的数值乘以 10^-n))乘以 10^e, 这里 n 第二个 DecimalDigits 码点数，e 是 ExponentPart 的数值.
StrUnsignedDecimalLiteral:::.DecimalDigits 的数值是 DecimalDigits 的数值乘上 10^-n, 这里的 n 是 DecimalDigits 码点数.
StrUnsignedDecimalLiteral:::.DecimalDigitsExponentPart 的数值是 DecimalDigits 的数值乘以 10^e-n, 这里 n 是 DecimalDigits 的码点数，e 是 ExponentPart 的数值.
StrUnsignedDecimalLiteral:::DecimalDigits 的数值是 DecimalDigits 的数值.
StrUnsignedDecimalLiteral:::DecimalDigitsExponentPart 的数值是 DecimalDigits 的数值乘上 10^e, e 是 ExponentPart 的数值.

一旦确定了字符串数字字面量确切的数值，它将被舍入为一个 Number 类型的值。如果数值为 0，那么它舍入的值为 +0，除非字符串数字字面量的第一个非空格码点是 "-"，这种情况舍入值为 -0。否则，舍入的值必须为一个数字类型的数值（定义在 6.1.6 ）除非这个字面量包含一个 StrUnsignedDecimalLiteral 并且拥有超过 20 位的有效数值，这种情况下，这个数值可以是由 0 替换字面量第 20 位后面的每个有效数字的字面量数值，也可以是在替换后从第 20 位数位递增的字面量数值。如果数字不是一个 ExponentPart 的部分，并且

这个数字不是 0; 或者
数字的左侧是一个非零的数字且他的右边是一个不在 ExponentPart 的非零数字。那么这个数字是有效的。

7.1.4 ToInteger ( `argument` )

抽象操作 ToInteger 把 argument 转换为整数数值。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, return +0.
If number is +0, -0, +∞, or -∞, return number.
Return the number value that is the same sign as number and whose magnitude is floor(abs(number)).

7.1.5 ToInt32 ( `argument` )

抽象操作 ToInt32 把 argument 转换为范围为 -2³¹ 到 2³¹-1的 2³² 个整数之一,这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int32bit be int modulo 2³².
If int32bit ≥ 2³¹, return int32bit - 2³²; otherwise return int32bit.

Note

鉴于以上 ToInt32 的定义:

ToInt32 是等幂的: 其作用在同一个元素上产生的结果相同。
对于任意 x，ToInt32(ToUint32(x)) 等于 ToInt32(x)。 (这是为了保证 +∞ 和 -∞ 会被映射为 +0。)
ToInt32 把 -0 映射为 +0.

7.1.6 ToUint32 ( `argument` )

抽象操作 ToUint32 将 argument 转换为 0 到 2³²-1 的 2³² 个整数之一。该抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int32bit be int modulo 2³².
Return int32bit.

Note

鉴于以上 ToUint32 的定义：

ToUint32 与ToInt32 只有第五步不同。
ToUint32 是等幂的: 其作用在同一个元素上产生的结果相同。
对于任意 x，ToUint32(ToInt32(x)) 等于 ToUint32(x)。 (这是为了保证 +∞ 和 -∞ 会被映射为 +0。)
ToUint32 把 -0 映射为 +0。

7.1.7 ToInt16 ( `argument` )

抽象操作 ToInt16 将 argument 转换为 -32768 到 32767 的 2¹⁶ 个整数之一。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int16bit be int modulo 2¹⁶.
If int16bit ≥ 2¹⁵, return int16bit - 2¹⁶; otherwise return int16bit.

7.1.8 ToUint16 ( `argument` )

抽象操作 ToUint16 将 argument 转换为 0 到 2¹⁶-1 的 2¹⁶ 个整数之一。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int16bit be int modulo 2¹⁶.
Return int16bit.

Note

鉴于以上 ToUint16 的定义：

ToUint32 与 ToUint16 不同之处在于第四步时用 2¹⁶ 替换了 2³²。
ToUint16 把 -0 映射为 +0.

7.1.9 ToInt8 ( `argument` )

抽象操作 ToInt8 负责将 argument 转换为 -128 到 127 的 2⁸ 个整数之一。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int8bit be int modulo 2⁸.
If int8bit ≥ 2⁷, return int8bit - 2⁸; otherwise return int8bit.

7.1.10 ToUint8 ( `argument` )

抽象操作 ToUint8 负责将 argument 转换为 0 到 255 的 2⁸ 个整数之一。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, +0, -0, +∞, or -∞, return +0.
Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
Let int8bit be int modulo 2⁸.
Return int8bit.

7.1.11 ToUint8Clamp ( `argument` )

抽象操作 ToUint8Clamp 负责将 argument 转换为 0 到 255 的 2⁸ 个整数之一。这个抽象操作算法如下：

Let number be ? ToNumber(argument).
If number is NaN, return +0.
If number ≤ 0, return +0.
If number ≥ 255, return 255.
Let f be floor(number).
If f + 0.5 < number, return f + 1.
If number < f + 0.5, return f.
If f is odd, return f + 1.
Return f.

Note

与其他 ECMAScript 整数转换抽象操作不同。 ToUint8Clamp 对非整数值使用取整而不是截断，并且不会把 +∞ 转换为 0。

7.1.12 ToString ( `argument` )

抽象操作 ToString 负责将 argument 转换为一个 String 类型值，如下Table 11:

Table 11: ToString Conversions

参数类型	结果
Undefined	返回 `"undefined"`.
Null	返回 `"null"`.
Boolean	如果 `argument` 是 true, 返回 `"true"`. 如果 `argument` 是 false, 返回 `"false"`.
Number	返回 NumberToString(`argument`).
String	返回 `argument`.
Symbol	抛出一个 TypeError 异常.
Object	应用如下步骤： Let `primValue` be ? ToPrimitive(`argument`, hint String). Return ? ToString(`primValue`).

7.1.12.1 NumberToString ( `m` )

抽象操作 NumberToString 负责将一个 Number 类型值 m 转换为 String 类型，步骤如下：

If m is NaN, return the String "NaN".
If m is +0 or -0, return the String "0".
If m is less than zero, return the string-concatenation of "-" and ! NumberToString(-m).
If m is +∞, return the String "Infinity".
Otherwise, let n, k, and s be integers such that k ≥ 1, 10^k-1 ≤ s < 10^k, the Number value for s × 10^n-k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
If k ≤ n ≤ 21, return the string-concatenation of:
- the code units of the k digits of the decimal representation of s (in order, with no leading zeroes)
- n-k occurrences of the code unit 0x0030 (DIGIT ZERO)
If 0 < n ≤ 21, return the string-concatenation of:
- the code units of the most significant n digits of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k-n digits of the decimal representation of s
If -6 < n ≤ 0, return the string-concatenation of:
- the code unit 0x0030 (DIGIT ZERO)
- the code unit 0x002E (FULL STOP)
- -n occurrences of the code unit 0x0030 (DIGIT ZERO)
- the code units of the k digits of the decimal representation of s
Otherwise, if k = 1, return the string-concatenation of:
- the code unit of the single digit of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n-1 is positive or negative
- the code units of the decimal representation of the integer abs(n-1) (with no leading zeroes)
Return the string-concatenation of:
- the code units of the most significant digit of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k-1 digits of the decimal representation of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n-1 is positive or negative
- the code units of the decimal representation of the integer abs(n-1) (with no leading zeroes)

Note 1

以下结论作为实现的参考可能是有用的，不过他们并不作为规范的要求。

对于任何非 -0 的 Number 类型值 x，他的 ToNumber(ToString(x)) 结果恰好为 Number 值 x。
s 的最少有效数位并不总是由第五步中的要求列表唯一确定。

Note 2

对于提供了比上述转换规则更准确的实现，建议其中第五步参考下面的另一个版本：

Otherwise, let n, k, and s be integers such that k ≥ 1, 10^k-1 ≤ s < 10^k, the Number value for s × 10^n-k is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10^n-k is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.

Note 3

David M. Gay 编写的二进制转换为十进制中关于浮点数的文章和代码对于 ECMAScript 实现者可能会有帮助：

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. 相关文章可以在
http://ampl.com/REFS/abstracts.html#rounding 找到. 相关代码在
http://netlib.sandia.gov/fp/dtoa.c 和
http://netlib.sandia.gov/fp/g_fmt.c ，这些代码也可以在其他 netlib 镜像网站找到。

7.1.13 ToObject ( `argument` )

抽象操作 ToObject 负责将 argument 转换为 Object 类型的值，见表Table 12：

Table 12: ToObject Conversions

参数类型	结果
Undefined	抛出一个 TypeError 异常。
Null	抛出一个 TypeError 异常。
Boolean	返回一个内部槽 [[BooleanData]] 为 `argument` 的新 Boolean 对象。见 19.3 关于 Boolean 对象的描述。
Number	返回一个内部槽 [[NumberData]] 为 `argument` 的新 Number 对象。见 20.1 关于 Number 对象的描述。
String	返回一个内部槽 [[StringData]] 为 `argument` 的新 String 对象。见 21.1 关于 String 对象的描述。
Symbol	返回一个内部槽 [[SymbolData]] 为 `argument` 的新 Symbol 对象。见 19.4 关于 Symbol 对象的描述。
Object	返回 `argument`。

7.1.14 ToPropertyKey ( `argument` )

抽象操作 ToPropertyKey 负责将 argument 转换为一个可用作属性 key 的值，其步骤如下：

Let key be ? ToPrimitive(argument, hint String).
If Type(key) is Symbol, then
1. Return key.
Return ! ToString(key).

7.1.15 ToLength ( `argument` )

抽象操作 ToLength 负责将 argument 转换为一个可用作类数组的长度的整数，其步骤如下：

Let len be ? ToInteger(argument).
If len ≤ +0, return +0.
Return min(len, 2⁵³-1).

7.1.16 CanonicalNumericIndexString ( `argument` )

如果 argument 为是一可以通过 ToString 产生的数字字符串，或者是一个为 "-0" 的字符串，则抽象操作 CanonicalNumericIndexString 返回转换 argument 得到的一个数字。否则，返回 undefined。这个抽象操作算法如下：

Assert: Type(argument) is String.
If argument is "-0", return -0.
Let n be ! ToNumber(argument).
If SameValue(! ToString(n), argument) is false, return undefined.
Return n.

一个规范的数字字符串是指 CanonicalNumericIndexString 抽象操作返回值不为 undefined 的字符串。

7.1.17 ToIndex ( `value` )

抽象操作 ToIndex 负责将 value 参数转换为一个有效的整数索引值。这个抽象操作算法如下：

If value is undefined, then
1. Let index be 0.
Else,
1. Let integerIndex be ? ToInteger(value).
2. If integerIndex < 0, throw a RangeError exception.
3. Let index be ! ToLength(integerIndex).
4. If SameValueZero(integerIndex, index) is false, throw a RangeError exception.
Return index.

7.2 测试与比较操作

7.2.1 RequireObjectCoercible ( `argument` )

抽象操作 RequireObjectCoercible 尝试使用 ToObject 将 argument 转换为一个对象，如果转换失败，则抛出一个错误，成功则返回 argument。如下表Table 13中定义的：

Table 13: RequireObjectCoercible Results

参数类型	结果
Undefined	抛出一个 TypeError 异常。
Null	抛出一个 TypeError 异常。
Boolean	返回 `argument`.
Number	返回 `argument`.
String	返回 `argument`.
Symbol	返回 `argument`.
Object	返回 `argument`.

7.2.2 IsArray ( `argument` )

抽象操作 IsArray 接受一个参数 argument，他的执行步骤如下：

If Type(argument) is not Object, return false.
If argument is an Array exotic object, return true.
If argument is a Proxy exotic object, then
1. If argument.[[ProxyHandler]] is null, throw a TypeError exception.
2. Let target be argument.[[ProxyTarget]].
3. Return ? IsArray(target).
Return false.

7.2.3 IsCallable ( `argument` )

抽象操作 IsCallable 用于判断 argument 是否为一个拥有 [[call]] 内部方法的可调用函数，argument 必须是 ECMAScript 语言值。

If Type(argument) is not Object, return false.
If argument has a [[Call]] internal method, return true.
Return false.

7.2.4 IsConstructor ( `argument` )

抽象操作 IsConstructor 用于判断 argument 是否为一个拥有 [[Construct]] 内部方法的函数对象，argument 必须是 ECMAScript 语言值。

If Type(argument) is not Object, return false.
If argument has a [[Construct]] internal method, return true.
Return false.

7.2.5 IsExtensible ( `O` )

抽象操作 IsExtensible 用于判断对象 O 是否可以添加新的属性，返回一个 Boolean 值。该抽象操作的执行步骤如下：

Assert: Type(O) is Object.
Return ? O.[[IsExtensible]]().

7.2.6 IsInteger ( `argument` )

抽象操作 IsInteger 用于判断 argument 是否为一个有限整数数值。

If Type(argument) is not Number, return false.
If argument is NaN, +∞, or -∞, return false.
If floor(abs(argument)) ≠ abs(argument), return false.
Return true.

7.2.7 IsPropertyKey ( `argument` )

抽象操作 IsPropertyKey 用于判断 argument 是否可用作为对象的属性的 key，argument 必须是一个 ECMAScript 语言值。

If Type(argument) is String, return true.
If Type(argument) is Symbol, return true.
Return false.

7.2.8 IsRegExp ( `argument` )

参数为 argument 的抽象操作 IsRegExp 执行步骤如下：

If Type(argument) is not Object, return false.
Let matcher be ? Get(argument, @@match).
If matcher is not undefined, return ToBoolean(matcher).
If argument has a [[RegExpMatcher]] internal slot, return true.
Return false.

7.2.9 IsStringPrefix ( `p`, `q` )

抽象操作 IsStringPrefix 确定字符串 p 是不是字符串 q 的前缀。

Assert: Type(p) is String.
Assert: Type(q) is String.
If q can be the string-concatenation of p and some other String r, return true. Otherwise, return false.
NOTE: Any String is a prefix of itself, because r may be the empty String.

7.2.10 SameValue ( `x`, `y` )

内部比较抽象操作 SameValue(x, y) 返回 true 或 false，其中 x 和 y 都是 ECMAScript 语言值。它将执行比较如下：

If Type(x) is different from Type(y), return false.
If Type(x) is Number, then
1. If x is NaN and y is NaN, return true.
2. If x is +0 and y is -0, return false.
3. If x is -0 and y is +0, return false.
4. If x is the same Number value as y, return true.
5. Return false.
Return SameValueNonNumber(x, y).

Note

这个算法在比较有符号的 0 以及比较 NaN 时和严格相等比较算法不同。

7.2.11 SameValueZero ( `x`, `y` )

内部比较抽象操作 SameValueZero(x, y) 返回 true 或 false，其中 x 和 y 都是ECMAScript语言值。它将执行比较如下：

If Type(x) is different from Type(y), return false.
If Type(x) is Number, then
1. If x is NaN and y is NaN, return true.
2. If x is +0 and y is -0, return true.
3. If x is -0 and y is +0, return true.
4. If x is the same Number value as y, return true.
5. Return false.
Return SameValueNonNumber(x, y).

Note

SameValueZero 与 SameValue 仅在处理 +0 和 -0 时不同

7.2.12 SameValueNonNumber ( `x`, `y` )

内部比较抽象操作 SameValueNonNumber(x, y) 返回 true 或 false，其中 x 和 y 都是 Number 值。它将执行比较如下：

Assert: Type(x) is not Number.
Assert: Type(x) is the same as Type(y).
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is String, then
1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
If Type(x) is Boolean, then
1. If x and y are both true or both false, return true; otherwise, return false.
If Type(x) is Symbol, then
1. If x and y are both the same Symbol value, return true; otherwise, return false.
If x and y are the same Object value, return true. Otherwise, return false.

7.2.13 抽象关系比较

比较x < y，x 和 y 都是 ECMAScript 语言值，将可能产生true, false, 或 undefined（表示其中至少有一个操作数为 NaN）。除了 x 和 y 之外，算法会使用一个名为 LeftFirst 的 Boolean 类型标志，作为一个参数。这个标志用于控制对 x 和 y 执行潜在可见副作用操作的顺序。这是有必要的，因为 ECMAScript 规定计算是从左向右的。LeftFirst 的默认值为 true，表示 x 参数相应表达式在 y 参数相应的表达式的左测。如果 LeftFirst 为 false，则必须在 x 之前执行 y。具体比较执行如下：

If the LeftFirst flag is true, then
1. Let px be ? ToPrimitive(x, hint Number).
2. Let py be ? ToPrimitive(y, hint Number).
Else the order of evaluation needs to be reversed to preserve left to right evaluation,
1. Let py be ? ToPrimitive(y, hint Number).
2. Let px be ? ToPrimitive(x, hint Number).
If Type(px) is String and Type(py) is String, then
1. If IsStringPrefix(py, px) is true, return false.
2. If IsStringPrefix(px, py) is true, return true.
3. Let k be the smallest nonnegative integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
4. Let m be the integer that is the numeric value of the code unit at index k within px.
5. Let n be the integer that is the numeric value of the code unit at index k within py.
6. If m < n, return true. Otherwise, return false.
Else,
1. NOTE: Because px and py are primitive values evaluation order is not important.
2. Let nx be ? ToNumber(px).
3. Let ny be ? ToNumber(py).
4. If nx is NaN, return undefined.
5. If ny is NaN, return undefined.
6. If nx and ny are the same Number value, return false.
7. If nx is +0 and ny is -0, return false.
8. If nx is -0 and ny is +0, return false.
9. If nx is +∞, return false.
10. If ny is +∞, return true.
11. If ny is -∞, return false.
12. If nx is -∞, return true.
13. If the mathematical value of nx is less than the mathematical value of ny—note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.

Note 1

算法中的第三步和第七步不同是加法运算 + (12.8.3)使用逻辑与操作而不是使用逻辑或操作。

Note 2

字符串类型的比较使用码元值序列的简单词典排序。这里没有尝试使用 Unicode 规范定义的更复杂的，基于语义的字符或字符串相等，以及排序顺序进行比较。因此，根据 Unicode 标准在规范上相等的字符串值可能测试为不相等。实际上，该算法假设了两个值是已经标准化的值。另外还需注意，对于包含补充字符的字符串，UTF-16 码元值序列与普通码元值序列的词典排序是不同的。

7.2.14 抽象相等比较

比较 x == y，产生 true，false，其中 x 与 y 都是 ECMAScript 语言值。具体比较算法如下：

If Type(x) is the same as Type(y), then
1. Return the result of performing Strict Equality Comparison x === y.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String, return the result of the comparison x == ! ToNumber(y).
If Type(x) is String and Type(y) is Number, return the result of the comparison ! ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ! ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).
If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
Return false.

7.2.15 严格相等比较

比较 x === y，产生 true，false，其中 x 和 y 都是ECMAScript语言值。具体比较算法如下：

If Type(x) is different from Type(y), return false.
If Type(x) is Number, then
1. If x is NaN, return false.
2. If y is NaN, return false.
3. If x is the same Number value as y, return true.
4. If x is +0 and y is -0, return true.
5. If x is -0 and y is +0, return true.
6. Return false.
Return SameValueNonNumber(x, y).

Note

该算法与SameValue算法不同之处在于对有符号的 0 和 NaN 的处理。

7.3 对象具有的操作

7.3.1 Get ( `O`, `P` )

抽象操作 Get 用于检索对象特定属性的值。该操作接受两个参数 O, P，其中 O 是操作的对象，P 是对象的属性 key。该抽象操作执行以下步骤：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return ? O.[[Get]](P, O).

7.3.2 GetV ( `V`, `P` )

抽象操作 GetV 用于检索一个 ECMAScript 语言值指定属性的值。如果这个 ECMAScript 值不是对象，则使用合适该值类型的属性包装器对象来执行属性的查找。使用参数 V 和 P 调用操作([[Get]])，其中 V 是传入的 ECMAScript 值，P 是需要检索的属性 key。该抽象操作的执行步骤如下：

Assert: IsPropertyKey(P) is true.
Let O be ? ToObject(V).
Return ? O.[[Get]](P, V).

7.3.3 Set ( `O`, `P`, `V`, `Throw` )

抽象操作 Set 用于设置对象的特定属性的值。该操作使用参数 O， P， V， Throw 调用，其中 O 是被操作的对象，P 是制定的属性， V 是属性的新值，Throw 是一个 Boolean 类型的标志。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Assert: Type(Throw) is Boolean.
Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is true, throw a TypeError exception.
Return success.

7.3.4 CreateDataProperty ( `O`, `P`, `V` )

抽象操作 CreateDataProperty 用于给对象创建一个新的自身属性。这个操作使用参数 O， P 和 V 调用，其中 O 是被操作的对象，P 是属性 key 值，V 是属性的值。这个抽象操作执行以下步骤：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).

Note

该抽象操作创建一个和通过 ECMAScript 语言赋值运算符创建的属性相同特性的属性。通常，这个属性（开始）是不存在的。如果该属性存在并且是不可配置的，或者对象 O 是不可扩展的，则内置方法 [[DefineOwnProperty]] 将返回 false。

7.3.5 CreateMethodProperty ( `O`, `P`, `V` )

抽象操作 CreateMethodProperty 用于给对象创建一个新的自身属性。接受参数 O，P，V，其中 O 是目标对象，P 是属性 key，V 是属性值。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).

Note

这个抽象操作创建的属性和那些内部方法或者使用类定于语法定义的方法使用相同的默认特性。该抽象操作创建一个和通过 ECMAScript 语言赋值运算符创建的属性相同特性的属性。通常，这个属性（开始）是不存在的。如果该属性存在并且是不可配置的，或者对象 O 是不可扩展的，则内置方法 [[DefineOwnProperty]] 将返回 false。

7.3.6 CreateDataPropertyOrThrow ( `O`, `P`, `V` )

抽象操作 CreateDataPropertyOrThrow 用于给对象自身添加一个新属性，如果更新属性的请求不能执行将抛出 TypeError 异常。接受参数 O，P，V，其中 O 是目标对象，P 是属性 key，V 是属性值。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? CreateDataProperty(O, P, V).
If success is false, throw a TypeError exception.
Return success.

Note

7.3.7 DefinePropertyOrThrow ( `O`, `P`, `desc` )

抽象操作 DefinePropertyOrThrow 通过调用对象内部方法 [[DefineOwnProperty]] 的方式定义一个自身属性，如果更新属性的请求不能执行将抛出 TypeError 异常。该操作接受参数 O，P，desc，其中 O 是目标对象，P 是属性 key，desc 是属性的 Property Descriptor。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError exception.
Return success.

7.3.8 DeletePropertyOrThrow ( `O`, `P` )

抽象操作 DeletePropertyOrThrow 用于删除指定对象属性。如果属性是不可配置的将抛出（TypeError）异常。参数 O 表示目标对象，P 是指定需要删除的属性 key。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be ? O.[[Delete]](P).
If success is false, throw a TypeError exception.
Return success.

7.3.9 GetMethod ( `V`, `P` )

抽象操作 GetMethod 用于从一个 ECMAScript 语言值获取指定属性的值，该值应该是一个函数。参数 V 表示一个被操作的 ECMAScript 语言值，P 表示指定的属性 key。该操作的执行步骤如下：

Assert: IsPropertyKey(P) is true.
Let func be ? GetV(V, P).
If func is either undefined or null, return undefined.
If IsCallable(func) is false, throw a TypeError exception.
Return func.

7.3.10 HasProperty ( `O`, `P` )

抽象操作 HasProperty 用于确定一个对象是否拥有特定的属性。这个属性可以是自身的也可以是继承的。该抽象操作返回一个 Boolean 值。参数 O 表示目标对象，P 是指定的属性 key。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return ? O.[[HasProperty]](P).

7.3.11 HasOwnProperty ( `O`, `P` )

抽象操作 HasOwnProperty 用于确定对象自身是否拥有特定的属性。返回一个 Boolean 值。参数 O 表示目标对象，P 是指定的属性 key。该操作的执行步骤如下：

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return false.
Return true.

7.3.12 Call ( `F`, `V` [ , `argumentsList` ] )

抽象操作 Call 负责调用一个函数对象的 [[Call]] 内部方法。该操作接受一个函数对象 F，一个代表 [[Call]] 的 this 值的 ECMAScript 语言值 V，以及传递给内部方法的可选参数 argumentsList。如果 argument 没提供，则默认使用一个空的 List。该操作的执行步骤如下：

If argumentsList is not present, set argumentsList to a new empty List.
If IsCallable(F) is false, throw a TypeError exception.
Return ? F.[[Call]](V, argumentsList).

7.3.13 Construct ( `F` [ , `argumentsList` [ , `newTarget` ]] )

抽象操作 Construct 用于调用函数对象内部方法 [[Construct]]。该抽象操作使用参数 F, 可选参数 argumentsList, 以及可选参数 newTarget。其中 F 是一个函数对象. argumentsList 和 newTarget 是作为内部方法相应参数传递的值。如果 argumentsList 未提供则默认使用一个空 List 作为其值。如果 newTarget 未提供，则使用 F 作为其值。该抽象操作执行步骤如下：

If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty List.
Assert: IsConstructor(F) is true.
Assert: IsConstructor(newTarget) is true.
Return ? F.[[Construct]](argumentsList, newTarget).

Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.14 SetIntegrityLevel ( `O`, `level` )

抽象操作 SetIntegrityLevel 用于固定一个对象自身属性集合。这个抽象操作执行步骤如下：

Assert: Type(O) is Object.
Assert: level is either "sealed" or "frozen".
Let status be ? O.[[PreventExtensions]]().
If status is false, return false.
Let keys be ? O.[[OwnPropertyKeys]]().
If level is "sealed", then
1. For each element k of keys, do
  1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]: false }).
Else level is "frozen",
1. For each element k of keys, do
  1. Let currentDesc be ? O.[[GetOwnProperty]](k).
  2. If currentDesc is not undefined, then
    1. If IsAccessorDescriptor(currentDesc) is true, then
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false }.
    2. Else,
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
    3. Perform ? DefinePropertyOrThrow(O, k, desc).
Return true.

7.3.15 TestIntegrityLevel ( `O`, `level` )

抽象操作 TestIntegrityLevel 用于检查对象自身属性是否被固定。这个抽象操作执行步骤如下：

Assert: Type(O) is Object.
Assert: level is either "sealed" or "frozen".
Let status be ? IsExtensible(O).
If status is true, return false.
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
1. Let currentDesc be ? O.[[GetOwnProperty]](k).
2. If currentDesc is not undefined, then
  1. If currentDesc.[[Configurable]] is true, return false.
  2. If level is "frozen" and IsDataDescriptor(currentDesc) is true, then
    1. If currentDesc.[[Writable]] is true, return false.
Return true.

7.3.16 CreateArrayFromList ( `elements` )

抽象操作 CreateArrayFromList 用于创建一个数组对象，其元素由由一个 List 的参数提供。该抽象操作执行以下步骤：

Assert: elements is a List whose elements are all ECMAScript language values.
Let array be ! ArrayCreate(0).
Let n be 0.
For each element e of elements, do
1. Let status be CreateDataProperty(array, ! ToString(n), e).
2. Assert: status is true.
3. Increment n by 1.
Return array.

7.3.17 CreateListFromArrayLike ( `obj` [ , `elementTypes` ] )

抽象操作 CreateListFromArrayLike 用于创建一个 List 值，其元素通过类数组对象 obj 属性的索引提供。可选参数 elementTypes 是一个包含 ECMAScript 语言类型的名称列表，表示被创建的 List 元素值被允许的类型。该抽象操作的执行步骤如下：

If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, Object ».
If Type(obj) is not Object, throw a TypeError exception.
Let len be ? ToLength(? Get(obj, "length")).
Let list be a new empty List.
Let index be 0.
Repeat, while index < len
1. Let indexName be ! ToString(index).
2. Let next be ? Get(obj, indexName).
3. If Type(next) is not an element of elementTypes, throw a TypeError exception.
4. Append next as the last element of list.
5. Set index to index + 1.
Return list.

7.3.18 Invoke ( `V`, `P` [ , `argumentsList` ] )

抽象操作 Invoke 用于调用一个 ECMAScript 语言值的方法属性。该操作用参数以及可选参数 argumentsList 调用，其中 V 即作为属性的查找点也作为调用的 this 值， P 是属性 key， argumentsList 是传递给方法的参数列表。如果 argumentsList 未提供，则使用一个空的 List 作为他的值。该抽象操作执行步骤如下：

Assert: IsPropertyKey(P) is true.
If argumentsList is not present, set argumentsList to a new empty List.
Let func be ? GetV(V, P).
Return ? Call(func, V, argumentsList).

7.3.19 OrdinaryHasInstance ( `C`, `O` )

抽象操作 OrdinaryHasInstance 执行默认算法，确定对象 O 是否继承自构找函数 C 提供的实例对象继承路劲。该抽象操作执行步骤如下：

If IsCallable(C) is false, return false.
If C has a [[BoundTargetFunction]] internal slot, then
1. Let BC be C.[[BoundTargetFunction]].
2. Return ? InstanceofOperator(O, BC).
If Type(O) is not Object, return false.
Let P be ? Get(C, "prototype").
If Type(P) is not Object, throw a TypeError exception.
Repeat,
1. Set O to ? O.[[GetPrototypeOf]]().
2. If O is null, return false.
3. If SameValue(P, O) is true, return true.

7.3.20 SpeciesConstructor ( `O`, `defaultConstructor` )

抽象操作 SpeciesConstructor 用于检索用于创建新对象的构找函数，该新对象是从参数对象 O 派生的。如果在对象 O 属性中不能找到一个 constructor@@species 属性，则使用 defaultConstructor 作为构造函数。该抽象操作的执行步骤如下：

Assert: Type(O) is Object.
Let C be ? Get(O, "constructor").
If C is undefined, return defaultConstructor.
If Type(C) is not Object, throw a TypeError exception.
Let S be ? Get(C, @@species).
If S is either undefined or null, return defaultConstructor.
If IsConstructor(S) is true, return S.
Throw a TypeError exception.

7.3.21 EnumerableOwnPropertyNames ( `O`, `kind` )

当使用对象 O 和字符串 kind 作为参数调用抽象操作 EnumerableOwnPropertyNames 时执行以下步骤：

Assert: Type(O) is Object.
Let ownKeys be ? O.[[OwnPropertyKeys]]().
Let properties be a new empty List.
For each element key of ownKeys in List order, do
1. If Type(key) is String, then
  1. Let desc be ? O.[[GetOwnProperty]](key).
  2. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. If kind is "key", append key to properties.
    2. Else,
      1. Let value be ? Get(O, key).
      2. If kind is "value", append value to properties.
      3. Else,
        Assert: kind is "key+value".
        Let entry be CreateArrayFromList(« key, value »).
        Append entry to properties.
Order the elements of properties so they are in the same relative order as would be produced by the Iterator that would be returned if the EnumerateObjectProperties internal method were invoked with O.
Return properties.

7.3.22 GetFunctionRealm ( `obj` )

参数为 obj 的抽象操作 GetFunctionRealm 执行以下步骤：

Assert: obj is a callable object.
If obj has a [[Realm]] internal slot, then
1. Return obj.[[Realm]].
If obj is a Bound Function exotic object, then
1. Let target be obj.[[BoundTargetFunction]].
2. Return ? GetFunctionRealm(target).
If obj is a Proxy exotic object, then
1. If obj.[[ProxyHandler]] is null, throw a TypeError exception.
2. Let proxyTarget be obj.[[ProxyTarget]].
3. Return ? GetFunctionRealm(proxyTarget).
Return the current Realm Record.

Note

第五步只有在 obj 是一个没有 [[Realm]] 内部槽的非标准函数怪异对象时执行。

7.3.23 CopyDataProperties ( `target`, `source`, `excludedItems` )

When the abstract operation CopyDataProperties is called with arguments target, source, and excludedItems, the following steps are taken:

Assert: Type(target) is Object.
Assert: excludedItems is a List of property keys.
If source is undefined or null, return target.
Let from be ! ToObject(source).
Let keys be ? from.[[OwnPropertyKeys]]().
For each element nextKey of keys in List order, do
1. Let excluded be false.
2. For each element e of excludedItems in List order, do
  1. If SameValue(e, nextKey) is true, then
    1. Set excluded to true.
3. If excluded is false, then
  1. Let desc be ? from.[[GetOwnProperty]](nextKey).
  2. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. Let propValue be ? Get(from, nextKey).
    2. Perform ! CreateDataProperty(target, nextKey, propValue).
Return target.

Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.4 迭代器对象具有的操作

参考常见的迭代接口 (25.1).

7.4.1 GetIterator ( `obj` [ , `hint` [, `method` ] )

参数为 obj 和可选参数 hint, method 的抽象操作 GetIterator 执行以下步骤：

If hint is not present, set hint to sync.
Assert: hint is either sync or async.
If method is not present, then
1. If hint is async, then
  1. Set method to ? GetMethod(obj, @@asyncIterator).
  2. If method is undefined, then
    1. Let syncMethod be ? GetMethod(obj, @@iterator).
    2. Let syncIteratorRecord be ? GetIterator(obj, sync, syncMethod).
    3. Return ? CreateAsyncFromSyncIterator(syncIteratorRecord).
2. Otherwise, set method to ? GetMethod(obj, @@iterator).
Let iterator be ? Call(method, obj).
If Type(iterator) is not Object, throw a TypeError exception.
Let nextMethod be ? GetV(iterator, "next").
Let iteratorRecord be Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
Return iteratorRecord.

7.4.2 IteratorNext ( `iteratorRecord` [ , `value` ] )

参数为 iteratorRecord 和可选参数 value 的抽象操作 IteratorNext 执行以下步骤：

If value is not present, then
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « »).
Else,
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »).
If Type(result) is not Object, throw a TypeError exception.
Return result.

7.4.3 IteratorComplete ( `iterResult` )

参数为 iterResult 的抽象操作 IteratorComplete 执行以下步骤：

Assert: Type(iterResult) is Object.
Return ToBoolean(? Get(iterResult, "done")).

7.4.4 IteratorValue ( `iterResult` )

使用参数为 iterResult 的抽象操作 IteratorValue 执行以下步骤：

Assert: Type(iterResult) is Object.
Return ? Get(iterResult, "value").

7.4.5 IteratorStep ( `iteratorRecord` )

使用参数 iteratorRecord 的抽象操作 IteratorStep 通过调用 iteratorRecord.[[NextMethod]] 从 iteratorRecord.[[Iterator]] 中请求 next 值，并返回 false 或一个 IteratorResult 对象。false 表示迭代结束，IteratorResult 对象是一个可用的 next 值。其执行步骤如下：

Let result be ? IteratorNext(iteratorRecord).
Let done be ? IteratorComplete(result).
If done is true, return false.
Return result.

7.4.6 IteratorClose ( `iteratorRecord`, `completion` )

参数为 iteratorRecord 和 completion 的抽象操作 IteratorClose 用于通知迭代器，它应该执行它在达到其完成状态时通常会执行的一些操作：

Assert: Type(iteratorRecord.[[Iterator]]) is Object.
Assert: completion is a Completion Record.
Let iterator be iteratorRecord.[[Iterator]].
Let return be ? GetMethod(iterator, "return").
If return is undefined, return Completion(completion).
Let innerResult be Call(return, iterator, « »).
If completion.[[Type]] is throw, return Completion(completion).
If innerResult.[[Type]] is throw, return Completion(innerResult).
If Type(innerResult.[[Value]]) is not Object, throw a TypeError exception.
Return Completion(completion).

7.4.7 AsyncIteratorClose ( `iteratorRecord`, `completion` )

The abstract operation AsyncIteratorClose with arguments iteratorRecord and completion is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state:

Assert: Type(iteratorRecord.[[Iterator]]) is Object.
Assert: completion is a Completion Record.
Let iterator be iteratorRecord.[[Iterator]].
Let return be ? GetMethod(iterator, "return").
If return is undefined, return Completion(completion).
Let innerResult be Call(return, iterator, « »).
If innerResult.[[Type]] is normal, set innerResult to Await(innerResult.[[Value]]).
If completion.[[Type]] is throw, return Completion(completion).
If innerResult.[[Type]] is throw, return Completion(innerResult).
If Type(innerResult.[[Value]]) is not Object, throw a TypeError exception.
Return Completion(completion).

7.4.8 CreateIterResultObject ( `value`, `done` )

参数为 value 和 done 的抽象操作 CreateIterResultObject 创建一个支持 IteratorResult 接口的对象，它执行以下操作：

Assert: Type(done) is Boolean.
Let obj be ObjectCreate(%ObjectPrototype%).
Perform CreateDataProperty(obj, "value", value).
Perform CreateDataProperty(obj, "done", done).
Return obj.

7.4.9 CreateListIteratorRecord ( `list` )

参数为 list 的抽象操作 CreateListIteratorRecord 创建一个迭代器 (25.1.1.2) 对象记录，其 next 方法返回元素连续的 list。该抽象操作执行以下步骤：

Let iterator be ObjectCreate(%IteratorPrototype%, « [[IteratedList]], [[ListIteratorNextIndex]] »).
Set iterator.[[IteratedList]] to list.
Set iterator.[[ListIteratorNextIndex]] to 0.
Let steps be the algorithm steps defined in ListIterator next (7.4.9.1).
Let next be CreateBuiltinFunction(steps, « »).
Return Record { [[Iterator]]: iterator, [[NextMethod]]: next, [[Done]]: false }.

Note

这个迭代器对象列表从不直接通过 ECMAScript 代码访问。

7.4.9.1 ListIterator next ( )

ListIterator next 方法是一个标准的内置函数方法(章节 17)，它执行以下步骤：

Let O be the this value.
Assert: Type(O) is Object.
Assert: O has an [[IteratedList]] internal slot.
Let list be O.[[IteratedList]].
Let index be O.[[ListIteratorNextIndex]].
Let len be the number of elements of list.
If index ≥ len, then
1. Return CreateIterResultObject(undefined, true).
Set O.[[ListIteratorNextIndex]] to index+1.
Return CreateIterResultObject(list[index], false).

8 Executable Code and Execution Contexts

8.1 Lexical Environments

A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.

An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment. It is referred to as the Lexical Environment's EnvironmentRecord.

The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the current evaluation of the surrounding function.

A global environment is a Lexical Environment which does not have an outer environment. The global environment's outer environment reference is null. A global environment's EnvironmentRecord may be prepopulated with identifier bindings and includes an associated global object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

A module environment is a Lexical Environment that contains the bindings for the top level declarations of a Module. It also contains the bindings that are explicitly imported by the Module. The outer environment of a module environment is a global environment.

A function environment is a Lexical Environment that corresponds to the invocation of an ECMAScript function object. A function environment may establish a new this binding. A function environment also captures the state necessary to support super method invocations.

Lexical Environments and Environment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

8.1.1 Environment Records

There are two primary kinds of Environment Record values used in this specification: declarative Environment Records and object Environment Records. Declarative Environment Records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object Environment Records are used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object. Global Environment Records and function Environment Records are specializations that are used for specifically for Script global declarations and for top-level declarations within functions.

For specification purposes Environment Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses, declarative Environment Record, object Environment Record, and global Environment Record. Function Environment Records and module Environment Records are subclasses of declarative Environment Record. The abstract class includes the abstract specification methods defined in Table 14. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 14: Abstract Methods of Environment Records

Method	Purpose
HasBinding(N)	Determine if an Environment Record has a binding for the String value `N`. Return true if it does and false if it does not.
CreateMutableBinding(N, D)	Create a new but uninitialized mutable binding in an Environment Record. The String value `N` is the text of the bound name. If the Boolean argument `D` is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)	Create a new but uninitialized immutable binding in an Environment Record. The String value `N` is the text of the bound name. If `S` is true then attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)	Set the value of an already existing but uninitialized binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)	Set the value of an already existing mutable binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and may be a value of any ECMAScript language type. `S` is a Boolean flag. If `S` is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S)	Returns the value of an already existing binding from an Environment Record. The String value `N` is the text of the bound name. `S` is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If `S` is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of `S`.
DeleteBinding(N)	Delete a binding from an Environment Record. The String value `N` is the text of the bound name. If a binding for `N` exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding()	Determine if an Environment Record establishes a `this` binding. Return true if it does and false if it does not.
HasSuperBinding()	Determine if an Environment Record establishes a `super` method binding. Return true if it does and false if it does not.
WithBaseObject()	If this Environment Record is associated with a `with` statement, return the with object. Otherwise, return undefined.

8.1.1.1 Declarative Environment Records

Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.

8.1.1.1.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for declarative Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:

Let envRec be the declarative Environment Record for which the method was invoked.
If envRec has a binding for the name that is the value of N, return true.
Return false.

8.1.1.1.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for declarative Environment Records creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument D has the value true the new binding is marked as being subject to deletion.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return NormalCompletion(empty).

8.1.1.1.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding for declarative Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
Return NormalCompletion(empty).

8.1.1.1.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for declarative Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.
Return NormalCompletion(empty).

8.1.1.1.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for declarative Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true.

Let envRec be the declarative Environment Record for which the method was invoked.
If envRec does not have a binding for N, then
1. If S is true, throw a ReferenceError exception.
2. Perform envRec.CreateMutableBinding(N, true).
3. Perform envRec.InitializeBinding(N, V).
4. Return NormalCompletion(empty).
If the binding for N in envRec is a strict binding, set S to true.
If the binding for N in envRec has not yet been initialized, throw a ReferenceError exception.
Else if the binding for N in envRec is a mutable binding, change its bound value to V.
Else,
1. Assert: This is an attempt to change the value of an immutable binding.
2. If S is true, throw a TypeError exception.
Return NormalCompletion(empty).

Note

An example of ECMAScript code that results in a missing binding at step 2 is:

function f(){eval("var x; x = (delete x, 0);")}

8.1.1.1.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for declarative Environment Records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

8.1.1.1.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for declarative Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.

Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec has a binding for the name that is the value of N.
If the binding for N in envRec cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.

8.1.1.1.8 HasThisBinding ( )

Regular declarative Environment Records do not provide a this binding.

Return false.

8.1.1.1.9 HasSuperBinding ( )

Regular declarative Environment Records do not provide a super binding.

Return false.

8.1.1.1.10 WithBaseObject ( )

Declarative Environment Records always return undefined as their WithBaseObject.

Return undefined.

8.1.1.2 Object Environment Records

Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object Environment Records.

Object Environment Records created for with statements (13.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a withEnvironment Boolean value that is associated with each object Environment Record. By default, the value of withEnvironment is false for any object Environment Record.

The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.

8.1.1.2.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for object Environment Records determines if its associated binding object has a property whose name is the value of the argument N:

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Let foundBinding be ? HasProperty(bindings, N).
If foundBinding is false, return false.
If the withEnvironment flag of envRec is false, return true.
Let unscopables be ? Get(bindings, @@unscopables).
If Type(unscopables) is Object, then
1. Let blocked be ToBoolean(? Get(unscopables, N)).
2. If blocked is true, return false.
Return true.

8.1.1.2.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for object Environment Records creates in an Environment Record's associated binding object a property whose name is the String value and initializes it to the value undefined. If Boolean argument D has the value true the new property's [[Configurable]] attribute is set to true; otherwise it is set to false.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? DefinePropertyOrThrow(bindings, N, PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).

Note

Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.

8.1.1.2.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding is never used within this specification in association with object Environment Records.

8.1.1.2.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for object Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the object Environment Record for which the method was invoked.
Assert: envRec must have an uninitialized binding for N.
Record that the binding for N in envRec has been initialized.
Return ? envRec.SetMutableBinding(N, V, false).

Note

In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, implementations do not need to explicitly track the initialization state of individual object Environment Record bindings.

8.1.1.2.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for object Environment Records attempts to set the value of the Environment Record's associated binding object's property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? Set(bindings, N, V, S).

8.1.1.2.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for object Environment Records returns the value of its associated binding object's property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Let value be ? HasProperty(bindings, N).
If value is false, then
1. If S is false, return the value undefined; otherwise throw a ReferenceError exception.
Return ? Get(bindings, N).

8.1.1.2.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for object Environment Records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.

Let envRec be the object Environment Record for which the method was invoked.
Let bindings be the binding object for envRec.
Return ? bindings.[[Delete]](N).

8.1.1.2.8 HasThisBinding ( )

Regular object Environment Records do not provide a this binding.

Return false.

8.1.1.2.9 HasSuperBinding ( )

Regular object Environment Records do not provide a super binding.

Return false.

8.1.1.2.10 WithBaseObject ( )

Object Environment Records return undefined as their WithBaseObject unless their withEnvironment flag is true.

Let envRec be the object Environment Record for which the method was invoked.
If the withEnvironment flag of envRec is true, return the binding object for envRec.
Otherwise, return undefined.

8.1.1.3 Function Environment Records

A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.

Function Environment Records have the additional state fields listed in Table 15.

Table 15: Additional Fields of Function Environment Records

Field Name	Value	Meaning
[[ThisValue]]	Any	This is the this value used for this invocation of the function.
[[ThisBindingStatus]]	`"lexical"` \| `"initialized"` \| `"uninitialized"`	If the value is `"lexical"`, this is an ArrowFunction and does not have a local this value.
[[FunctionObject]]	Object	The function object whose invocation caused this Environment Record to be created.
[[HomeObject]]	Object \| undefined	If the associated function has `super` property accesses and is not an ArrowFunction, [[HomeObject]] is the object that the function is bound to as a method. The default value for [[HomeObject]] is undefined.
[[NewTarget]]	Object \| undefined	If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] `newTarget` parameter. Otherwise, its value is undefined.

Function Environment Records support all of the declarative Environment Record methods listed in Table 14 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed in Table 16:

Table 16: Additional Methods of Function Environment Records

Method	Purpose
BindThisValue(V)	Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()	Return the value of this Environment Record's `this` binding. Throws a ReferenceError if the `this` binding has not been initialized.
GetSuperBase()	Return the object that is the base for `super` property accesses bound in this Environment Record. The object is derived from this Environment Record's [[HomeObject]] field. The value undefined indicates that `super` property accesses will produce runtime errors.

The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:

8.1.1.3.1 BindThisValue ( `V` )

Let envRec be the function Environment Record for which the method was invoked.
Assert: envRec.[[ThisBindingStatus]] is not "lexical".
If envRec.[[ThisBindingStatus]] is "initialized", throw a ReferenceError exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to "initialized".
Return V.

8.1.1.3.2 HasThisBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
If envRec.[[ThisBindingStatus]] is "lexical", return false; otherwise, return true.

8.1.1.3.3 HasSuperBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
If envRec.[[ThisBindingStatus]] is "lexical", return false.
If envRec.[[HomeObject]] has the value undefined, return false; otherwise, return true.

8.1.1.3.4 GetThisBinding ( )

Let envRec be the function Environment Record for which the method was invoked.
Assert: envRec.[[ThisBindingStatus]] is not "lexical".
If envRec.[[ThisBindingStatus]] is "uninitialized", throw a ReferenceError exception.
Return envRec.[[ThisValue]].

8.1.1.3.5 GetSuperBase ( )

Let envRec be the function Environment Record for which the method was invoked.
Let home be envRec.[[HomeObject]].
If home has the value undefined, return undefined.
Assert: Type(home) is Object.
Return ? home.[[GetPrototypeOf]]().

8.1.1.4 Global Environment Records

A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A global Environment Record provides the bindings for built-in globals (clause 18), properties of the global object, and for all top-level declarations (13.2.8, 13.2.10) that occur within a Script.

A global Environment Record is logically a single record but it is specified as a composite encapsulating an object Environment Record and a declarative Environment Record. The object Environment Record has as its base object the global object of the associated Realm Record. This global object is the value returned by the global Environment Record's GetThisBinding concrete method. The object Environment Record component of a global Environment Record contains the bindings for all built-in globals (clause 18) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the declarative Environment Record component of the global Environment Record.

Properties may be created directly on a global object. Hence, the object Environment Record component of a global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableDeclaration declarations and bindings created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.

Global Environment Records have the additional fields listed in Table 17 and the additional methods listed in Table 18.

Table 17: Additional Fields of Global Environment Records

Field Name	Value	Meaning
[[ObjectRecord]]	Object Environment Record	Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings in global code for the associated realm.
[[GlobalThisValue]]	Object	The value returned by `this` in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]]	Declarative Environment Record	Contains bindings for all declarations in global code for the associated realm code except for FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration `bindings`.
[[VarNames]]	List of String	The string names bound by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration declarations in global code for the associated realm.

Table 18: Additional Methods of Global Environment Records

Method	Purpose
GetThisBinding()	Return the value of this Environment Record's `this` binding.
HasVarDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, or AsyncGeneratorDeclaration.
HasLexicalDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N)	Determines if the argument is the name of a global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)	Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument `N`.
CanDeclareGlobalFunction (N)	Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument `N`.
CreateGlobalVarBinding(N, D)	Used to create and initialize to undefined a global `var` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `var`. The String value `N` is the bound name. If `D` is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)	Create and initialize a global `function` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `function`. The String value `N` is the bound name. `V` is the initialization value. If the Boolean argument `D` is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.

8.1.1.4.1 HasBinding ( `N` )

The concrete Environment Record method HasBinding for global Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).

8.1.1.4.2 CreateMutableBinding ( `N`, `D` )

The concrete Environment Record method CreateMutableBinding for global Environment Records creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If Boolean argument D has the value true the new binding is marked as being subject to deletion.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
Return DclRec.CreateMutableBinding(N, D).

8.1.1.4.3 CreateImmutableBinding ( `N`, `S` )

The concrete Environment Record method CreateImmutableBinding for global Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If the Boolean argument S has the value true the new binding is marked as a strict binding.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, throw a TypeError exception.
Return DclRec.CreateImmutableBinding(N, S).

8.1.1.4.4 InitializeBinding ( `N`, `V` )

The concrete Environment Record method InitializeBinding for global Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.InitializeBinding(N, V).
Assert: If the binding exists, it must be in the object Environment Record.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.InitializeBinding(N, V).

8.1.1.4.5 SetMutableBinding ( `N`, `V`, `S` )

The concrete Environment Record method SetMutableBinding for global Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, a TypeError is thrown if S is true. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N, V, S).

8.1.1.4.6 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for global Environment Records returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N, S).

8.1.1.4.7 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for global Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) is true, then
1. Return DclRec.DeleteBinding(N).
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? HasOwnProperty(globalObject, N).
If existingProp is true, then
1. Let status be ? ObjRec.DeleteBinding(N).
2. If status is true, then
  1. Let varNames be envRec.[[VarNames]].
  2. If N is an element of varNames, remove that element from the varNames.
3. Return status.
Return true.

8.1.1.4.8 HasThisBinding ( )

Return true.

8.1.1.4.9 HasSuperBinding ( )

Return false.

8.1.1.4.10 WithBaseObject ( )

Global Environment Records always return undefined as their WithBaseObject.

Return undefined.

8.1.1.4.11 GetThisBinding ( )

Let envRec be the global Environment Record for which the method was invoked.
Return envRec.[[GlobalThisValue]].

8.1.1.4.12 HasVarDeclaration ( `N` )

The concrete Environment Record method HasVarDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration:

Let envRec be the global Environment Record for which the method was invoked.
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains N, return true.
Return false.

8.1.1.4.13 HasLexicalDeclaration ( `N` )

The concrete Environment Record method HasLexicalDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration:

Let envRec be the global Environment Record for which the method was invoked.
Let DclRec be envRec.[[DeclarativeRecord]].
Return DclRec.HasBinding(N).

8.1.1.4.14 HasRestrictedGlobalProperty ( `N` )

The concrete Environment Record method HasRestrictedGlobalProperty for global Environment Records determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexical binding:

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true, return false.
Return true.

Note

Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property undefined is an example of such a property.

8.1.1.4.15 CanDeclareGlobalVar ( `N` )

The concrete Environment Record method CanDeclareGlobalVar for global Environment Records determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let hasProperty be ? HasOwnProperty(globalObject, N).
If hasProperty is true, return true.
Return ? IsExtensible(globalObject).

8.1.1.4.16 CanDeclareGlobalFunction ( `N` )

The concrete Environment Record method CanDeclareGlobalFunction for global Environment Records determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true, return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
Return false.

8.1.1.4.17 CreateGlobalVarBinding ( `N`, `D` )

The concrete Environment Record method CreateGlobalVarBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let hasProperty be ? HasOwnProperty(globalObject, N).
Let extensible be ? IsExtensible(globalObject).
If hasProperty is false and extensible is true, then
1. Perform ? ObjRec.CreateMutableBinding(N, D).
2. Perform ? ObjRec.InitializeBinding(N, undefined).
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames does not contain N, then
1. Append N to varDeclaredNames.
Return NormalCompletion(empty).

8.1.1.4.18 CreateGlobalFunctionBinding ( `N`, `V`, `D` )

The concrete Environment Record method CreateGlobalFunctionBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced.

Let envRec be the global Environment Record for which the method was invoked.
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be the binding object for ObjRec.
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined or existingProp.[[Configurable]] is true, then
1. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }.
Else,
1. Let desc be the PropertyDescriptor { [[Value]]: V }.
Perform ? DefinePropertyOrThrow(globalObject, N, desc).
Record that the binding for N in ObjRec has been initialized.
Perform ? Set(globalObject, N, V, false).
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames does not contain N, then
1. Append N to varDeclaredNames.
Return NormalCompletion(empty).

Note

Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Steps 8-9 are equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

8.1.1.5 Module Environment Records

A module Environment Record is a declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.

Module Environment Records support all of the declarative Environment Record methods listed in Table 14 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed in Table 19:

Table 19: Additional Methods of Module Environment Records

Method	Purpose
CreateImportBinding(N, M, N2)	Create an immutable indirect binding in a module Environment Record. The String value `N` is the text of the bound name. `M` is a Module Record, and `N2` is a binding that exists in M's module Environment Record.
GetThisBinding()	Return the value of this Environment Record's `this` binding.

The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:

8.1.1.5.1 GetBindingValue ( `N`, `S` )

The concrete Environment Record method GetBindingValue for module Environment Records returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown.

Assert: S is true.
Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N is an indirect binding, then
1. Let M and N2 be the indirection values provided when this binding for N was created.
2. Let targetEnv be M.[[Environment]].
3. If targetEnv is undefined, throw a ReferenceError exception.
4. Let targetER be targetEnv's EnvironmentRecord.
5. Return ? targetER.GetBindingValue(N2, true).
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

Note

S will always be true because a Module is always strict mode code.

8.1.1.5.2 DeleteBinding ( `N` )

The concrete Environment Record method DeleteBinding for module Environment Records refuses to delete bindings.

Assert: This method is never invoked. See 12.5.3.1.

Note

Module Environment Records are only used within strict code and an early error rule prevents the delete operator, in strict code, from being applied to a Reference that would resolve to a module Environment Record binding. See 12.5.3.1.

8.1.1.5.3 HasThisBinding ( )

Module Environment Records provide a this binding.

Return true.

8.1.1.5.4 GetThisBinding ( )

Return undefined.

8.1.1.5.5 CreateImportBinding ( `N`, `M`, `N2` )

The concrete Environment Record method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. M is a Module Record, and N2 is the name of a binding that exists in M's module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding.

Let envRec be the module Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Assert: M is a Module Record.
Assert: When M.[[Environment]] is instantiated it will have a direct binding for N2.
Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
Return NormalCompletion(empty).

8.1.2 Lexical Environment Operations

The following abstract operations are used in this specification to operate upon lexical environments:

8.1.2.1 GetIdentifierReference ( `lex`, `name`, `strict` )

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, a String name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:

If lex is the value null, then
1. Return a value of type Reference whose base value component is undefined, whose referenced name component is name, and whose strict reference flag is strict.
Let envRec be lex's EnvironmentRecord.
Let exists be ? envRec.HasBinding(name).
If exists is true, then
1. Return a value of type Reference whose base value component is envRec, whose referenced name component is name, and whose strict reference flag is strict.
Else,
1. Let outer be the value of lex's outer environment reference.
2. Return ? GetIdentifierReference(outer, name, strict).

8.1.2.2 NewDeclarativeEnvironment ( `E` )

When the abstract operation NewDeclarativeEnvironment is called with a Lexical Environment as argument E the following steps are performed:

Let env be a new Lexical Environment.
Let envRec be a new declarative Environment Record containing no bindings.
Set env's EnvironmentRecord to envRec.
Set the outer lexical environment reference of env to E.
Return env.

8.1.2.3 NewObjectEnvironment ( `O`, `E` )

When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical Environment E as arguments, the following steps are performed:

Let env be a new Lexical Environment.
Let envRec be a new object Environment Record containing O as the binding object.
Set env's EnvironmentRecord to envRec.
Set the outer lexical environment reference of env to E.
Return env.

8.1.2.4 NewFunctionEnvironment ( `F`, `newTarget` )

When the abstract operation NewFunctionEnvironment is called with arguments F and newTarget the following steps are performed:

Assert: F is an ECMAScript function.
Assert: Type(newTarget) is Undefined or Object.
Let env be a new Lexical Environment.
Let envRec be a new function Environment Record containing no bindings.
Set envRec.[[FunctionObject]] to F.
If F.[[ThisMode]] is lexical, set envRec.[[ThisBindingStatus]] to "lexical".
Else, set envRec.[[ThisBindingStatus]] to "uninitialized".
Let home be F.[[HomeObject]].
Set envRec.[[HomeObject]] to home.
Set envRec.[[NewTarget]] to newTarget.
Set env's EnvironmentRecord to envRec.
Set the outer lexical environment reference of env to F.[[Environment]].
Return env.

8.1.2.5 NewGlobalEnvironment ( `G`, `thisValue` )

When the abstract operation NewGlobalEnvironment is called with arguments G and thisValue, the following steps are performed:

Let env be a new Lexical Environment.
Let objRec be a new object Environment Record containing G as the binding object.
Let dclRec be a new declarative Environment Record containing no bindings.
Let globalRec be a new global Environment Record.
Set globalRec.[[ObjectRecord]] to objRec.
Set globalRec.[[GlobalThisValue]] to thisValue.
Set globalRec.[[DeclarativeRecord]] to dclRec.
Set globalRec.[[VarNames]] to a new empty List.
Set env's EnvironmentRecord to globalRec.
Set the outer lexical environment reference of env to null.
Return env.

8.1.2.6 NewModuleEnvironment ( `E` )

When the abstract operation NewModuleEnvironment is called with a Lexical Environment argument E the following steps are performed:

Let env be a new Lexical Environment.
Let envRec be a new module Environment Record containing no bindings.
Set env's EnvironmentRecord to envRec.
Set the outer lexical environment reference of env to E.
Return env.

8.2 Realms

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

A realm is represented in this specification as a Realm Record with the fields specified in Table 20:

Table 20: Realm Record Fields

Field Name	Value	Meaning
[[Intrinsics]]	Record whose field names are intrinsic keys and whose values are objects	The intrinsic values used by code associated with this realm
[[GlobalObject]]	Object	The global object for this realm
[[GlobalEnv]]	Lexical Environment	The global environment for this realm
[[TemplateMap]]	A List of Record { [[Site]]: Parse Node, [[Array]]: Object }.	Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a TemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function. Note Once a Parse Node becomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a Realm Record.

8.2.1 CreateRealm ( )

The abstract operation CreateRealm with no arguments performs the following steps:

Let realmRec be a new Realm Record.
Perform CreateIntrinsics(realmRec).
Set realmRec.[[GlobalObject]] to undefined.
Set realmRec.[[GlobalEnv]] to undefined.
Set realmRec.[[TemplateMap]] to a new empty List.
Return realmRec.

8.2.2 CreateIntrinsics ( `realmRec` )

The abstract operation CreateIntrinsics with argument realmRec performs the following steps:

Let intrinsics be a new Record.
Set realmRec.[[Intrinsics]] to intrinsics.
Let objProto be ObjectCreate(null).
Set intrinsics.[[%ObjectPrototype%]] to objProto.
Let throwerSteps be the algorithm steps specified in 9.2.9.1 for the %ThrowTypeError% function.
Let thrower be CreateBuiltinFunction(throwerSteps, « », realmRec, null).
Set intrinsics.[[%ThrowTypeError%]] to thrower.
Let noSteps be an empty sequence of algorithm steps.
Let funcProto be CreateBuiltinFunction(noSteps, « », realmRec, objProto).
Set intrinsics.[[%FunctionPrototype%]] to funcProto.
Call thrower.[[SetPrototypeOf]](funcProto).
Perform AddRestrictedFunctionProperties(funcProto, realmRec).
Set fields of intrinsics with the values listed in Table 7 that have not already been handled above. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 18-26. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(<steps>, <slots>, realmRec, <prototype>) where <steps> is the definition of that function provided by this specification, <slots> is a list of the names, if any, of the function's specified internal slots, and <prototype> is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Return intrinsics.

8.2.3 SetRealmGlobalObject ( `realmRec`, `globalObj`, `thisValue` )

The abstract operation SetRealmGlobalObject with arguments realmRec, globalObj, and thisValue performs the following steps:

If globalObj is undefined, then
1. Let intrinsics be realmRec.[[Intrinsics]].
2. Set globalObj to ObjectCreate(intrinsics.[[%ObjectPrototype%]]).
Assert: Type(globalObj) is Object.
If thisValue is undefined, set thisValue to globalObj.
Set realmRec.[[GlobalObject]] to globalObj.
Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
Set realmRec.[[GlobalEnv]] to newGlobalEnv.
Return realmRec.

8.2.4 SetDefaultGlobalBindings ( `realmRec` )

The abstract operation SetDefaultGlobalBindings with argument realmRec performs the following steps:

Let global be realmRec.[[GlobalObject]].
For each property of the Global Object specified in clause 18, do
1. Let name be the String value of the property name.
2. Let desc be the fully populated data property descriptor for the property containing the specified attributes for the property. For properties listed in 18.2, 18.3, or 18.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
3. Perform ? DefinePropertyOrThrow(global, name, desc).
Return global.

8.3 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per agent that is actually executing code. This is known as the agent's running execution context. All references to the running execution context in this specification denote the running execution context of the surrounding agent.

The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 21.

Table 21: State Components for All Execution Contexts

Component	Purpose
code evaluation state	Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function	If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
Realm	The Realm Record from which associated code accesses ECMAScript resources.
ScriptOrModule	The Module Record or Script Record from which associated code originates. If there is no originating script or module, as is the case for the original execution context created in InitializeHostDefinedRealm, the value is null.

Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.

The value of the Realm component of the running execution context is also called the current Realm Record. The value of the Function component of the running execution context is also called the active function object.

Execution contexts for ECMAScript code have the additional state components listed in Table 22.

Table 22: Additional State Components for ECMAScript Code Execution Contexts

Component	Purpose
LexicalEnvironment	Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.
VariableEnvironment	Identifies the Lexical Environment whose EnvironmentRecord holds bindings created by VariableStatements within this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments.

Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 23.

Table 23: Additional State Components for Generator Execution Contexts

Component	Purpose
Generator	The GeneratorObject that this execution context is evaluating.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

8.3.1 GetActiveScriptOrModule ( )

The GetActiveScriptOrModule abstract operation is used to determine the running script or module, based on the running execution context. GetActiveScriptOrModule performs the following steps:

If the execution context stack is empty, return null.
Let ec be the topmost execution context on the execution context stack whose ScriptOrModule component is not null.
If no such execution context exists, return null. Otherwise, return ec's ScriptOrModule component.

8.3.2 ResolveBinding ( `name` [ , `env` ] )

The ResolveBinding abstract operation is used to determine the binding of name passed as a String value. The optional argument env can be used to explicitly provide the Lexical Environment that is to be searched for the binding. During execution of ECMAScript code, ResolveBinding is performed using the following algorithm:

If env is not present or if env is undefined, then
1. Set env to the running execution context's LexicalEnvironment.
Assert: env is a Lexical Environment.
If the code matching the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let strict be false.
Return ? GetIdentifierReference(env, name, strict).

Note

The result of ResolveBinding is always a Reference value with its referenced name component equal to the name argument.

8.3.3 GetThisEnvironment ( )

The abstract operation GetThisEnvironment finds the Environment Record that currently supplies the binding of the keyword this. GetThisEnvironment performs the following steps:

Let lex be the running execution context's LexicalEnvironment.
Repeat,
1. Let envRec be lex's EnvironmentRecord.
2. Let exists be envRec.HasThisBinding().
3. If exists is true, return envRec.
4. Let outer be the value of lex's outer environment reference.
5. Assert: outer is not null.
6. Set lex to outer.

Note

The loop in step 2 will always terminate because the list of environments always ends with the global environment which has a this binding.

8.3.4 ResolveThisBinding ( )

The abstract operation ResolveThisBinding determines the binding of the keyword this using the LexicalEnvironment of the running execution context. ResolveThisBinding performs the following steps:

Let envRec be GetThisEnvironment().
Return ? envRec.GetThisBinding().

8.3.5 GetNewTarget ( )

The abstract operation GetNewTarget determines the NewTarget value using the LexicalEnvironment of the running execution context. GetNewTarget performs the following steps:

Let envRec be GetThisEnvironment().
Assert: envRec has a [[NewTarget]] field.
Return envRec.[[NewTarget]].

8.3.6 GetGlobalObject ( )

The abstract operation GetGlobalObject returns the global object used by the currently running execution context. GetGlobalObject performs the following steps:

Let ctx be the running execution context.
Let currentRealm be ctx's Realm.
Return currentRealm.[[GlobalObject]].

8.4 Jobs and Job Queues

A Job is an abstract operation that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.

Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 24. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.

Table 24: PendingJob Record Fields

Field Name	Value	Meaning
[[Job]]	The name of a Job abstract operation	This is the abstract operation that is performed when execution of this PendingJob is initiated.
[[Arguments]]	A List	The List of argument values that are to be passed to [[Job]] when it is activated.
[[Realm]]	A Realm Record	The Realm Record for the initial execution context when this PendingJob is initiated.
[[ScriptOrModule]]	A Script Record or Module Record	The script or module for the initial execution context when this PendingJob is initiated.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a pending Job.

A Job Queue is a FIFO queue of PendingJob records. Each Job Queue has a name and the full set of available Job Queues are defined by an ECMAScript implementation. Every ECMAScript implementation has at least the Job Queues defined in Table 25.

Each agent has its own set of named Job Queues. All references to a named job queue in this specification denote the named job queue of the surrounding agent.

Table 25: Required Job Queues

Name	Purpose
ScriptJobs	Jobs that validate and evaluate ECMAScript Script and Module source text. See clauses 10 and 15.
PromiseJobs	Jobs that are responses to the settlement of a Promise (see 25.6).

A request for the future execution of a Job is made by enqueueing, on a Job Queue, a PendingJob record that includes a Job abstract operation name and any necessary argument values. When there is no running execution context and the execution context stack is empty, the ECMAScript implementation removes the first PendingJob from a Job Queue and uses the information contained in it to create an execution context and starts execution of the associated Job abstract operation.

The PendingJob records from a single Job Queue are always initiated in FIFO order. This specification does not define the order in which multiple Job Queues are serviced. An ECMAScript implementation may interweave the FIFO evaluation of the PendingJob records of a Job Queue with the evaluation of the PendingJob records of one or more other Job Queues. An implementation must define what occurs when there are no running execution context and all Job Queues are empty.

Note

Typically an ECMAScript implementation will have its Job Queues pre-initialized with at least one PendingJob and one of those Jobs will be the first to be executed. An implementation might choose to free all resources and terminate if the current Job completes and all Job Queues are empty. Alternatively, it might choose to wait for a some implementation specific agent or mechanism to enqueue new PendingJob requests.

The following abstract operations are used to create and manage Jobs and Job Queues:

8.4.1 EnqueueJob ( `queueName`, `job`, `arguments` )

The EnqueueJob abstract operation requires three arguments: queueName, job, and arguments. It performs the following steps:

Assert: Type(queueName) is String and its value is the name of a Job Queue recognized by this implementation.
Assert: job is the name of a Job.
Assert: arguments is a List that has the same number of elements as the number of parameters required by job.
Let callerContext be the running execution context.
Let callerRealm be callerContext's Realm.
Let callerScriptOrModule be callerContext's ScriptOrModule.
Let pending be PendingJob { [[Job]]: job, [[Arguments]]: arguments, [[Realm]]: callerRealm, [[ScriptOrModule]]: callerScriptOrModule, [[HostDefined]]: undefined }.
Perform any implementation or host environment defined processing of pending. This may include modifying the [[HostDefined]] field or any other field of pending.
Add pending at the back of the Job Queue named by queueName.
Return NormalCompletion(empty).

8.5 InitializeHostDefinedRealm ( )

The abstract operation InitializeHostDefinedRealm performs the following steps:

Let realm be CreateRealm().
Let newContext be a new execution context.
Set the Function of newContext to null.
Set the Realm of newContext to realm.
Set the ScriptOrModule of newContext to null.
Push newContext onto the execution context stack; newContext is now the running execution context.
If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in an implementation-defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in an implementation-defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
Perform SetRealmGlobalObject(realm, global, thisValue).
Let globalObj be ? SetDefaultGlobalBindings(realm).
Create any implementation-defined global object properties on globalObj.
Return NormalCompletion(empty).

8.6 RunJobs ( )

The abstract operation RunJobs performs the following steps:

Perform ? InitializeHostDefinedRealm().
In an implementation-dependent manner, obtain the ECMAScript source texts (see clause 10) and any associated host-defined values for zero or more ECMAScript scripts and/or ECMAScript modules. For each such sourceText and hostDefined, do
1. If sourceText is the source code of a script, then
  1. Perform EnqueueJob("ScriptJobs", ScriptEvaluationJob, « sourceText, hostDefined »).
2. Else sourceText is the source code of a module,
  1. Perform EnqueueJob("ScriptJobs", TopLevelModuleEvaluationJob, « sourceText, hostDefined »).
Repeat,
1. Suspend the running execution context and remove it from the execution context stack.
2. Assert: The execution context stack is now empty.
3. Let nextQueue be a non-empty Job Queue chosen in an implementation-defined manner. If all Job Queues are empty, the result is implementation-defined.
4. Let nextPending be the PendingJob record at the front of nextQueue. Remove that record from nextQueue.
5. Let newContext be a new execution context.
6. Set newContext's Function to null.
7. Set newContext's Realm to nextPending.[[Realm]].
8. Set newContext's ScriptOrModule to nextPending.[[ScriptOrModule]].
9. Push newContext onto the execution context stack; newContext is now the running execution context.
10. Perform any implementation or host environment defined job initialization using nextPending.
11. Let result be the result of performing the abstract operation named by nextPending.[[Job]] using the elements of nextPending.[[Arguments]] as its arguments.
12. If result is an abrupt completion, perform HostReportErrors(« result.[[Value]] »).

8.7 Agents

An agent comprises a set of ECMAScript execution contexts, an execution context stack, a running execution context, a set of named job queues, an Agent Record, and an executing thread. Except for the executing thread, the constituents of an agent belong exclusively to that agent.

An agent's executing thread executes the jobs in the agent's job queues on the agent's execution contexts independently of other agents, except that an executing thread may be used as the executing thread by multiple agents, provided none of the agents sharing the thread have an Agent Record whose [[CanBlock]] property is true.

Note 1

Some web browsers share a single executing thread across multiple unrelated tabs of a browser window, for example.

While an agent's executing thread executes the jobs in the agent's job queues, the agent is the surrounding agent for the code in those jobs. The code uses the surrounding agent to access the specification level execution objects held within the agent: the running execution context, the execution context stack, the named job queues, and the Agent Record's fields.

Table 26: Agent Record Fields

Field Name	Value	Meaning
[[LittleEndian]]	Boolean	The default value computed for the isLittleEndian parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-dependent and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
[[CanBlock]]	Boolean	Determines whether the agent can block or not.
[[Signifier]]	Any globally-unique value	Uniquely identifies the agent within its agent cluster.
[[IsLockFree1]]	Boolean	true if atomic operations on one-byte values are lock-free, false otherwise.
[[IsLockFree2]]	Boolean	true if atomic operations on two-byte values are lock-free, false otherwise.
[[CandidateExecution]]	A candidate execution Record	See the memory model.

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.

Note 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.

There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.

That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.

Note 3

An agent is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

8.7.1 AgentSignifier ( )

The abstract operation AgentSignifier takes no arguments. It performs the following steps:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[Signifier]].

8.7.2 AgentCanSuspend ( )

The abstract operation AgentCanSuspend takes no arguments. It performs the following steps:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[CanBlock]].

Note

In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.

8.8 Agent Clusters

An agent cluster is a maximal set of agents that can communicate by operating on shared memory.

Note 1

Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.

Every agent belongs to exactly one agent cluster.

Note 2

The agents in a cluster need not all be alive at some particular point in time. If agent A creates another agent B, after which A terminates and B creates agent C, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.

All agents within a cluster must have the same value for the [[LittleEndian]] property in their respective Agent Records.

Note 3

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.

All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respective Agent Records; similarly for the [[IsLockFree2]] property.

All agents within a cluster must have different values for the [[Signifier]] property in their respective Agent Records.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.

Note 4

The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.

An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an agent is terminated not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify the agent that was terminated.

Note 5

Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.

Prior to any evaluation of any ECMAScript code by any agent in a cluster, the [[CandidateExecution]] field of the Agent Record for all agents in the cluster is set to the initial candidate execution. The initial candidate execution is an empty candidate execution whose [[EventLists]] field is a List containing, for each agent, an Agent Events Record whose [[AgentSignifier]] field is that agent's signifier and whose [[EventList]] field is an empty List.

Note 6

All agents in an agent cluster share the same candidate execution in its Agent Record's [[CandidateExecution]] field. The candidate execution is a specification mechanism used by the memory model.

Note 7

An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

8.9 Forward Progress

For an agent to make forward progress is for it to perform an evaluation step according to this specification.

An agent becomes blocked when its running execution context waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] property is true can become blocked in this sense. An unblocked agent is one that is not blocked.

Implementations must ensure that:

every unblocked agent with a dedicated executing thread eventually makes forward progress
in a set of agents that share an executing thread, one agent eventually makes forward progress
an agent does not cause another agent to become blocked except via explicit APIs that provide blocking.

Note

This, along with the liveness guarantee in the memory model, ensures that all "SeqCst" writes eventually become observable to all agents.

9 Ordinary and Exotic Objects Behaviours

9.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (and visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.

Every ordinary object has a Boolean-valued [[Extensible]] internal slot that controls whether or not properties may be added to the object. If the value of the [[Extensible]] internal slot is false then additional properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Prototype]] internal slot of the object may not be modified. Once the value of an object's [[Extensible]] internal slot has been set to false it may not be subsequently changed to true.

In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.

Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.

9.1.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of O is called, the following steps are taken:

Return ! OrdinaryGetPrototypeOf(O).

9.1.1.1 OrdinaryGetPrototypeOf ( `O` )

When the abstract operation OrdinaryGetPrototypeOf is called with Object O, the following steps are taken:

Return O.[[Prototype]].

9.1.2 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of O is called with argument V, the following steps are taken:

Return ! OrdinarySetPrototypeOf(O, V).

9.1.2.1 OrdinarySetPrototypeOf ( `O`, `V` )

When the abstract operation OrdinarySetPrototypeOf is called with Object O and value V, the following steps are taken:

Assert: Either Type(V) is Object or Type(V) is Null.
Let extensible be O.[[Extensible]].
Let current be O.[[Prototype]].
If SameValue(V, current) is true, return true.
If extensible is false, return false.
Let p be V.
Let done be false.
Repeat, while done is false,
1. If p is null, set done to true.
2. Else if SameValue(p, O) is true, return false.
3. Else,
  1. If p.[[GetPrototypeOf]] is not the ordinary object internal method defined in 9.1.1, set done to true.
  2. Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.

Note

The loop in step 8 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].

9.1.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of O is called, the following steps are taken:

Return ! OrdinaryIsExtensible(O).

9.1.3.1 OrdinaryIsExtensible ( `O` )

When the abstract operation OrdinaryIsExtensible is called with Object O, the following steps are taken:

Return O.[[Extensible]].

9.1.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of O is called, the following steps are taken:

Return ! OrdinaryPreventExtensions(O).

9.1.4.1 OrdinaryPreventExtensions ( `O` )

When the abstract operation OrdinaryPreventExtensions is called with Object O, the following steps are taken:

Set O.[[Extensible]] to false.
Return true.

9.1.5 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of O is called with property key P, the following steps are taken:

Return ! OrdinaryGetOwnProperty(O, P).

9.1.5.1 OrdinaryGetOwnProperty ( `O`, `P` )

When the abstract operation OrdinaryGetOwnProperty is called with Object O and with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If O does not have an own property with key P, return undefined.
Let D be a newly created Property Descriptor with no fields.
Let X be O's own property whose key is P.
If X is a data property, then
1. Set D.[[Value]] to the value of X's [[Value]] attribute.
2. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
Else X is an accessor property,
1. Set D.[[Get]] to the value of X's [[Get]] attribute.
2. Set D.[[Set]] to the value of X's [[Set]] attribute.
Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
Return D.

9.1.6 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of O is called with property key P and Property Descriptor Desc, the following steps are taken:

Return ? OrdinaryDefineOwnProperty(O, P, Desc).

9.1.6.1 OrdinaryDefineOwnProperty ( `O`, `P`, `Desc` )

When the abstract operation OrdinaryDefineOwnProperty is called with Object O, property key P, and Property Descriptor Desc, the following steps are taken:

Let current be ? O.[[GetOwnProperty]](P).
Let extensible be O.[[Extensible]].
Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

9.1.6.2 IsCompatiblePropertyDescriptor ( `Extensible`, `Desc`, `Current` )

When the abstract operation IsCompatiblePropertyDescriptor is called with Boolean value Extensible, and Property Descriptors Desc, and Current, the following steps are taken:

Return ValidateAndApplyPropertyDescriptor(undefined, undefined, Extensible, Desc, Current).

9.1.6.3 ValidateAndApplyPropertyDescriptor ( `O`, `P`, `extensible`, `Desc`, `current` )

When the abstract operation ValidateAndApplyPropertyDescriptor is called with Object O, property key P, Boolean value extensible, and Property Descriptors Desc, and current, the following steps are taken:

Note

If undefined is passed as O, only validation is performed and no object updates are performed.

Assert: If O is not undefined, then IsPropertyKey(P) is true.
If current is undefined, then
1. If extensible is false, return false.
2. Assert: extensible is true.
3. If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
  1. If O is not undefined, create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
4. Else Desc must be an accessor Property Descriptor,
  1. If O is not undefined, create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
5. Return true.
If every field in Desc is absent, return true.
If current.[[Configurable]] is false, then
1. If Desc.[[Configurable]] is present and its value is true, return false.
2. If Desc.[[Enumerable]] is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other, return false.
If IsGenericDescriptor(Desc) is true, no further validation is required.
Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
1. If current.[[Configurable]] is false, return false.
2. If IsDataDescriptor(current) is true, then
  1. If O is not undefined, convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
3. Else,
  1. If O is not undefined, convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
1. If current.[[Configurable]] is false and current.[[Writable]] is false, then
  1. If Desc.[[Writable]] is present and Desc.[[Writable]] is true, return false.
  2. If Desc.[[Value]] is present and SameValue(Desc.[[Value]], current.[[Value]]) is false, return false.
  3. Return true.
Else IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true,
1. If current.[[Configurable]] is false, then
  1. If Desc.[[Set]] is present and SameValue(Desc.[[Set]], current.[[Set]]) is false, return false.
  2. If Desc.[[Get]] is present and SameValue(Desc.[[Get]], current.[[Get]]) is false, return false.
  3. Return true.
If O is not undefined, then
1. For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
Return true.

9.1.7 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of O is called with property key P, the following steps are taken:

Return ? OrdinaryHasProperty(O, P).

9.1.7.1 OrdinaryHasProperty ( `O`, `P` )

When the abstract operation OrdinaryHasProperty is called with Object O and with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let hasOwn be ? O.[[GetOwnProperty]](P).
If hasOwn is not undefined, return true.
Let parent be ? O.[[GetPrototypeOf]]().
If parent is not null, then
1. Return ? parent.[[HasProperty]](P).
Return false.

9.1.8 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Return ? OrdinaryGet(O, P, Receiver).

9.1.8.1 OrdinaryGet ( `O`, `P`, `Receiver` )

When the abstract operation OrdinaryGet is called with Object O, property key P, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. If parent is null, return undefined.
3. Return ? parent.[[Get]](P, Receiver).
If IsDataDescriptor(desc) is true, return desc.[[Value]].
Assert: IsAccessorDescriptor(desc) is true.
Let getter be desc.[[Get]].
If getter is undefined, return undefined.
Return ? Call(getter, Receiver).

9.1.9 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Return ? OrdinarySet(O, P, V, Receiver).

9.1.9.1 OrdinarySet ( `O`, `P`, `V`, `Receiver` )

When the abstract operation OrdinarySet is called with Object O, property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let ownDesc be ? O.[[GetOwnProperty]](P).
Return OrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).

9.1.9.2 OrdinarySetWithOwnDescriptor ( `O`, `P`, `V`, `Receiver`, `ownDesc` )

When the abstract operation OrdinarySetWithOwnDescriptor is called with Object O, property key P, value V, ECMAScript language value Receiver, and Property Descriptor (or undefined) ownDesc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If ownDesc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. If parent is not null, then
  1. Return ? parent.[[Set]](P, V, Receiver).
3. Else,
  1. Set ownDesc to the PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
If IsDataDescriptor(ownDesc) is true, then
1. If ownDesc.[[Writable]] is false, return false.
2. If Type(Receiver) is not Object, return false.
3. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
4. If existingDescriptor is not undefined, then
  1. If IsAccessorDescriptor(existingDescriptor) is true, return false.
  2. If existingDescriptor.[[Writable]] is false, return false.
  3. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
  4. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
5. Else Receiver does not currently have a property P,
  1. Return ? CreateDataProperty(Receiver, P, V).
Assert: IsAccessorDescriptor(ownDesc) is true.
Let setter be ownDesc.[[Set]].
If setter is undefined, return false.
Perform ? Call(setter, Receiver, « V »).
Return true.

9.1.10 [[Delete]] ( `P` )

When the [[Delete]] internal method of O is called with property key P, the following steps are taken:

Return ? OrdinaryDelete(O, P).

9.1.10.1 OrdinaryDelete ( `O`, `P` )

When the abstract operation OrdinaryDelete is called with Object O and property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return true.
If desc.[[Configurable]] is true, then
1. Remove the own property with name P from O.
2. Return true.
Return false.

9.1.11 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of O is called, the following steps are taken:

Return ! OrdinaryOwnPropertyKeys(O).

9.1.11.1 OrdinaryOwnPropertyKeys ( `O` )

When the abstract operation OrdinaryOwnPropertyKeys is called with Object O, the following steps are taken:

Let keys be a new empty List.
For each own property key P of O that is an integer index, in ascending numeric index order, do
1. Add P as the last element of keys.
For each own property key P of O that is a String but is not an integer index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O that is a Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.1.12 ObjectCreate ( `proto` [ , `internalSlotsList` ] )

The abstract operation ObjectCreate with argument proto (an object or null) is used to specify the runtime creation of new ordinary objects. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty List is used. This abstract operation performs the following steps:

If internalSlotsList is not present, set internalSlotsList to a new empty List.
Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in 9.1.
Set obj.[[Prototype]] to proto.
Set obj.[[Extensible]] to true.
Return obj.

9.1.13 OrdinaryCreateFromConstructor ( `constructor`, `intrinsicDefaultProto` [ , `internalSlotsList` ] )

The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's prototype property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. The optional internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty List is used. This abstract operation performs the following steps:

Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Let proto be ? GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
Return ObjectCreate(proto, internalSlotsList).

9.1.14 GetPrototypeFromConstructor ( `constructor`, `intrinsicDefaultProto` )

The abstract operation GetPrototypeFromConstructor determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's prototype property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. This abstract operation performs the following steps:

Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Assert: IsCallable(constructor) is true.
Let proto be ? Get(constructor, "prototype").
If Type(proto) is not Object, then
1. Let realm be ? GetFunctionRealm(constructor).
2. Set proto to realm's intrinsic object named intrinsicDefaultProto.
Return proto.

Note

If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.

9.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (10.2.1) or non-strict code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.

ECMAScript function objects have the additional internal slots listed in Table 27.

Table 27: Internal Slots of ECMAScript Function Objects

Internal Slot	Type	Description
[[Environment]]	Lexical Environment	The Lexical Environment that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[FormalParameters]]	Parse Node	The root parse node of the source text that defines the function's formal parameter list.
[[FunctionKind]]	String	Either `"normal"`, `"classConstructor"`, `"generator"`, or `"async"`.
[[ECMAScriptCode]]	Parse Node	The root parse node of the source text that defines the function's body.
[[ConstructorKind]]	String	Either `"base"` or `"derived"`.
[[Realm]]	Realm Record	The realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]]	Script Record or Module Record	The script or module in which the function was created.
[[ThisMode]]	(lexical, strict, global)	Defines how `this` references are interpreted within the formal parameters and code body of the function. lexical means that `this` refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.
[[Strict]]	Boolean	true if this is a strict function, false if this is a non-strict function.
[[HomeObject]]	Object	If the function uses `super`, this is the object whose [[GetPrototypeOf]] provides the object where `super` property lookups begin.

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.

9.2.1 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Assert: F is an ECMAScript function object.
If F.[[FunctionKind]] is "classConstructor", throw a TypeError exception.
Let callerContext be the running execution context.
Let calleeContext be PrepareForOrdinaryCall(F, undefined).
Assert: calleeContext is now the running execution context.
Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
Let result be OrdinaryCallEvaluateBody(F, argumentsList).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
If result.[[Type]] is return, return NormalCompletion(result.[[Value]]).
ReturnIfAbrupt(result).
Return NormalCompletion(undefined).

Note

When calleeContext is removed from the execution context stack in step 8 it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.

9.2.1.1 PrepareForOrdinaryCall ( `F`, `newTarget` )

When the abstract operation PrepareForOrdinaryCall is called with function object F and ECMAScript language value newTarget, the following steps are taken:

Assert: Type(newTarget) is Undefined or Object.
Let callerContext be the running execution context.
Let calleeContext be a new ECMAScript code execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
Let localEnv be NewFunctionEnvironment(F, newTarget).
Set the LexicalEnvironment of calleeContext to localEnv.
Set the VariableEnvironment of calleeContext to localEnv.
If callerContext is not already suspended, suspend callerContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
NOTE: Any exception objects produced after this point are associated with calleeRealm.
Return calleeContext.

9.2.1.2 OrdinaryCallBindThis ( `F`, `calleeContext`, `thisArgument` )

When the abstract operation OrdinaryCallBindThis is called with function object F, execution context calleeContext, and ECMAScript value thisArgument, the following steps are taken:

Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return NormalCompletion(undefined).
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
If thisMode is strict, let thisValue be thisArgument.
Else,
1. If thisArgument is undefined or null, then
  1. Let globalEnv be calleeRealm.[[GlobalEnv]].
  2. Let globalEnvRec be globalEnv's EnvironmentRecord.
  3. Assert: globalEnvRec is a global Environment Record.
  4. Let thisValue be globalEnvRec.[[GlobalThisValue]].
2. Else,
  1. Let thisValue be ! ToObject(thisArgument).
  2. NOTE: ToObject produces wrapper objects using calleeRealm.
Let envRec be localEnv's EnvironmentRecord.
Assert: envRec is a function Environment Record.
Assert: The next step never returns an abrupt completion because envRec.[[ThisBindingStatus]] is not "initialized".
Return envRec.BindThisValue(thisValue).

9.2.1.3 OrdinaryCallEvaluateBody ( `F`, `argumentsList` )

When the abstract operation OrdinaryCallEvaluateBody is called with function object F and List argumentsList, the following steps are taken:

Return the result of EvaluateBody of the parsed code that is F.[[ECMAScriptCode]] passing F and argumentsList as the arguments.

9.2.2 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method for an ECMAScript function object F is called with parameters argumentsList and newTarget. argumentsList is a possibly empty List of ECMAScript language values. The following steps are taken:

Assert: F is an ECMAScript function object.
Assert: Type(newTarget) is Object.
Let callerContext be the running execution context.
Let kind be F.[[ConstructorKind]].
If kind is "base", then
1. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%ObjectPrototype%").
Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
Assert: calleeContext is now the running execution context.
If kind is "base", perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
Let constructorEnv be the LexicalEnvironment of calleeContext.
Let envRec be constructorEnv's EnvironmentRecord.
Let result be OrdinaryCallEvaluateBody(F, argumentsList).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
If result.[[Type]] is return, then
1. If Type(result.[[Value]]) is Object, return NormalCompletion(result.[[Value]]).
2. If kind is "base", return NormalCompletion(thisArgument).
3. If result.[[Value]] is not undefined, throw a TypeError exception.
Else, ReturnIfAbrupt(result).
Return ? envRec.GetThisBinding().

9.2.3 FunctionAllocate ( `functionPrototype`, `strict`, `functionKind` )

The abstract operation FunctionAllocate requires the three arguments functionPrototype, strict and functionKind. FunctionAllocate performs the following steps:

Assert: Type(functionPrototype) is Object.
Assert: functionKind is either "normal", "non-constructor", "generator", "async", or "async generator".
If functionKind is "normal", let needsConstruct be true.
Else, let needsConstruct be false.
If functionKind is "non-constructor", set functionKind to "normal".
Let F be a newly created ECMAScript function object with the internal slots listed in Table 27. All of those internal slots are initialized to undefined.
Set F's essential internal methods to the default ordinary object definitions specified in 9.1.
Set F.[[Call]] to the definition specified in 9.2.1.
If needsConstruct is true, then
1. Set F.[[Construct]] to the definition specified in 9.2.2.
2. Set F.[[ConstructorKind]] to "base".
Set F.[[Strict]] to strict.
Set F.[[FunctionKind]] to functionKind.
Set F.[[Prototype]] to functionPrototype.
Set F.[[Extensible]] to true.
Set F.[[Realm]] to the current Realm Record.
Return F.

9.2.4 FunctionInitialize ( `F`, `kind`, `ParameterList`, `Body`, `Scope` )

The abstract operation FunctionInitialize requires the arguments: a function object F, kind which is one of (Normal, Method, Arrow), a parameter list Parse Node specified by ParameterList, a body Parse Node specified by Body, a Lexical Environment specified by Scope. FunctionInitialize performs the following steps:

Let len be the ExpectedArgumentCount of ParameterList.
Perform ! SetFunctionLength(F, len).
Let Strict be F.[[Strict]].
Set F.[[Environment]] to Scope.
Set F.[[FormalParameters]] to ParameterList.
Set F.[[ECMAScriptCode]] to Body.
Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
If kind is Arrow, set F.[[ThisMode]] to lexical.
Else if Strict is true, set F.[[ThisMode]] to strict.
Else, set F.[[ThisMode]] to global.
Return F.

9.2.5 FunctionCreate ( `kind`, `ParameterList`, `Body`, `Scope`, `Strict` [ , `prototype` ] )

The abstract operation FunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list Parse Node specified by ParameterList, a body Parse Node specified by Body, a Lexical Environment specified by Scope, a Boolean flag Strict, and optionally, an object prototype. FunctionCreate performs the following steps:

If prototype is not present, then
1. Set prototype to the intrinsic object %FunctionPrototype%.
If kind is not Normal, let allocKind be "non-constructor".
Else, let allocKind be "normal".
Let F be FunctionAllocate(prototype, Strict, allocKind).
Return FunctionInitialize(F, kind, ParameterList, Body, Scope).

9.2.6 GeneratorFunctionCreate ( `kind`, `ParameterList`, `Body`, `Scope`, `Strict` )

The abstract operation GeneratorFunctionCreate requires the arguments: kind which is one of (Normal, Method), a parameter list Parse Node specified by ParameterList, a body Parse Node specified by Body, a Lexical Environment specified by Scope, and a Boolean flag Strict. GeneratorFunctionCreate performs the following steps:

Let functionPrototype be the intrinsic object %Generator%.
Let F be FunctionAllocate(functionPrototype, Strict, "generator").
Return FunctionInitialize(F, kind, ParameterList, Body, Scope).

9.2.7 AsyncGeneratorFunctionCreate ( `kind`, `ParameterList`, `Body`, `Scope`, `Strict` )

The abstract operation AsyncGeneratorFunctionCreate requires the arguments: kind which is one of (Normal, Method), a parameter list Parse Node specified by ParameterList, a body Parse Node specified by Body, a Lexical Environment specified by Scope, and a Boolean flag Strict. AsyncGeneratorFunctionCreate performs the following steps:

Let functionPrototype be the intrinsic object %AsyncGenerator%.
Let F be ! FunctionAllocate(functionPrototype, Strict, "generator").
Return ! FunctionInitialize(F, kind, ParameterList, Body, Scope).

9.2.8 AsyncFunctionCreate ( `kind`, `parameters`, `body`, `Scope`, `Strict` )

The abstract operation AsyncFunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list Parse Node specified by parameters, a body Parse Node specified by body, a Lexical Environment specified by Scope, and a Boolean flag Strict. AsyncFunctionCreate performs the following steps:

Let functionPrototype be the intrinsic object %AsyncFunctionPrototype%.
Let F be ! FunctionAllocate(functionPrototype, Strict, "async").
Return ! FunctionInitialize(F, kind, parameters, body, Scope).

9.2.9 AddRestrictedFunctionProperties ( `F`, `realm` )

The abstract operation AddRestrictedFunctionProperties is called with a function object F and Realm Record realm as its argument. It performs the following steps:

Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
Perform ! DefinePropertyOrThrow(F, "caller", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
Return ! DefinePropertyOrThrow(F, "arguments", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).

9.2.9.1 %ThrowTypeError% ( )

The %ThrowTypeError% intrinsic is an anonymous built-in function object that is defined once for each realm. When %ThrowTypeError% is called it performs the following steps:

Throw a TypeError exception.

The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is false.

The length property of a %ThrowTypeError% function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

9.2.10 MakeConstructor ( `F` [ , `writablePrototype` [ , `prototype` ] ] )

The abstract operation MakeConstructor requires a Function argument F and optionally, a Boolean writablePrototype and an object prototype. If prototype is provided it is assumed to already contain, if needed, a "constructor" property whose value is F. This operation converts F into a constructor by performing the following steps:

Assert: F is an ECMAScript function object.
Assert: IsConstructor(F) is true.
Assert: F is an extensible object that does not have a prototype own property.
If writablePrototype is not present, set writablePrototype to true.
If prototype is not present, then
1. Set prototype to ObjectCreate(%ObjectPrototype%).
2. Perform ! DefinePropertyOrThrow(prototype, "constructor", PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false }).
Return NormalCompletion(undefined).

9.2.11 MakeClassConstructor ( `F` )

The abstract operation MakeClassConstructor with argument F performs the following steps:

Assert: F is an ECMAScript function object.
Assert: F.[[FunctionKind]] is "normal".
Set F.[[FunctionKind]] to "classConstructor".
Return NormalCompletion(undefined).

9.2.12 MakeMethod ( `F`, `homeObject` )

The abstract operation MakeMethod with arguments F and homeObject configures F as a method by performing the following steps:

Assert: F is an ECMAScript function object.
Assert: Type(homeObject) is Object.
Set F.[[HomeObject]] to homeObject.
Return NormalCompletion(undefined).

9.2.13 SetFunctionName ( `F`, `name` [ , `prefix` ] )

The abstract operation SetFunctionName requires a Function argument F, a String or Symbol argument name and optionally a String argument prefix. This operation adds a name property to F by performing the following steps:

Assert: F is an extensible object that does not have a name own property.
Assert: Type(name) is either Symbol or String.
Assert: If prefix is present, then Type(prefix) is String.
If Type(name) is Symbol, then
1. Let description be name's [[Description]] value.
2. If description is undefined, set name to the empty String.
3. Else, set name to the string-concatenation of "[", description, and "]".
If prefix is present, then
1. Set name to the string-concatenation of prefix, the code unit 0x0020 (SPACE), and name.
Return ! DefinePropertyOrThrow(F, "name", PropertyDescriptor { [[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).

9.2.14 SetFunctionLength ( `F`, `length` )

The abstract operation SetFunctionLength requires a Function argument F and a Number argument length. This operation adds a length property to F by performing the following steps:

Assert: F is an extensible object that does not have a length own property.
Assert: Type(length) is Number.
Assert: length ≥ 0 and ! ToInteger(length) is equal to length.
Return ! DefinePropertyOrThrow(F, "length", PropertyDescriptor{[[Value]]: length, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true}).

9.2.15 FunctionDeclarationInstantiation ( `func`, `argumentsList` )

Note 1

When an execution context is established for evaluating an ECMAScript function a new function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

FunctionDeclarationInstantiation is performed as follows using arguments func and argumentsList. func is the function object for which the execution context is being established.

Let calleeContext be the running execution context.
Let env be the LexicalEnvironment of calleeContext.
Let envRec be env's EnvironmentRecord.
Let code be func.[[ECMAScriptCode]].
Let strict be func.[[Strict]].
Let formals be func.[[FormalParameters]].
Let parameterNames be the BoundNames of formals.
If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
Let simpleParameterList be IsSimpleParameterList of formals.
Let hasParameterExpressions be ContainsExpression of formals.
Let varNames be the VarDeclaredNames of code.
Let varDeclarations be the VarScopedDeclarations of code.
Let lexicalNames be the LexicallyDeclaredNames of code.
Let functionNames be a new empty List.
Let functionsToInitialize be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. Let fn be the sole element of the BoundNames of d.
  3. If fn is not an element of functionNames, then
    1. Insert fn as the first element of functionNames.
    2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
    3. Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If func.[[ThisMode]] is lexical, then
1. NOTE: Arrow functions never have an arguments objects.
2. Set argumentsObjectNeeded to false.
Else if "arguments" is an element of parameterNames, then
1. Set argumentsObjectNeeded to false.
Else if hasParameterExpressions is false, then
1. If "arguments" is an element of functionNames or if "arguments" is an element of lexicalNames, then
  1. Set argumentsObjectNeeded to false.
For each String paramName in parameterNames, do
1. Let alreadyDeclared be envRec.HasBinding(paramName).
2. NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
3. If alreadyDeclared is false, then
  1. Perform ! envRec.CreateMutableBinding(paramName, false).
  2. If hasDuplicates is true, then
    1. Perform ! envRec.InitializeBinding(paramName, undefined).
If argumentsObjectNeeded is true, then
1. If strict is true or if simpleParameterList is false, then
  1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
2. Else,
  1. NOTE: mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
  2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, envRec).
3. If strict is true, then
  1. Perform ! envRec.CreateImmutableBinding("arguments", false).
4. Else,
  1. Perform ! envRec.CreateMutableBinding("arguments", false).
5. Call envRec.InitializeBinding("arguments", ao).
6. Let parameterBindings be a new List of parameterNames with "arguments" appended.
Else,
1. Let parameterBindings be parameterNames.
Let iteratorRecord be CreateListIteratorRecord(argumentsList).
If hasDuplicates is true, then
1. Perform ? IteratorBindingInitialization for formals with iteratorRecord and undefined as arguments.
Else,
1. Perform ? IteratorBindingInitialization for formals with iteratorRecord and env as arguments.
If hasParameterExpressions is false, then
1. NOTE: Only a single lexical environment is needed for the parameters and top-level vars.
2. Let instantiatedVarNames be a copy of the List parameterBindings.
3. For each n in varNames, do
  1. If n is not an element of instantiatedVarNames, then
    1. Append n to instantiatedVarNames.
    2. Perform ! envRec.CreateMutableBinding(n, false).
    3. Call envRec.InitializeBinding(n, undefined).
4. Let varEnv be env.
5. Let varEnvRec be envRec.
Else,
1. NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
2. Let varEnv be NewDeclarativeEnvironment(env).
3. Let varEnvRec be varEnv's EnvironmentRecord.
4. Set the VariableEnvironment of calleeContext to varEnv.
5. Let instantiatedVarNames be a new empty List.
6. For each n in varNames, do
  1. If n is not an element of instantiatedVarNames, then
    1. Append n to instantiatedVarNames.
    2. Perform ! varEnvRec.CreateMutableBinding(n, false).
    3. If n is not an element of parameterBindings or if n is an element of functionNames, let initialValue be undefined.
    4. Else,
      1. Let initialValue be ! envRec.GetBindingValue(n, false).
    5. Call varEnvRec.InitializeBinding(n, initialValue).
    6. NOTE: vars whose names are the same as a formal parameter, initially have the same value as the corresponding initialized parameter.
NOTE: Annex B.3.3.1 adds additional steps at this point.
If strict is false, then
1. Let lexEnv be NewDeclarativeEnvironment(varEnv).
2. NOTE: Non-strict functions use a separate lexical Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
Else, let lexEnv be varEnv.
Let lexEnvRec be lexEnv's EnvironmentRecord.
Set the LexicalEnvironment of calleeContext to lexEnv.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations, do
1. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! lexEnvRec.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! lexEnvRec.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be the result of performing InstantiateFunctionObject for f with argument lexEnv.
3. Perform ! varEnvRec.SetMutableBinding(fn, fo, false).
Return NormalCompletion(empty).

Note 2

B.3.3 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.

Note 3

Parameter Initializers may contain direct eval expressions. Any top level declarations of such evals are only visible to the eval code (10.2). The creation of the environment for such declarations is described in 14.1.19.

9.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as implementation provided function exotic objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.

If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 9.1. All such function exotic objects also have [[Prototype]], [[Extensible]], [[Realm]], and [[ScriptOrModule]] internal slots.

Unless otherwise specified every built-in function object has the %FunctionPrototype% object as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the this value, the [[Call]] argumentsList provides the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the this value is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[FunctionKind]] internal slot to have the value "classConstructor".

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor's named parameters.

Built-in functions that are not constructors do not have a prototype property unless otherwise specified in the description of a particular function.

If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

9.3.1 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method for a built-in function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Let callerContext be the running execution context.
If callerContext is not already suspended, suspend callerContext.
Let calleeContext be a new ECMAScript code execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
Perform any necessary implementation-defined initialization of calleeContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
Let result be the Completion Record that is the result of evaluating F in an implementation-defined manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
Return result.

Note

When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.

9.3.2 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method for built-in function object F is called with parameters argumentsList and newTarget. The steps performed are the same as [[Call]] (see 9.3.1) except that step 10 is replaced by:

Let result be the Completion Record that is the result of evaluating F in an implementation-defined manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.

9.3.3 CreateBuiltinFunction ( `steps`, `internalSlotsList` [ , `realm` [ , `prototype` ] ] )

The abstract operation CreateBuiltinFunction takes arguments steps, internalSlotsList, realm, and prototype. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. CreateBuiltinFunction returns a built-in function object created by the following steps:

Assert: steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
If realm is not present, set realm to the current Realm Record.
Assert: realm is a Realm Record.
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%FunctionPrototype%]].
Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList. The initial value of each of those internal slots is undefined.
Set func.[[Realm]] to realm.
Set func.[[Prototype]] to prototype.
Set func.[[Extensible]] to true.
Set func.[[ScriptOrModule]] to null.
Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.

9.4 Built-in Exotic Object Internal Methods and Slots

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:

9.4.1 Bound Function Exotic Objects

A bound function is an exotic object that wraps another function object. A bound function is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function generally results in a call of its wrapped function.

Bound function objects do not have the internal slots of ECMAScript function objects defined in Table 27. Instead they have the internal slots defined in Table 28.

Table 28: Internal Slots of Bound Function Exotic Objects

Internal Slot	Type	Description
[[BoundTargetFunction]]	Callable Object	The wrapped function object.
[[BoundThis]]	Any	The value that is always passed as the this value when calling the wrapped function.
[[BoundArguments]]	List of Any	A list of values whose elements are used as the first arguments to any call to the wrapped function.

Bound function objects provide all of the essential internal methods as specified in 9.1. However, they use the following definitions for the essential internal methods of function objects.

9.4.1.1 [[Call]] ( `thisArgument`, `argumentsList` )

When the [[Call]] internal method of a bound function exotic object, F, which was created using the bind function is called with parameters thisArgument and argumentsList, a List of ECMAScript language values, the following steps are taken:

Let target be F.[[BoundTargetFunction]].
Let boundThis be F.[[BoundThis]].
Let boundArgs be F.[[BoundArguments]].
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
Return ? Call(target, boundThis, args).

9.4.1.2 [[Construct]] ( `argumentsList`, `newTarget` )

When the [[Construct]] internal method of a bound function exotic object, F that was created using the bind function is called with a list of arguments argumentsList and newTarget, the following steps are taken:

Let target be F.[[BoundTargetFunction]].
Assert: IsConstructor(target) is true.
Let boundArgs be F.[[BoundArguments]].
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
If SameValue(F, newTarget) is true, set newTarget to target.
Return ? Construct(target, args, newTarget).

9.4.1.3 BoundFunctionCreate ( `targetFunction`, `boundThis`, `boundArgs` )

The abstract operation BoundFunctionCreate with arguments targetFunction, boundThis and boundArgs is used to specify the creation of new Bound Function exotic objects. It performs the following steps:

Assert: Type(targetFunction) is Object.
Let proto be ? targetFunction.[[GetPrototypeOf]]().
Let obj be a newly created object.
Set obj's essential internal methods to the default ordinary object definitions specified in 9.1.
Set obj.[[Call]] as described in 9.4.1.1.
If IsConstructor(targetFunction) is true, then
1. Set obj.[[Construct]] as described in 9.4.1.2.
Set obj.[[Prototype]] to proto.
Set obj.[[Extensible]] to true.
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.

9.4.2 Array Exotic Objects

An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 2³². The value of the length property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the length property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.

Note

A String property name P is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³²-1.

Array exotic objects always have a non-configurable property named "length".

Array exotic objects provide an alternative definition for the [[DefineOwnProperty]] internal method. Except for that internal method, Array exotic objects provide all of the other essential internal methods as specified in 9.1.

9.4.2.1 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of an Array exotic object A is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If P is "length", then
1. Return ? ArraySetLength(A, Desc).
Else if P is an array index, then
1. Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
2. Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
3. Let oldLen be oldLenDesc.[[Value]].
4. Let index be ! ToUint32(P).
5. If index ≥ oldLen and oldLenDesc.[[Writable]] is false, return false.
6. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
7. If succeeded is false, return false.
8. If index ≥ oldLen, then
  1. Set oldLenDesc.[[Value]] to index + 1.
  2. Let succeeded be OrdinaryDefineOwnProperty(A, "length", oldLenDesc).
  3. Assert: succeeded is true.
9. Return true.
Return OrdinaryDefineOwnProperty(A, P, Desc).

9.4.2.2 ArrayCreate ( `length` [ , `proto` ] )

The abstract operation ArrayCreate with argument length (either 0 or a positive integer) and optional argument proto is used to specify the creation of new Array exotic objects. It performs the following steps:

Assert: length is an integer Number ≥ 0.
If length is -0, set length to +0.
If length>2³²-1, throw a RangeError exception.
If proto is not present, set proto to the intrinsic object %ArrayPrototype%.
Let A be a newly created Array exotic object.
Set A's essential internal methods except for [[DefineOwnProperty]] to the default ordinary object definitions specified in 9.1.
Set A.[[DefineOwnProperty]] as specified in 9.4.2.1.
Set A.[[Prototype]] to proto.
Set A.[[Extensible]] to true.
Perform ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor{[[Value]]: length, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}).
Return A.

9.4.2.3 ArraySpeciesCreate ( `originalArray`, `length` )

The abstract operation ArraySpeciesCreate with arguments originalArray and length is used to specify the creation of a new Array object using a constructor function that is derived from originalArray. It performs the following steps:

Assert: length is an integer Number ≥ 0.
If length is -0, set length to +0.
Let isArray be ? IsArray(originalArray).
If isArray is false, return ? ArrayCreate(length).
Let C be ? Get(originalArray, "constructor").
If IsConstructor(C) is true, then
1. Let thisRealm be the current Realm Record.
2. Let realmC be ? GetFunctionRealm(C).
3. If thisRealm and realmC are not the same Realm Record, then
  1. If SameValue(C, realmC.[[Intrinsics]].[[%Array%]]) is true, set C to undefined.
If Type(C) is Object, then
1. Set C to ? Get(C, @@species).
2. If C is null, set C to undefined.
If C is undefined, return ? ArrayCreate(length).
If IsConstructor(C) is false, throw a TypeError exception.
Return ? Construct(C, « length »).

Note

If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.

9.4.2.4 ArraySetLength ( `A`, `Desc` )

When the abstract operation ArraySetLength is called with an Array exotic object A, and Property Descriptor Desc, the following steps are taken:

If Desc.[[Value]] is absent, then
1. Return OrdinaryDefineOwnProperty(A, "length", Desc).
Let newLenDesc be a copy of Desc.
Let newLen be ? ToUint32(Desc.[[Value]]).
Let numberLen be ? ToNumber(Desc.[[Value]]).
If newLen ≠ numberLen, throw a RangeError exception.
Set newLenDesc.[[Value]] to newLen.
Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
Let oldLen be oldLenDesc.[[Value]].
If newLen ≥ oldLen, then
1. Return OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If oldLenDesc.[[Writable]] is false, return false.
If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
Else,
1. Need to defer setting the [[Writable]] attribute to false in case any elements cannot be deleted.
2. Let newWritable be false.
3. Set newLenDesc.[[Writable]] to true.
Let succeeded be ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If succeeded is false, return false.
Repeat, while newLen < oldLen,
1. Set oldLen to oldLen - 1.
2. Let deleteSucceeded be ! A.[[Delete]](! ToString(oldLen)).
3. If deleteSucceeded is false, then
  1. Set newLenDesc.[[Value]] to oldLen + 1.
  2. If newWritable is false, set newLenDesc.[[Writable]] to false.
  3. Perform ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  4. Return false.
If newWritable is false, then
1. Return OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Writable]]: false }). This call will always return true.
Return true.

Note

In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2^nd Edition of this specification.

9.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual integer-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have a data property named "length" whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

String exotic objects provide alternative definitions for the following internal methods. All of the other String exotic object essential internal methods that are not defined below are as specified in 9.1.

9.4.3.1 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a String exotic object S is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let desc be OrdinaryGetOwnProperty(S, P).
If desc is not undefined, return desc.
Return ! StringGetOwnProperty(S, P).

9.4.3.2 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a String exotic object S is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let stringDesc be ! StringGetOwnProperty(S, P).
If stringDesc is not undefined, then
1. Let extensible be S.[[Extensible]].
2. Return ! IsCompatiblePropertyDescriptor(extensible, Desc, stringDesc).
Return ! OrdinaryDefineOwnProperty(S, P, Desc).

9.4.3.3 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a String exotic object O is called, the following steps are taken:

Let keys be a new empty List.
Let str be the String value of O.[[StringData]].
Let len be the length of str.
For each integer i starting with 0 such that i < len, in ascending order, do
1. Add ! ToString(i) as the last element of keys.
For each own property key P of O such that P is an integer index and ToInteger(P) ≥ len, in ascending numeric index order, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is String and P is not an integer index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.4.3.4 StringCreate ( `value`, `prototype` )

The abstract operation StringCreate with arguments value and prototype is used to specify the creation of new String exotic objects. It performs the following steps:

Assert: Type(value) is String.
Let S be a newly created String exotic object.
Set S.[[StringData]] to value.
Set S's essential internal methods to the default ordinary object definitions specified in 9.1.
Set S.[[GetOwnProperty]] as specified in 9.4.3.1.
Set S.[[DefineOwnProperty]] as specified in 9.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in 9.4.3.3.
Set S.[[Prototype]] to prototype.
Set S.[[Extensible]] to true.
Let length be the number of code unit elements in value.
Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor { [[Value]]: length, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Return S.

9.4.3.5 StringGetOwnProperty ( `S`, `P` )

The abstract operation StringGetOwnProperty called with arguments S and P performs the following steps:

Assert: S is an Object that has a [[StringData]] internal slot.
Assert: IsPropertyKey(P) is true.
If Type(P) is not String, return undefined.
Let index be ! CanonicalNumericIndexString(P).
If index is undefined, return undefined.
If IsInteger(index) is false, return undefined.
If index = -0, return undefined.
Let str be the String value of S.[[StringData]].
Let len be the length of str.
If index < 0 or len ≤ index, return undefined.
Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at index index.
Return a PropertyDescriptor { [[Value]]: resultStr, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }.

9.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (19.1.3.6) to identify them as such.

Arguments exotic objects provide alternative definitions for the following internal methods. All of the other arguments exotic object essential internal methods that are not defined below are as specified in 9.1

Note 1

The integer-indexed data properties of an arguments exotic object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If the arguments object is an ordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

Note 2

The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

Note 3

Ordinary arguments objects define a non-configurable accessor property named "callee" which throws a TypeError exception on access. The "callee" property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.

Note 4

ECMAScript implementations of arguments exotic objects have historically contained an accessor property named "caller". Prior to ECMAScript 2017, this specification included the definition of a throwing "caller" property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing "caller" accessor.

9.4.4.1 [[GetOwnProperty]] ( `P` )

The [[GetOwnProperty]] internal method of an arguments exotic object when called with a property key P performs the following steps:

Let args be the arguments object.
Let desc be OrdinaryGetOwnProperty(args, P).
If desc is undefined, return desc.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
If isMapped is true, then
1. Set desc.[[Value]] to Get(map, P).
Return desc.

9.4.4.2 [[DefineOwnProperty]] ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of an arguments exotic object when called with a property key P and Property Descriptor Desc performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be HasOwnProperty(map, P).
Let newArgDesc be Desc.
If isMapped is true and IsDataDescriptor(Desc) is true, then
1. If Desc.[[Value]] is not present and Desc.[[Writable]] is present and its value is false, then
  1. Set newArgDesc to a copy of Desc.
  2. Set newArgDesc.[[Value]] to Get(map, P).
Let allowed be ? OrdinaryDefineOwnProperty(args, P, newArgDesc).
If allowed is false, return false.
If isMapped is true, then
1. If IsAccessorDescriptor(Desc) is true, then
  1. Call map.[[Delete]](P).
2. Else,
  1. If Desc.[[Value]] is present, then
    1. Let setStatus be Set(map, P, Desc.[[Value]], false).
    2. Assert: setStatus is true because formal parameters mapped by argument objects are always writable.
  2. If Desc.[[Writable]] is present and its value is false, then
    1. Call map.[[Delete]](P).
Return true.

9.4.4.3 [[Get]] ( `P`, `Receiver` )

The [[Get]] internal method of an arguments exotic object when called with a property key P and ECMAScript language value Receiver performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
If isMapped is false, then
1. Return ? OrdinaryGet(args, P, Receiver).
Else map contains a formal parameter mapping for P,
1. Return Get(map, P).

9.4.4.4 [[Set]] ( `P`, `V`, `Receiver` )

The [[Set]] internal method of an arguments exotic object when called with property key P, value V, and ECMAScript language value Receiver performs the following steps:

Let args be the arguments object.
If SameValue(args, Receiver) is false, then
1. Let isMapped be false.
Else,
1. Let map be args.[[ParameterMap]].
2. Let isMapped be ! HasOwnProperty(map, P).
If isMapped is true, then
1. Let setStatus be Set(map, P, V, false).
2. Assert: setStatus is true because formal parameters mapped by argument objects are always writable.
Return ? OrdinarySet(args, P, V, Receiver).

9.4.4.5 [[Delete]] ( `P` )

The [[Delete]] internal method of an arguments exotic object when called with a property key P performs the following steps:

Let args be the arguments object.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
Let result be ? OrdinaryDelete(args, P).
If result is true and isMapped is true, then
1. Call map.[[Delete]](P).
Return result.

9.4.4.6 CreateUnmappedArgumentsObject ( `argumentsList` )

The abstract operation CreateUnmappedArgumentsObject called with an argument argumentsList performs the following steps:

Let len be the number of elements in argumentsList.
Let obj be ObjectCreate(%ObjectPrototype%, « [[ParameterMap]] »).
Set obj.[[ParameterMap]] to undefined.
Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let index be 0.
Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform CreateDataProperty(obj, ! ToString(index), val).
3. Let index be index + 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false }).
Return obj.

9.4.4.7 CreateMappedArgumentsObject ( `func`, `formals`, `argumentsList`, `env` )

The abstract operation CreateMappedArgumentsObject is called with object func, Parse Node formals, List argumentsList, and Environment Record env. The following steps are performed:

Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
Set obj.[[GetOwnProperty]] as specified in 9.4.4.1.
Set obj.[[DefineOwnProperty]] as specified in 9.4.4.2.
Set obj.[[Get]] as specified in 9.4.4.3.
Set obj.[[Set]] as specified in 9.4.4.4.
Set obj.[[Delete]] as specified in 9.4.4.5.
Set the remainder of obj's essential internal methods to the default ordinary object definitions specified in 9.1.
Set obj.[[Prototype]] to %ObjectPrototype%.
Set obj.[[Extensible]] to true.
Let map be ObjectCreate(null).
Set obj.[[ParameterMap]] to map.
Let parameterNames be the BoundNames of formals.
Let numberOfParameters be the number of elements in parameterNames.
Let index be 0.
Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform CreateDataProperty(obj, ! ToString(index), val).
3. Let index be index + 1.
Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let mappedNames be a new empty List.
Let index be numberOfParameters - 1.
Repeat, while index ≥ 0,
1. Let name be parameterNames[index].
2. If name is not an element of mappedNames, then
  1. Add name as an element of the list mappedNames.
  2. If index < len, then
    1. Let g be MakeArgGetter(name, env).
    2. Let p be MakeArgSetter(name, env).
    3. Perform map.[[DefineOwnProperty]](! ToString(index), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]: false, [[Configurable]]: true }).
3. Let index be index - 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Return obj.

9.4.4.7.1 MakeArgGetter ( `name`, `env` )

The abstract operation MakeArgGetter called with String name and Environment Record env creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps:

Let steps be the steps of an ArgGetter function as specified below.
Let getter be CreateBuiltinFunction(steps, « [[Name]], [[Env]] »).
Set getter.[[Name]] to name.
Set getter.[[Env]] to env.
Return getter.

An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:

Let name be f.[[Name]].
Let env be f.[[Env]].
Return env.GetBindingValue(name, false).

Note

ArgGetter functions are never directly accessible to ECMAScript code.

9.4.4.7.2 MakeArgSetter ( `name`, `env` )

The abstract operation MakeArgSetter called with String name and Environment Record env creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps:

Let steps be the steps of an ArgSetter function as specified below.
Let setter be CreateBuiltinFunction(steps, « [[Name]], [[Env]] »).
Set setter.[[Name]] to name.
Set setter.[[Env]] to env.
Return setter.

An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:

Let name be f.[[Name]].
Let env be f.[[Env]].
Return env.SetMutableBinding(name, value, false).

Note

ArgSetter functions are never directly accessible to ECMAScript code.

9.4.5 Integer-Indexed Exotic Objects

An Integer-Indexed exotic object is an exotic object that performs special handling of integer index property keys.

Integer-Indexed exotic objects have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.

Integer-Indexed exotic objects provide alternative definitions for the following internal methods. All of the other Integer-Indexed exotic object essential internal methods that are not defined below are as specified in 9.1.

9.4.5.1 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Let value be ? IntegerIndexedElementGet(O, numericIndex).
  2. If value is undefined, return undefined.
  3. Return a PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.
Return OrdinaryGetOwnProperty(O, P).

9.4.5.2 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Let buffer be O.[[ViewedArrayBuffer]].
  2. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  3. If IsInteger(numericIndex) is false, return false.
  4. If numericIndex = -0, return false.
  5. If numericIndex < 0, return false.
  6. If numericIndex ≥ O.[[ArrayLength]], return false.
  7. Return true.
Return ? OrdinaryHasProperty(O, P).

9.4.5.3 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of an Integer-Indexed exotic object O is called with property key P, and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. If IsInteger(numericIndex) is false, return false.
  2. If numericIndex = -0, return false.
  3. If numericIndex < 0, return false.
  4. Let length be O.[[ArrayLength]].
  5. If numericIndex ≥ length, return false.
  6. If IsAccessorDescriptor(Desc) is true, return false.
  7. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is true, return false.
  8. If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] is false, return false.
  9. If Desc has a [[Writable]] field and if Desc.[[Writable]] is false, return false.
  10. If Desc has a [[Value]] field, then
    1. Let value be Desc.[[Value]].
    2. Return ? IntegerIndexedElementSet(O, numericIndex, value).
  11. Return true.
Return ! OrdinaryDefineOwnProperty(O, P, Desc).

9.4.5.4 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of an Integer-Indexed exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Return ? IntegerIndexedElementGet(O, numericIndex).
Return ? OrdinaryGet(O, P, Receiver).

9.4.5.5 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of an Integer-Indexed exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is String, then
1. Let numericIndex be ! CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, then
  1. Return ? IntegerIndexedElementSet(O, numericIndex, V).
Return ? OrdinarySet(O, P, V, Receiver).

9.4.5.6 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of an Integer-Indexed exotic object O is called, the following steps are taken:

Let keys be a new empty List.
Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let len be O.[[ArrayLength]].
For each integer i starting with 0 such that i < len, in ascending order, do
1. Add ! ToString(i) as the last element of keys.
For each own property key P of O such that Type(P) is String and P is not an integer index, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
For each own property key P of O such that Type(P) is Symbol, in ascending chronological order of property creation, do
1. Add P as the last element of keys.
Return keys.

9.4.5.7 IntegerIndexedObjectCreate ( `prototype`, `internalSlotsList` )

The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer-Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:

Assert: internalSlotsList contains the names [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]].
Let A be a newly created object with an internal slot for each name in internalSlotsList.
Set A's essential internal methods to the default ordinary object definitions specified in 9.1.
Set A.[[GetOwnProperty]] as specified in 9.4.5.1.
Set A.[[HasProperty]] as specified in 9.4.5.2.
Set A.[[DefineOwnProperty]] as specified in 9.4.5.3.
Set A.[[Get]] as specified in 9.4.5.4.
Set A.[[Set]] as specified in 9.4.5.5.
Set A.[[OwnPropertyKeys]] as specified in 9.4.5.6.
Set A.[[Prototype]] to prototype.
Set A.[[Extensible]] to true.
Return A.

9.4.5.8 IntegerIndexedElementGet ( `O`, `index` )

The abstract operation IntegerIndexedElementGet with arguments O and index performs the following steps:

Assert: Type(index) is Number.
Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If IsInteger(index) is false, return undefined.
If index = -0, return undefined.
Let length be O.[[ArrayLength]].
If index < 0 or index ≥ length, return undefined.
Let offset be O.[[ByteOffset]].
Let arrayTypeName be the String value of O.[[TypedArrayName]].
Let elementSize be the Number value of the Element Size value specified in Table 56 for arrayTypeName.
Let indexedPosition be (index × elementSize) + offset.
Let elementType be the String value of the Element Type value in Table 56 for arrayTypeName.
Return GetValueFromBuffer(buffer, indexedPosition, elementType, true, "Unordered").

9.4.5.9 IntegerIndexedElementSet ( `O`, `index`, `value` )

The abstract operation IntegerIndexedElementSet with arguments O, index, and value performs the following steps:

Assert: Type(index) is Number.
Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let numValue be ? ToNumber(value).
Let buffer be O.[[ViewedArrayBuffer]].
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If IsInteger(index) is false, return false.
If index = -0, return false.
Let length be O.[[ArrayLength]].
If index < 0 or index ≥ length, return false.
Let offset be O.[[ByteOffset]].
Let arrayTypeName be the String value of O.[[TypedArrayName]].
Let elementSize be the Number value of the Element Size value specified in Table 56 for arrayTypeName.
Let indexedPosition be (index × elementSize) + offset.
Let elementType be the String value of the Element Type value in Table 56 for arrayTypeName.
Perform SetValueInBuffer(buffer, indexedPosition, elementType, numValue, true, "Unordered").
Return true.

9.4.6 Module Namespace Exotic Objects

A module namespace object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace objects are not extensible.

Module namespace objects have the internal slots defined in Table 29.

Table 29: Internal Slots of Module Namespace Exotic Objects

Internal Slot	Type	Description
[[Module]]	Module Record	The Module Record whose exports this namespace exposes.
[[Exports]]	List of String	A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using `Array.prototype.sort` using undefined as `comparefn`.
[[Prototype]]	Null	This slot always contains the value null (see 9.4.6.1).

Module namespace exotic objects provide alternative definitions for all of the internal methods except [[GetPrototypeOf]], which behaves as defined in 9.1.1.

9.4.6.1 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of a module namespace exotic object O is called with argument V, the following steps are taken:

Return ? SetImmutablePrototype(O, V).

9.4.6.2 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a module namespace exotic object O is called, the following steps are taken:

Return false.

9.4.6.3 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a module namespace exotic object O is called, the following steps are taken:

Return true.

9.4.6.4 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

If Type(P) is Symbol, return OrdinaryGetOwnProperty(O, P).
Let exports be O.[[Exports]].
If P is not an element of exports, return undefined.
Let value be ? O.[[Get]](P, O).
Return PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.

9.4.6.5 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a module namespace exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:

Return false.

9.4.6.6 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

If Type(P) is Symbol, return OrdinaryHasProperty(O, P).
Let exports be O.[[Exports]].
If P is an element of exports, return true.
Return false.

9.4.6.7 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of a module namespace exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is Symbol, then
1. Return ? OrdinaryGet(O, P, Receiver).
Let exports be O.[[Exports]].
If P is not an element of exports, return undefined.
Let m be O.[[Module]].
Let binding be ! m.ResolveExport(P, « »).
Assert: binding is a ResolvedBinding Record.
Let targetModule be binding.[[Module]].
Assert: targetModule is not undefined.
Let targetEnv be targetModule.[[Environment]].
If targetEnv is undefined, throw a ReferenceError exception.
Let targetEnvRec be targetEnv's EnvironmentRecord.
Return ? targetEnvRec.GetBindingValue(binding.[[BindingName]], true).

Note

ResolveExport is idempotent and side-effect free. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.

9.4.6.8 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of a module namespace exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Return false.

9.4.6.9 [[Delete]] ( `P` )

When the [[Delete]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
If Type(P) is Symbol, then
1. Return ? OrdinaryDelete(O, P).
Let exports be O.[[Exports]].
If P is an element of exports, return false.
Return true.

9.4.6.10 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a module namespace exotic object O is called, the following steps are taken:

Let exports be a copy of O.[[Exports]].
Let symbolKeys be ! OrdinaryOwnPropertyKeys(O).
Append all the entries of symbolKeys to the end of exports.
Return exports.

9.4.6.11 ModuleNamespaceCreate ( `module`, `exports` )

The abstract operation ModuleNamespaceCreate with arguments module, and exports is used to specify the creation of new module namespace exotic objects. It performs the following steps:

Assert: module is a Module Record.
Assert: module.[[Namespace]] is undefined.
Assert: exports is a List of String values.
Let M be a newly created object.
Set M's essential internal methods to the definitions specified in 9.4.6.
Set M.[[Module]] to module.
Let sortedExports be a new List containing the same values as the list exports where the values are ordered as if an Array of the same values had been sorted using Array.prototype.sort using undefined as comparefn.
Set M.[[Exports]] to sortedExports.
Create own properties of M corresponding to the definitions in 26.3.
Set module.[[Namespace]] to M.
Return M.

9.4.7 Immutable Prototype Exotic Objects

An immutable prototype exotic object is an exotic object that has a [[Prototype]] internal slot that will not change once it is initialized.

Immutable prototype exotic objects have the same internal slots as ordinary objects. They are exotic only in the following internal methods. All other internal methods of immutable prototype exotic objects that are not explicitly defined below are instead defined as in ordinary objects.

9.4.7.1 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of an immutable prototype exotic object O is called with argument V, the following steps are taken:

Return ? SetImmutablePrototype(O, V).

9.4.7.2 SetImmutablePrototype ( `O`, `V` )

When the SetImmutablePrototype abstract operation is called with arguments O and V, the following steps are taken:

Assert: Either Type(V) is Object or Type(V) is Null.
Let current be ? O.[[GetPrototypeOf]]().
If SameValue(V, current) is true, return true.
Return false.

9.5 Proxy Object Internal Methods and Internal Slots

A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 30) of a handler object may be used to augment the implementation for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.

Table 30: Proxy Handler Methods

Internal Method	Handler Method
[[GetPrototypeOf]]	`getPrototypeOf`
[[SetPrototypeOf]]	`setPrototypeOf`
[[IsExtensible]]	`isExtensible`
[[PreventExtensions]]	`preventExtensions`
[[GetOwnProperty]]	`getOwnPropertyDescriptor`
[[DefineOwnProperty]]	`defineProperty`
[[HasProperty]]	`has`
[[Get]]	`get`
[[Set]]	`set`
[[Delete]]	`deleteProperty`
[[OwnPropertyKeys]]	`ownKeys`
[[Call]]	`apply`
[[Construct]]	`construct`

When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy object to throw a TypeError exception.

Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.

9.5.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "getPrototypeOf").
If trap is undefined, then
1. Return ? target.[[GetPrototypeOf]]().
Let handlerProto be ? Call(trap, handler, « target »).
If Type(handlerProto) is neither Object nor Null, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return handlerProto.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
Return handlerProto.

Note

[[GetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.

9.5.2 [[SetPrototypeOf]] ( `V` )

When the [[SetPrototypeOf]] internal method of a Proxy exotic object O is called with argument V, the following steps are taken:

Assert: Either Type(V) is Object or Type(V) is Null.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "setPrototypeOf").
If trap is undefined, then
1. Return ? target.[[SetPrototypeOf]](V).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, V »)).
If booleanTrapResult is false, return false.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return true.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(V, targetProto) is false, throw a TypeError exception.
Return true.

Note

[[SetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

9.5.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "isExtensible").
If trap is undefined, then
1. Return ? target.[[IsExtensible]]().
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
Let targetResult be ? target.[[IsExtensible]]().
If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
Return booleanTrapResult.

Note

[[IsExtensible]] for proxy objects enforces the following invariants:

The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.

9.5.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "preventExtensions").
If trap is undefined, then
1. Return ? target.[[PreventExtensions]]().
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
If booleanTrapResult is true, then
1. Let targetIsExtensible be ? target.[[IsExtensible]]().
2. If targetIsExtensible is true, throw a TypeError exception.
Return booleanTrapResult.

Note

[[PreventExtensions]] for proxy objects enforces the following invariants:

The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the proxy object only returns true if [[IsExtensible]] applied to the proxy object's target object is false.

9.5.5 [[GetOwnProperty]] ( `P` )

When the [[GetOwnProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "getOwnPropertyDescriptor").
If trap is undefined, then
1. Return ? target.[[GetOwnProperty]](P).
Let trapResultObj be ? Call(trap, handler, « target, P »).
If Type(trapResultObj) is neither Object nor Undefined, throw a TypeError exception.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If trapResultObj is undefined, then
1. If targetDesc is undefined, return undefined.
2. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
3. Let extensibleTarget be ? IsExtensible(target).
4. Assert: Type(extensibleTarget) is Boolean.
5. If extensibleTarget is false, throw a TypeError exception.
6. Return undefined.
Let extensibleTarget be ? IsExtensible(target).
Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
Call CompletePropertyDescriptor(resultDesc).
Let valid be IsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc).
If valid is false, throw a TypeError exception.
If resultDesc.[[Configurable]] is false, then
1. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
  1. Throw a TypeError exception.
Return resultDesc.

Note

[[GetOwnProperty]] for proxy objects enforces the following invariants:

The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exist as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, if it does not exist as an own property of the target object or if it exists as a configurable own property of the target object.

9.5.6 [[DefineOwnProperty]] ( `P`, `Desc` )

When the [[DefineOwnProperty]] internal method of a Proxy exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "defineProperty").
If trap is undefined, then
1. Return ? target.[[DefineOwnProperty]](P, Desc).
Let descObj be FromPropertyDescriptor(Desc).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, descObj »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
Let extensibleTarget be ? IsExtensible(target).
If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is false, then
1. Let settingConfigFalse be true.
Else, let settingConfigFalse be false.
If targetDesc is undefined, then
1. If extensibleTarget is false, throw a TypeError exception.
2. If settingConfigFalse is true, throw a TypeError exception.
Else targetDesc is not undefined,
1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc, targetDesc) is false, throw a TypeError exception.
2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
Return true.

Note

[[DefineOwnProperty]] for proxy objects enforces the following invariants:

The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

9.5.7 [[HasProperty]] ( `P` )

When the [[HasProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "has").
If trap is undefined, then
1. Return ? target.[[HasProperty]](P).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
If booleanTrapResult is false, then
1. Let targetDesc be ? target.[[GetOwnProperty]](P).
2. If targetDesc is not undefined, then
  1. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
  2. Let extensibleTarget be ? IsExtensible(target).
  3. If extensibleTarget is false, throw a TypeError exception.
Return booleanTrapResult.

Note

[[HasProperty]] for proxy objects enforces the following invariants:

The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

9.5.8 [[Get]] ( `P`, `Receiver` )

When the [[Get]] internal method of a Proxy exotic object O is called with property key P and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "get").
If trap is undefined, then
1. Return ? target.[[Get]](P, Receiver).
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]] is undefined, then
  1. If trapResult is not undefined, throw a TypeError exception.
Return trapResult.

Note

[[Get]] for proxy objects enforces the following invariants:

The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.

9.5.9 [[Set]] ( `P`, `V`, `Receiver` )

When the [[Set]] internal method of a Proxy exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "set").
If trap is undefined, then
1. Return ? target.[[Set]](P, V, Receiver).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) is true, then
  1. If targetDesc.[[Set]] is undefined, throw a TypeError exception.
Return true.

Note

[[Set]] for proxy objects enforces the following invariants:

The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.

9.5.10 [[Delete]] ( `P` )

When the [[Delete]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "deleteProperty").
If trap is undefined, then
1. Return ? target.[[Delete]](P).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is undefined, return true.
If targetDesc.[[Configurable]] is false, throw a TypeError exception.
Return true.

Note

[[Delete]] for proxy objects enforces the following invariants:

The result of [[Delete]] is a Boolean value.
A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.

9.5.11 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a Proxy exotic object O is called, the following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "ownKeys").
If trap is undefined, then
1. Return ? target.[[OwnPropertyKeys]]().
Let trapResultArray be ? Call(trap, handler, « target »).
Let trapResult be ? CreateListFromArrayLike(trapResultArray, « String, Symbol »).
If trapResult contains any duplicate entries, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
Let targetKeys be ? target.[[OwnPropertyKeys]]().
Assert: targetKeys is a List containing only String and Symbol values.
Assert: targetKeys contains no duplicate entries.
Let targetConfigurableKeys be a new empty List.
Let targetNonconfigurableKeys be a new empty List.
For each element key of targetKeys, do
1. Let desc be ? target.[[GetOwnProperty]](key).
2. If desc is not undefined and desc.[[Configurable]] is false, then
  1. Append key as an element of targetNonconfigurableKeys.
3. Else,
  1. Append key as an element of targetConfigurableKeys.
If extensibleTarget is true and targetNonconfigurableKeys is empty, then
1. Return trapResult.
Let uncheckedResultKeys be a new List which is a copy of trapResult.
For each key that is an element of targetNonconfigurableKeys, do
1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If extensibleTarget is true, return trapResult.
For each key that is an element of targetConfigurableKeys, do
1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If uncheckedResultKeys is not empty, throw a TypeError exception.
Return trapResult.

Note

[[OwnPropertyKeys]] for proxy objects enforces the following invariants:

The result of [[OwnPropertyKeys]] is a List.
The returned List contains no duplicate entries.
The Type of each result List element is either String or Symbol.
The result List must contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

9.5.12 [[Call]] ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of a Proxy exotic object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "apply").
If trap is undefined, then
1. Return ? Call(target, thisArgument, argumentsList).
Let argArray be CreateArrayFromList(argumentsList).
Return ? Call(trap, handler, « target, thisArgument, argArray »).

Note

A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

9.5.13 [[Construct]] ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of a Proxy exotic object O is called with parameters argumentsList which is a possibly empty List of ECMAScript language values and newTarget. The following steps are taken:

Let handler be O.[[ProxyHandler]].
If handler is null, throw a TypeError exception.
Assert: Type(handler) is Object.
Let target be O.[[ProxyTarget]].
Let trap be ? GetMethod(handler, "construct").
If trap is undefined, then
1. Assert: IsConstructor(target) is true.
2. Return ? Construct(target, argumentsList, newTarget).
Let argArray be CreateArrayFromList(argumentsList).
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
If Type(newObj) is not Object, throw a TypeError exception.
Return newObj.

Note 1

A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

Note 2

[[Construct]] for proxy objects enforces the following invariants:

The result of [[Construct]] must be an Object.

9.5.14 ProxyCreate ( `target`, `handler` )

The abstract operation ProxyCreate with arguments target and handler is used to specify the creation of new Proxy exotic objects. It performs the following steps:

If Type(target) is not Object, throw a TypeError exception.
If target is a Proxy exotic object and target.[[ProxyHandler]] is null, throw a TypeError exception.
If Type(handler) is not Object, throw a TypeError exception.
If handler is a Proxy exotic object and handler.[[ProxyHandler]] is null, throw a TypeError exception.
Let P be a newly created object.
Set P's essential internal methods (except for [[Call]] and [[Construct]]) to the definitions specified in 9.5.
If IsCallable(target) is true, then
1. Set P.[[Call]] as specified in 9.5.12.
2. If IsConstructor(target) is true, then
  1. Set P.[[Construct]] as specified in 9.5.13.
Set P.[[ProxyTarget]] to target.
Set P.[[ProxyHandler]] to handler.
Return P.

10 ECMAScript Language: Source Code

10.1 Source Text

Syntax

any Unicode code point

ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

Note

In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.

10.1.1 Static Semantics: UTF16Encoding ( `cp` )

The UTF16Encoding of a numeric code point value, cp, is determined as follows:

Assert: 0 ≤ cp ≤ 0x10FFFF.
If cp ≤ 0xFFFF, return cp.
Let cu1 be floor((cp - 0x10000) / 0x400) + 0xD800.
Let cu2 be ((cp - 0x10000) modulo 0x400) + 0xDC00.
Return the code unit sequence consisting of cu1 followed by cu2.

10.1.2 Static Semantics: UTF16Decode ( `lead`, `trail` )

Two code units, lead and trail, that form a UTF-16 代理对 are converted to a code point by performing the following steps:

Assert: lead is a 首代理 and trail is a 尾代理.
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.

10.2 Types of Source Code

There are four types of ECMAScript code:

Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 9.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

Note

Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3), Generator Function Definitions (14.4), Async Function Definitions (14.7), Async Generator Function Definitions (14.5), and Async Arrow Functions (14.8). Function code is also derived from the arguments to the Function constructor (19.2.1.1), the GeneratorFunction constructor (25.2.1.1), and the AsyncFunction constructor (25.7.1.1).

10.2.1 Strict Mode Code

An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:

Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive.
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or AsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

ECMAScript code that is not strict mode code is called non-strict code.

10.2.2 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some implementation-defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.

11 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

Note

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementRegExpOrTemplateTail

TemplateSubstitutionTail

InputElementTemplateTail

TemplateSubstitutionTail

11.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName after the first character.

U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 11.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.

Table 31: Format-Control Code Point Usage

Code Point	Name	Abbreviation	Usage
`U+200C`	ZERO WIDTH NON-JOINER	<ZWNJ>	IdentifierPart
`U+200D`	ZERO WIDTH JOINER	<ZWJ>	IdentifierPart
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>	WhiteSpace

11.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 32.

Table 32: White Space Code Points

Code Point	Name	Abbreviation
`U+0009`	CHARACTER TABULATION	<TAB>
`U+000B`	LINE TABULATION	<VT>
`U+000C`	FORM FEED (FF)	<FF>
`U+0020`	SPACE	<SP>
`U+00A0`	NO-BREAK SPACE	<NBSP>
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>
Other category “Zs”	Any other Unicode “Space_Separator” code point	<USP>

ECMAScript implementations must recognize as WhiteSpace code points listed in the “Space_Separator” (“Zs”) category.

Note

Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Space_Separator” (“Zs”).

Syntax

WhiteSpace

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<USP>

11.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 33.

Table 33: Line Terminator Code Points

Code Point	Unicode Name	Abbreviation
`U+000A`	LINE FEED (LF)	<LF>
`U+000D`	CARRIAGE RETURN (CR)	<CR>
`U+2028`	LINE SEPARATOR	<LS>
`U+2029`	PARAGRAPH SEPARATOR	<PS>

Only the Unicode code points in Table 33 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator

<LF>

<CR>

<LS>

<PS>

LineTerminatorSequence

<LF>

<CR>

[lookahead ≠ <LF>]

<LS>

<PS>

<CR>

<LF>

11.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 11.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

opt

MultiLineNotAsteriskChar

MultiLineNotForwardSlashOrAsteriskChar

opt

PostAsteriskCommentChars

opt

PostAsteriskCommentChars

opt

PostAsteriskCommentChars

opt

MultiLineNotAsteriskChar

MultiLineNotForwardSlashOrAsteriskChar

but not *

but not one of / or *

SingleLineComment

SingleLineCommentChars

opt

SingleLineCommentChars

SingleLineCommentChar

SingleLineCommentChars

opt

SingleLineCommentChar

but not LineTerminator

11.5 Tokens

Syntax

Note

The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

11.6 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

Note 1

This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the CodePoint of the UnicodeEscapeSequence (see 11.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierNames that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

Syntax

UnicodeIDContinue

<ZWNJ>

<ZWJ>

UnicodeIDStart

any Unicode code point with the Unicode property “ID_Start”

UnicodeIDContinue

any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 11.8.4.

Note 2

The nonterminal IdentifierPart derives _ via UnicodeIDContinue.

Note 3

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

11.6.1 Identifier Names

11.6.1.1 Static Semantics: Early Errors

IdentifierStart

It is a Syntax Error if SV(UnicodeEscapeSequence) is none of "$", or "_", or the UTF16Encoding of a code point matched by the UnicodeIDStart lexical grammar production.

It is a Syntax Error if SV(UnicodeEscapeSequence) is none of "$", or "_", or the UTF16Encoding of either <ZWNJ> or <ZWJ>, or the UTF16Encoding of a Unicode code point that would be matched by the UnicodeIDContinue lexical grammar production.

11.6.1.2 Static Semantics: StringValue

Return the String value consisting of the sequence of code units corresponding to IdentifierName. In determining the sequence any occurrences of \ UnicodeEscapeSequence are first replaced with the code point represented by the UnicodeEscapeSequence and then the code points of the entire IdentifierName are converted to code units by UTF16Encoding each code point.

11.6.2 Reserved Words

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax

Note

The ReservedWord definitions are specified as literal sequences of specific SourceCharacter elements. A code point in a ReservedWord cannot be expressed by a \ UnicodeEscapeSequence.

11.6.2.1 Keywords

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.

Syntax

Keyword

one of

await

break

case

catch

class

const

continue

debugger

default

delete

else

export

extends

finally

for

function

import

instanceof

new

return

super

switch

this

throw

try

typeof

var

void

while

with

yield

Note

In some contexts yield and await are given the semantics of an Identifier. See 12.1.1. In strict mode code, let and static are treated as reserved words through static semantic restrictions (see 12.1.1, 13.3.1.1, 13.7.5.1, and 14.6.1) rather than the lexical grammar.

11.6.2.2 Future Reserved Words

The following tokens are reserved for use as keywords in future language extensions.

Syntax

FutureReservedWord

enum

Note

Use of the following tokens within strict mode code is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:

`implements`	`package`	`protected`
`interface`	`private`	`public`

11.7 Punctuators

Syntax

Punctuator

one of

{

(

)

[

]

...

;

===

!==

>>>

**=

<<=

>>=

>>>=

DivPunctuator

RightBracePunctuator

}

11.8 Literals

11.8.1 Null Literals

Syntax

NullLiteral

null

11.8.2 Boolean Literals

Syntax

BooleanLiteral

true

false

11.8.3 Numeric Literals

Syntax

DecimalIntegerLiteral

opt

opt

opt

DecimalIntegerLiteral

ExponentPart

opt

DecimalIntegerLiteral

opt

one of

one of

one of

one of

one of

one of

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

A conforming implementation, when processing strict mode code, must not extend, as described in B.1.1, the syntax of NumericLiteral to include LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include NonOctalDecimalIntegerLiteral.

11.8.3.1 Static Semantics: MV

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

The MV of NumericLiteral::DecimalLiteral is the MV of DecimalLiteral.
The MV of NumericLiteral::BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.
The MV of NumericLiteral::OctalIntegerLiteral is the MV of OctalIntegerLiteral.
The MV of NumericLiteral::HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteral. is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10^-n), where n is the number of code points in DecimalDigits.
The MV of DecimalLiteral::DecimalIntegerLiteral.ExponentPart is the MV of DecimalIntegerLiteral × 10^e, where e is the MV of ExponentPart.
The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigitsExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10^-n)) × 10^e, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral::.DecimalDigits is the MV of DecimalDigits × 10^-n, where n is the number of code points in DecimalDigits.
The MV of DecimalLiteral::.DecimalDigitsExponentPart is the MV of DecimalDigits × 10^e-n, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral::DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral::DecimalIntegerLiteralExponentPart is the MV of DecimalIntegerLiteral × 10^e, where e is the MV of ExponentPart.
The MV of DecimalIntegerLiteral::0 is 0.
The MV of DecimalIntegerLiteral::NonZeroDigit is the MV of NonZeroDigit.
The MV of DecimalIntegerLiteral::NonZeroDigitDecimalDigits is (the MV of NonZeroDigit × 10ⁿ) plus the MV of DecimalDigits, where n is the number of code points in DecimalDigits.
The MV of DecimalDigits::DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits::DecimalDigitsDecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.
The MV of ExponentPart::ExponentIndicatorSignedInteger is the MV of SignedInteger.
The MV of SignedInteger::DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger::+DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger::-DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit::0 or of HexDigit::0 or of OctalDigit::0 or of BinaryDigit::0 is 0.
The MV of DecimalDigit::1 or of NonZeroDigit::1 or of HexDigit::1 or of OctalDigit::1 or of BinaryDigit::1 is 1.
The MV of DecimalDigit::2 or of NonZeroDigit::2 or of HexDigit::2 or of OctalDigit::2 is 2.
The MV of DecimalDigit::3 or of NonZeroDigit::3 or of HexDigit::3 or of OctalDigit::3 is 3.
The MV of DecimalDigit::4 or of NonZeroDigit::4 or of HexDigit::4 or of OctalDigit::4 is 4.
The MV of DecimalDigit::5 or of NonZeroDigit::5 or of HexDigit::5 or of OctalDigit::5 is 5.
The MV of DecimalDigit::6 or of NonZeroDigit::6 or of HexDigit::6 or of OctalDigit::6 is 6.
The MV of DecimalDigit::7 or of NonZeroDigit::7 or of HexDigit::7 or of OctalDigit::7 is 7.
The MV of DecimalDigit::8 or of NonZeroDigit::8 or of HexDigit::8 is 8.
The MV of DecimalDigit::9 or of NonZeroDigit::9 or of HexDigit::9 is 9.
The MV of HexDigit::a or of HexDigit::A is 10.
The MV of HexDigit::b or of HexDigit::B is 11.
The MV of HexDigit::c or of HexDigit::C is 12.
The MV of HexDigit::d or of HexDigit::D is 13.
The MV of HexDigit::e or of HexDigit::E is 14.
The MV of HexDigit::f or of HexDigit::F is 15.
The MV of BinaryIntegerLiteral::0bBinaryDigits is the MV of BinaryDigits.
The MV of BinaryIntegerLiteral::0BBinaryDigits is the MV of BinaryDigits.
The MV of BinaryDigits::BinaryDigit is the MV of BinaryDigit.
The MV of BinaryDigits::BinaryDigitsBinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.
The MV of OctalIntegerLiteral::0oOctalDigits is the MV of OctalDigits.
The MV of OctalIntegerLiteral::0OOctalDigits is the MV of OctalDigits.
The MV of OctalDigits::OctalDigit is the MV of OctalDigit.
The MV of OctalDigits::OctalDigitsOctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.
The MV of HexIntegerLiteral::0xHexDigits is the MV of HexDigits.
The MV of HexIntegerLiteral::0XHexDigits is the MV of HexDigits.
The MV of HexDigits::HexDigit is the MV of HexDigit.
The MV of HexDigits::HexDigitsHexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

it is not 0; or
there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

11.8.4 String Literals

Note 1

A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), U+2028 (LINE SEPARATOR), U+2029 (PARAGRAPH SEPARATOR), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

opt

opt

DoubleStringCharacter

opt

SingleStringCharacter

opt

DoubleStringCharacter

but not one of " or \ or LineTerminator

EscapeSequence

LineContinuation

SingleStringCharacter

but not one of ' or \ or LineTerminator

EscapeSequence

LineContinuation

LineTerminatorSequence

EscapeSequence

CharacterEscapeSequence

[lookahead ∉ DecimalDigit]

A conforming implementation, when processing strict mode code, must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence as described in B.1.2.

CharacterEscapeSequence

SingleEscapeCharacter

NonEscapeCharacter

SingleEscapeCharacter

one of

NonEscapeCharacter

but not one of EscapeCharacter or LineTerminator

EscapeCharacter

SingleEscapeCharacter

DecimalDigit

}

The definition of the nonterminal HexDigit is given in 11.8.3. SourceCharacter is defined in 10.1.

Note 2

A line terminator code point cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to cause a line terminator code point to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.

11.8.4.1 Static Semantics: StringValue

opt

opt

Return the String value whose elements are the SV of this StringLiteral.

11.8.4.2 Static Semantics: SV

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.

The SV of StringLiteral::"" is the empty code unit sequence.
The SV of StringLiteral::'' is the empty code unit sequence.
The SV of StringLiteral::"DoubleStringCharacters" is the SV of DoubleStringCharacters.
The SV of StringLiteral::'SingleStringCharacters' is the SV of SingleStringCharacters.
The SV of DoubleStringCharacters::DoubleStringCharacter is a sequence of up to two code units that is the SV of DoubleStringCharacter.
The SV of DoubleStringCharacters::DoubleStringCharacterDoubleStringCharacters is a sequence of up to two code units that is the SV of DoubleStringCharacter followed by the code units of the SV of DoubleStringCharacters in order.
The SV of SingleStringCharacters::SingleStringCharacter is a sequence of up to two code units that is the SV of SingleStringCharacter.
The SV of SingleStringCharacters::SingleStringCharacterSingleStringCharacters is a sequence of up to two code units that is the SV of SingleStringCharacter followed by the code units of the SV of SingleStringCharacters in order.
The SV of DoubleStringCharacter::SourceCharacterbut not one of " or \ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of DoubleStringCharacter::\EscapeSequence is the SV of the EscapeSequence.
The SV of DoubleStringCharacter::LineContinuation is the empty code unit sequence.
The SV of SingleStringCharacter::SourceCharacterbut not one of ' or \ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of SingleStringCharacter::\EscapeSequence is the SV of the EscapeSequence.
The SV of SingleStringCharacter::LineContinuation is the empty code unit sequence.
The SV of EscapeSequence::CharacterEscapeSequence is the SV of the CharacterEscapeSequence.
The SV of EscapeSequence::0 is the code unit 0x0000 (NULL).
The SV of EscapeSequence::HexEscapeSequence is the SV of the HexEscapeSequence.
The SV of EscapeSequence::UnicodeEscapeSequence is the SV of the UnicodeEscapeSequence.
The SV of CharacterEscapeSequence::SingleEscapeCharacter is the code unit whose value is determined by the SingleEscapeCharacter according to Table 34.

Table 34: String Single Character Escape Sequences

Escape Sequence	Code Unit Value	Unicode Character Name	Symbol
`\b`	`0x0008`	BACKSPACE	<BS>
`\t`	`0x0009`	CHARACTER TABULATION	<HT>
`\n`	`0x000A`	LINE FEED (LF)	<LF>
`\v`	`0x000B`	LINE TABULATION	<VT>
`\f`	`0x000C`	FORM FEED (FF)	<FF>
`\r`	`0x000D`	CARRIAGE RETURN (CR)	<CR>
`\"`	`0x0022`	QUOTATION MARK	`"`
`\'`	`0x0027`	APOSTROPHE	`'`
`\\`	`0x005C`	REVERSE SOLIDUS	`\`

The SV of CharacterEscapeSequence::NonEscapeCharacter is the SV of the NonEscapeCharacter.
The SV of NonEscapeCharacter::SourceCharacterbut not one of EscapeCharacter or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The SV of HexEscapeSequence::xHexDigitHexDigit is the code unit whose value is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.
The SV of UnicodeEscapeSequence::uHex4Digits is the SV of Hex4Digits.
The SV of Hex4Digits::HexDigitHexDigitHexDigitHexDigit is the code unit whose value is (0x1000 times the MV of the first HexDigit) plus (0x100 times the MV of the second HexDigit) plus (0x10 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.
The SV of UnicodeEscapeSequence::u{CodePoint} is the UTF16Encoding of the MV of CodePoint.

11.8.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 21.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 21.2.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (21.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionFirstChar

RegularExpressionChars

[empty]

RegularExpressionChars

RegularExpressionChar

RegularExpressionFirstChar

but not one of * or \ or / or [

RegularExpressionClass

RegularExpressionChar

but not one of \ or / or [

RegularExpressionClass

RegularExpressionClassChars

but not LineTerminator

RegularExpressionClass

[

]

RegularExpressionClassChars

[empty]

RegularExpressionClassChars

RegularExpressionClassChar

but not one of ] or \

[empty]

Note 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

11.8.5.1 Static Semantics: Early Errors

It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.

11.8.5.2 Static Semantics: BodyText

Return the source text that was recognized as RegularExpressionBody.

11.8.5.3 Static Semantics: FlagText

Return the source text that was recognized as RegularExpressionFlags.

11.8.6 Template Literal Lexical Components

Syntax

Template

TemplateCharacters

opt

TemplateCharacters

opt

TemplateSubstitutionTail

}

opt

}

opt

opt

[lookahead ≠ {]

LineTerminatorSequence

but not one of ` or \ or $ or LineTerminator

NotEscapeSequence

DecimalDigit

but not 0

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ≠ {]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

NotCodePoint

[lookahead ∉ HexDigit]

{

CodePoint

[lookahead ∉ HexDigit]

[lookahead ≠ }]

A conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.

Note

TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

11.8.6.1 Static Semantics: TV and TRV

A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.

The TV and TRV of NoSubstitutionTemplate::`` is the empty code unit sequence.
The TV and TRV of TemplateHead::`${ is the empty code unit sequence.
The TV and TRV of TemplateMiddle::}${ is the empty code unit sequence.
The TV and TRV of TemplateTail::}` is the empty code unit sequence.
The TV of NoSubstitutionTemplate::`TemplateCharacters` is the TV of TemplateCharacters.
The TV of TemplateHead::`TemplateCharacters${ is the TV of TemplateCharacters.
The TV of TemplateMiddle::}TemplateCharacters${ is the TV of TemplateCharacters.
The TV of TemplateTail::}TemplateCharacters` is the TV of TemplateCharacters.
The TV of TemplateCharacters::TemplateCharacter is the TV of TemplateCharacter.
The TV of TemplateCharacters::TemplateCharacterTemplateCharacters is undefined if either the TV of TemplateCharacter is undefined or the TV of TemplateCharacters is undefined. Otherwise, it is a sequence consisting of the code units of the TV of TemplateCharacter followed by the code units of the TV of TemplateCharacters.
The TV of TemplateCharacter::SourceCharacterbut not one of ` or \ or $ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The TV of TemplateCharacter::$ is the code unit 0x0024 (DOLLAR SIGN).
The TV of TemplateCharacter::\EscapeSequence is the SV of EscapeSequence.
The TV of TemplateCharacter::\NotEscapeSequence is undefined.
The TV of TemplateCharacter::LineContinuation is the TV of LineContinuation.
The TV of TemplateCharacter::LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TV of LineContinuation::\LineTerminatorSequence is the empty code unit sequence.
The TRV of NoSubstitutionTemplate::`TemplateCharacters` is the TRV of TemplateCharacters.
The TRV of TemplateHead::`TemplateCharacters${ is the TRV of TemplateCharacters.
The TRV of TemplateMiddle::}TemplateCharacters${ is the TRV of TemplateCharacters.
The TRV of TemplateTail::}TemplateCharacters` is the TRV of TemplateCharacters.
The TRV of TemplateCharacters::TemplateCharacter is the TRV of TemplateCharacter.
The TRV of TemplateCharacters::TemplateCharacterTemplateCharacters is a sequence consisting of the code units of the TRV of TemplateCharacter followed by the code units of the TRV of TemplateCharacters.
The TRV of TemplateCharacter::SourceCharacterbut not one of ` or \ or $ or LineTerminator is the UTF16Encoding of the code point value of SourceCharacter.
The TRV of TemplateCharacter::$ is the code unit 0x0024 (DOLLAR SIGN).
The TRV of TemplateCharacter::\EscapeSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of EscapeSequence.
The TRV of TemplateCharacter::\NotEscapeSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of NotEscapeSequence.
The TRV of TemplateCharacter::LineContinuation is the TRV of LineContinuation.
The TRV of TemplateCharacter::LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TRV of EscapeSequence::CharacterEscapeSequence is the TRV of the CharacterEscapeSequence.
The TRV of EscapeSequence::0 is the code unit 0x0030 (DIGIT ZERO).
The TRV of EscapeSequence::HexEscapeSequence is the TRV of the HexEscapeSequence.
The TRV of EscapeSequence::UnicodeEscapeSequence is the TRV of the UnicodeEscapeSequence.
The TRV of NotEscapeSequence::0DecimalDigit is the sequence consisting of the code unit 0x0030 (DIGIT ZERO) followed by the code units of the TRV of DecimalDigit.
The TRV of NotEscapeSequence::x[lookahead ∉ HexDigit] is the code unit 0x0078 (LATIN SMALL LETTER X).
The TRV of NotEscapeSequence::xHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by the code units of the TRV of HexDigit.
The TRV of NotEscapeSequence::u[lookahead ∉ HexDigit][lookahead ≠ {] is the code unit 0x0075 (LATIN SMALL LETTER U).
The TRV of NotEscapeSequence::uHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of HexDigit.
The TRV of NotEscapeSequence::uHexDigitHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first HexDigit followed by the code units of the TRV of the second HexDigit.
The TRV of NotEscapeSequence::uHexDigitHexDigitHexDigit[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first HexDigit followed by the code units of the TRV of the second HexDigit followed by the code units of the TRV of the third HexDigit.
The TRV of NotEscapeSequence::u{[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET).
The TRV of NotEscapeSequence::u{NotCodePoint[lookahead ∉ HexDigit] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of NotCodePoint.
The TRV of NotEscapeSequence::u{CodePoint[lookahead ∉ HexDigit][lookahead ≠ }] is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of CodePoint.
The TRV of DecimalDigit::one of0123456789 is the SV of the SourceCharacter that is that single code point.
The TRV of CharacterEscapeSequence::SingleEscapeCharacter is the TRV of the SingleEscapeCharacter.
The TRV of CharacterEscapeSequence::NonEscapeCharacter is the SV of the NonEscapeCharacter.
The TRV of SingleEscapeCharacter::one of'"\bfnrtv is the SV of the SourceCharacter that is that single code point.
The TRV of HexEscapeSequence::xHexDigitHexDigit is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by TRV of the first HexDigit followed by the TRV of the second HexDigit.
The TRV of UnicodeEscapeSequence::uHex4Digits is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by TRV of Hex4Digits.
The TRV of UnicodeEscapeSequence::u{CodePoint} is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by TRV of CodePoint followed by the code unit 0x007D (RIGHT CURLY BRACKET).
The TRV of Hex4Digits::HexDigitHexDigitHexDigitHexDigit is the sequence consisting of the TRV of the first HexDigit followed by the TRV of the second HexDigit followed by the TRV of the third HexDigit followed by the TRV of the fourth HexDigit.
The TRV of HexDigits::HexDigit is the TRV of HexDigit.
The TRV of HexDigits::HexDigitsHexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.
The TRV of a HexDigit is the SV of the SourceCharacter that is that HexDigit.
The TRV of LineContinuation::\LineTerminatorSequence is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of LineTerminatorSequence.
The TRV of LineTerminatorSequence::<LF> is the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence::<CR> is the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence::<LS> is the code unit 0x2028 (LINE SEPARATOR).
The TRV of LineTerminatorSequence::<PS> is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The TRV of LineTerminatorSequence::<CR><LF> is the sequence consisting of the code unit 0x000A (LINE FEED).

Note

TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.

11.9 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

11.9.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.

There are three basic rules of semicolon insertion:

When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
- The offending token is separated from the previous token by at least one LineTerminator.
- The offending token is }.
- The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (13.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 13.7.4).

Note

The following are the only restricted productions in the grammar:

[Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[no LineTerminator here]

[Yield, Await]

continue

;

continue

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

break

;

break

[no LineTerminator here]

[?Yield, ?Await]

;

[Yield, Await]

return

;

return

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[Yield, Await]

throw

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

[In, Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[In, Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.
An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.

11.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

Note 1

The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

Note 2

The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

12 ECMAScript Language: Expressions

12.1 Identifiers

Syntax

[Yield, Await]

[~Yield]

yield

[~Await]

await

[Yield, Await]

yield

await

[Yield, Await]

[~Yield]

yield

[~Await]

await

but not ReservedWord

Note

yield and await are permitted as BindingIdentifier in the grammar, and prohibited with static semantics below, to prohibit automatic semicolon insertion in cases such as

let
await 0;

12.1.1 Static Semantics: Early Errors

It is a Syntax Error if the code matched by this production is contained in strict mode code and the StringValue of Identifier is "arguments" or "eval".

yield

yield

yield

It is a Syntax Error if the code matched by this production is contained in strict mode code.

await

await

await

It is a Syntax Error if the goal symbol of the syntactic grammar is Module.

yield

It is a Syntax Error if this production has a _[Yield] parameter.

await

It is a Syntax Error if this production has an _[Await] parameter.

[Yield, Await]

[Yield, Await]

[Yield, Await]

It is a Syntax Error if this production has a _[Yield] parameter and StringValue of Identifier is "yield".
It is a Syntax Error if this production has an _[Await] parameter and StringValue of Identifier is "await".

but not ReservedWord

It is a Syntax Error if this phrase is contained in strict mode code and the StringValue of IdentifierName is: "implements", "interface", "let", "package", "private", "protected", "public", "static", or "yield".
It is a Syntax Error if the goal symbol of the syntactic grammar is Module and the StringValue of IdentifierName is "await".
It is a Syntax Error if StringValue of IdentifierName is the same String value as the StringValue of any ReservedWord except for yield or await.

Note

StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

12.1.2 Static Semantics: BoundNames

Return a new List containing the StringValue of Identifier.

yield

Return a new List containing "yield".

await

Return a new List containing "await".

12.1.3 Static Semantics: IsValidSimpleAssignmentTarget

If this IdentifierReference is contained in strict mode code and StringValue of Identifier is "eval" or "arguments", return false.
Return true.

yield

Return true.

await

Return true.

12.1.4 Static Semantics: StringValue

yield

yield

yield

Return "yield".

await

await

await

Return "await".

but not ReservedWord

Return the StringValue of IdentifierName.

12.1.5 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 9.2.15). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

Let name be StringValue of Identifier.
Return ? InitializeBoundName(name, value, environment).

yield

Return ? InitializeBoundName("yield", value, environment).

await

Return ? InitializeBoundName("await", value, environment).

12.1.5.1 Runtime Semantics: InitializeBoundName ( `name`, `value`, `environment` )

Assert: Type(name) is String.
If environment is not undefined, then
1. Let env be the EnvironmentRecord component of environment.
2. Perform env.InitializeBinding(name, value).
3. Return NormalCompletion(undefined).
Else,
1. Let lhs be ResolveBinding(name).
2. Return ? PutValue(lhs, value).

12.1.6 Runtime Semantics: Evaluation

Return ? ResolveBinding(StringValue of Identifier).

yield

Return ? ResolveBinding("yield").

await

Return ? ResolveBinding("await").

Note 1

The result of evaluating an IdentifierReference is always a value of type Reference.

Note 2

In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code.

12.2 Primary Expression

Syntax

[Yield, Await]

this

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ~Tagged]

[?Yield, ?Await]

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

)

(

)

(

...

[?Yield, ?Await]

)

(

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

(

[+In, ?Yield, ?Await]

...

[?Yield, ?Await]

)

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

[Yield, Await]

(

[+In, ?Yield, ?Await]

)

12.2.1 Semantics

12.2.1.1 Static Semantics: CoveredParenthesizedExpression

(

)

Return the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.

12.2.1.2 Static Semantics: HasName

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
If IsFunctionDefinition of expr is false, return false.
Return HasName of expr.

12.2.1.3 Static Semantics: IsFunctionDefinition

this

Return false.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return IsFunctionDefinition of expr.

12.2.1.4 Static Semantics: IsIdentifierRef

Return true.

this

Return false.

12.2.1.5 Static Semantics: IsValidSimpleAssignmentTarget

this

Return false.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return IsValidSimpleAssignmentTarget of expr.

12.2.2 The `this` Keyword

12.2.2.1 Runtime Semantics: Evaluation

this

Return ? ResolveThisBinding().

12.2.3 Identifier Reference

See 12.1 for IdentifierReference.

12.2.4 Literals

Syntax

12.2.4.1 Runtime Semantics: Evaluation

NullLiteral

Return null.

BooleanLiteral

If BooleanLiteral is the token false, return false.
If BooleanLiteral is the token true, return true.

NumericLiteral

Return the number whose value is MV of NumericLiteral as defined in 11.8.3.

Return the StringValue of StringLiteral as defined in 11.8.4.1.

12.2.5 Array Initializer

Note

An ArrayLiteral is an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

[Yield, Await]

[

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

]

[Yield, Await]

opt

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

opt

[+In, ?Yield, ?Await]

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

...

[+In, ?Yield, ?Await]

12.2.5.1 Static Semantics: ElisionWidth

Return the numeric value 1.

Let preceding be the ElisionWidth of Elision.
Return preceding+1.

12.2.5.2 Runtime Semantics: ArrayAccumulation

With parameters array and nextIndex.

opt

Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be ? GetValue(initResult).
Let created be CreateDataProperty(array, ToString(ToUint32(nextIndex+padding)), initValue).
Assert: created is true.
Return nextIndex+padding+1.

opt

SpreadElement

Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing ArrayAccumulation for SpreadElement with arguments array and nextIndex+padding.

opt

Let postIndex be the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(postIndex).
Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be ? GetValue(initResult).
Let created be CreateDataProperty(array, ToString(ToUint32(postIndex+padding)), initValue).
Assert: created is true.
Return postIndex+padding+1.

opt

SpreadElement

Let postIndex be the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(postIndex).
Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing ArrayAccumulation for SpreadElement with arguments array and postIndex+padding.

SpreadElement

...

Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return nextIndex.
3. Let nextValue be ? IteratorValue(next).
4. Let status be CreateDataProperty(array, ToString(ToUint32(nextIndex)), nextValue).
5. Assert: status is true.
6. Let nextIndex be nextIndex + 1.

Note

CreateDataProperty is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].

12.2.5.3 Runtime Semantics: Evaluation

[

opt

]

Let array be ! ArrayCreate(0).
Let pad be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Perform Set(array, "length", ToUint32(pad), false).
NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
Return array.

[

]

Let array be ! ArrayCreate(0).
Let len be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(len).
Perform Set(array, "length", ToUint32(len), false).
NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
Return array.

[

opt

]

Let array be ! ArrayCreate(0).
Let len be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(len).
Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
Perform Set(array, "length", ToUint32(padding+len), false).
NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
Return array.

12.2.6 Object Initializer

Note 1

An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

Syntax

ObjectLiteral

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[

[+In, ?Yield, ?Await]

]

CoverInitializedName

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

Note 2

MethodDefinition is defined in 14.3.

Note 3

In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.

12.2.6.1 Static Semantics: Early Errors

It is a Syntax Error if HasDirectSuper of MethodDefinition is true.

In addition to describing an actual object initializer the ObjectLiteral productions are also used as a cover grammar for ObjectAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

CoverInitializedName

Always throw a Syntax Error if code matches this production.

Note

This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.

12.2.6.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

Return false.

Return the result of ComputedPropertyName Contains symbol.

12.2.6.3 Static Semantics: Contains

With parameter symbol.

If symbol is MethodDefinition, return true.
Return the result of ComputedPropertyContains for MethodDefinition with argument symbol.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

12.2.6.4 Static Semantics: IsComputedPropertyKey

Return false.

Return true.

12.2.6.5 Static Semantics: PropName

Return StringValue of IdentifierReference.

...

Return empty.

Return PropName of PropertyName.

Return StringValue of IdentifierName.

Return the String value whose code units are the SV of the StringLiteral.

NumericLiteral

Let nbr be the result of forming the value of the NumericLiteral.
Return ! ToString(nbr).

[

]

Return empty.

12.2.6.6 Static Semantics: PropertyNameList

If PropName of PropertyDefinition is empty, return a new empty List.
Return a new List containing PropName of PropertyDefinition.

Let list be PropertyNameList of PropertyDefinitionList.
If PropName of PropertyDefinition is empty, return list.
Append PropName of PropertyDefinition to the end of list.
Return list.

12.2.6.7 Runtime Semantics: Evaluation

ObjectLiteral

{

}

Return ObjectCreate(%ObjectPrototype%).

ObjectLiteral

{

}

{

}

Let obj be ObjectCreate(%ObjectPrototype%).
Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with arguments obj and true.
Return obj.

Return StringValue of IdentifierName.

Return the String value whose code units are the SV of the StringLiteral.

NumericLiteral

Let nbr be the result of forming the value of the NumericLiteral.
Return ! ToString(nbr).

[

]

Let exprValue be the result of evaluating AssignmentExpression.
Let propName be ? GetValue(exprValue).
Return ? ToPropertyKey(propName).

12.2.6.8 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with arguments object and enumerable.
Return the result of performing PropertyDefinitionEvaluation of PropertyDefinition with arguments object and enumerable.

...

Let exprValue be the result of evaluating AssignmentExpression.
Let fromValue be ? GetValue(exprValue).
Let excludedNames be a new empty List.
Return ? CopyDataProperties(object, fromValue, excludedNames).

Let propName be StringValue of IdentifierReference.
Let exprValue be the result of evaluating IdentifierReference.
Let propValue be ? GetValue(exprValue).
Assert: enumerable is true.
Return CreateDataPropertyOrThrow(object, propName, propValue).

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
Let exprValueRef be the result of evaluating AssignmentExpression.
Let propValue be ? GetValue(exprValueRef).
If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. Let hasNameProperty be ? HasOwnProperty(propValue, "name").
2. If hasNameProperty is false, perform SetFunctionName(propValue, propKey).
Assert: enumerable is true.
Return CreateDataPropertyOrThrow(object, propKey, propValue).

Note

An alternative semantics for this production is given in B.3.1.

12.2.7 Function Defining Expressions

See 14.1 for PrimaryExpression:FunctionExpression .

See 14.4 for PrimaryExpression:GeneratorExpression .

See 14.6 for PrimaryExpression:ClassExpression .

See 14.7 for PrimaryExpression:AsyncFunctionExpression .

See 14.5 for PrimaryExpression:AsyncGeneratorExpression .

12.2.8 Regular Expression Literals

Syntax

See 11.8.5.

12.2.8.1 Static Semantics: Early Errors

It is a Syntax Error if BodyText of RegularExpressionLiteral cannot be recognized using the goal symbol Pattern of the ECMAScript RegExp grammar specified in 21.2.1.
It is a Syntax Error if FlagText of RegularExpressionLiteral contains any code points other than "g", "i", "m", "s", "u", or "y", or if it contains the same code point more than once.

12.2.8.2 Runtime Semantics: Evaluation

Let pattern be the String value consisting of the UTF16Encoding of each code point of BodyText of RegularExpressionLiteral.
Let flags be the String value consisting of the UTF16Encoding of each code point of FlagText of RegularExpressionLiteral.
Return RegExpCreate(pattern, flags).

12.2.9 Template Literals

Syntax

[Yield, Await, Tagged]

SubstitutionTemplate

[?Yield, ?Await, ?Tagged]

SubstitutionTemplate

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[?Yield, ?Await, ?Tagged]

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[+In, ?Yield, ?Await]

12.2.9.1 Static Semantics: Early Errors

[Yield, Await, Tagged]

It is a Syntax Error if the number of elements in the result of TemplateStrings of TemplateLiteral with argument false is greater than 2³²-1.
It is a Syntax Error if the _[Tagged] parameter was not set and NoSubstitutionTemplate Contains NotEscapeSequence.

SubstitutionTemplate

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateHead Contains NotEscapeSequence.

[Yield, Await, Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateTail Contains NotEscapeSequence.

[Yield, Await, Tagged]

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Tagged]

[+In, ?Yield, ?Await]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateMiddle Contains NotEscapeSequence.

12.2.9.2 Static Semantics: TemplateStrings

With parameter raw.

If raw is false, then
1. Let string be the TV of NoSubstitutionTemplate.
Else,
1. Let string be the TRV of NoSubstitutionTemplate.
Return a List containing the single element, string.

If raw is false, then
1. Let head be the TV of TemplateHead.
Else,
1. Let head be the TRV of TemplateHead.
Let tail be TemplateStrings of TemplateSpans with argument raw.
Return a List containing head followed by the elements, in order, of tail.

If raw is false, then
1. Let tail be the TV of TemplateTail.
Else,
1. Let tail be the TRV of TemplateTail.
Return a List containing the single element, tail.

Let middle be TemplateStrings of TemplateMiddleList with argument raw.
If raw is false, then
1. Let tail be the TV of TemplateTail.
Else,
1. Let tail be the TRV of TemplateTail.
Return a List containing the elements, in order, of middle followed by tail.

If raw is false, then
1. Let string be the TV of TemplateMiddle.
Else,
1. Let string be the TRV of TemplateMiddle.
Return a List containing the single element, string.

Let front be TemplateStrings of TemplateMiddleList with argument raw.
If raw is false, then
1. Let last be the TV of TemplateMiddle.
Else,
1. Let last be the TRV of TemplateMiddle.
Append last as the last element of the List front.
Return front.

12.2.9.3 Runtime Semantics: ArgumentListEvaluation

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Return a List containing the one element which is siteObj.

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Let firstSubRef be the result of evaluating Expression.
Let firstSub be ? GetValue(firstSubRef).
Let restSub be SubstitutionEvaluation of TemplateSpans.
ReturnIfAbrupt(restSub).
Assert: restSub is a List.
Return a List whose first element is siteObj, whose second elements is firstSub, and whose subsequent elements are the elements of restSub, in order. restSub may contain no elements.

12.2.9.4 Runtime Semantics: GetTemplateObject ( `templateLiteral` )

The abstract operation GetTemplateObject is called with a Parse Node, templateLiteral, as an argument. It performs the following steps:

Let rawStrings be TemplateStrings of templateLiteral with argument true.
Let realm be the current Realm Record.
Let templateRegistry be realm.[[TemplateMap]].
For each element e of templateRegistry, do
1. If e.[[Site]] is the same Parse Node as templateLiteral, then
  1. Return e.[[Array]].
Let cookedStrings be TemplateStrings of templateLiteral with argument false.
Let count be the number of elements in the List cookedStrings.
Assert: count ≤ 2³²-1.
Let template be ! ArrayCreate(count).
Let rawObj be ! ArrayCreate(count).
Let index be 0.
Repeat, while index < count
1. Let prop be ! ToString(index).
2. Let cookedValue be the String value cookedStrings[index].
3. Call template.[[DefineOwnProperty]](prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
4. Let rawValue be the String value rawStrings[index].
5. Call rawObj.[[DefineOwnProperty]](prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
6. Let index be index+1.
Perform SetIntegrityLevel(rawObj, "frozen").
Call template.[[DefineOwnProperty]]("raw", PropertyDescriptor { [[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetIntegrityLevel(template, "frozen").
Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to templateRegistry.
Return template.

Note 1

The creation of a template object cannot result in an abrupt completion.

Note 2

Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.9.6). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

Note 3

Future editions of this specification may define additional non-enumerable properties of template objects.

12.2.9.5 Runtime Semantics: SubstitutionEvaluation

Return a new empty List.

Return the result of SubstitutionEvaluation of TemplateMiddleList.

Let subRef be the result of evaluating Expression.
Let sub be ? GetValue(subRef).
Return a List containing only sub.

Let preceding be the result of SubstitutionEvaluation of TemplateMiddleList.
ReturnIfAbrupt(preceding).
Let nextRef be the result of evaluating Expression.
Let next be ? GetValue(nextRef).
Append next as the last element of the List preceding.
Return preceding.

12.2.9.6 Runtime Semantics: Evaluation

Return the String value whose code units are the elements of the TV of NoSubstitutionTemplate as defined in 11.8.6.

Let head be the TV of TemplateHead as defined in 11.8.6.
Let sub be the result of evaluating Expression.
ReturnIfAbrupt(sub).
Let middle be ? ToString(sub).
Let tail be the result of evaluating TemplateSpans.
ReturnIfAbrupt(tail).
Return the string-concatenation of head, middle, and tail.

Note 1

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

Let tail be the TV of TemplateTail as defined in 11.8.6.
Return the String value consisting of the code units of tail.

Let head be the result of evaluating TemplateMiddleList.
ReturnIfAbrupt(head).
Let tail be the TV of TemplateTail as defined in 11.8.6.
Return the string-concatenation of head and tail.

Let head be the TV of TemplateMiddle as defined in 11.8.6.
Let sub be the result of evaluating Expression.
ReturnIfAbrupt(sub).
Let middle be ? ToString(sub).
Return the sequence of code units consisting of the code units of head followed by the elements of middle.

Note 2

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

Let rest be the result of evaluating TemplateMiddleList.
ReturnIfAbrupt(rest).
Let middle be the TV of TemplateMiddle as defined in 11.8.6.
Let sub be the result of evaluating Expression.
ReturnIfAbrupt(sub).
Let last be ? ToString(sub).
Return the sequence of code units consisting of the elements of rest followed by the code units of middle followed by the elements of last.

Note 3

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

12.2.10 The Grouping Operator

12.2.10.1 Static Semantics: Early Errors

It is a Syntax Error if CoverParenthesizedExpressionAndArrowParameterList is not covering a ParenthesizedExpression.
All Early Error rules for ParenthesizedExpression and its derived productions also apply to CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.

12.2.10.2 Static Semantics: IsFunctionDefinition

(

)

Return IsFunctionDefinition of Expression.

12.2.10.3 Static Semantics: IsValidSimpleAssignmentTarget

(

)

Return IsValidSimpleAssignmentTarget of Expression.

12.2.10.4 Runtime Semantics: Evaluation

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return the result of evaluating expr.

(

)

Return the result of evaluating Expression. This may be of type Reference.

Note

This algorithm does not apply GetValue to the result of evaluating Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

12.3 Left-Hand-Side Expressions

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

new

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

super

[

[+In, ?Yield, ?Await]

]

super

MetaProperty

NewTarget

new

target

[Yield, Await]

[?Yield, ?Await]

new

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

SuperCall

[Yield, Await]

super

[?Yield, ?Await]

[Yield, Await]

(

)

(

[?Yield, ?Await]

)

(

[?Yield, ?Await]

)

[Yield, Await]

[+In, ?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

...

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production CallExpression:CoverCallExpressionAndAsyncArrowHead the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

CallMemberExpression

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.3.1 Static Semantics

12.3.1.1 Static Semantics: CoveredCallExpression

Return the CallMemberExpression that is covered by CoverCallExpressionAndAsyncArrowHead.

12.3.1.2 Static Semantics: Contains

With parameter symbol.

If MemberExpression Contains symbol is true, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

SuperProperty

super

If symbol is the ReservedWord super, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

If CallExpression Contains symbol is true, return true.
If symbol is a ReservedWord, return false.
If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true.
Return false.

12.3.1.3 Static Semantics: IsFunctionDefinition

[

]

new

new

Return false.

12.3.1.4 Static Semantics: IsDestructuring

If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral, return true.
Return false.

[

]

new

new

Return false.

12.3.1.5 Static Semantics: IsIdentifierRef

[

]

new

new

Return false.

12.3.1.6 Static Semantics: IsValidSimpleAssignmentTarget

[

]

[

]

Return true.

new

new

new

target

Return false.

12.3.2 Property Accessors

Note

Properties are accessed by name, using either the dot notation:

.

.

or the bracket notation:

[

]

[

]

The dot notation is explained by the following syntactic conversion:

.

is identical in its behaviour to

[ <identifier-name-string> ]

and similarly

.

is identical in its behaviour to

[ <identifier-name-string> ]

where <identifier-name-string> is the result of evaluating StringValue of IdentifierName.

12.3.2.1 Runtime Semantics: Evaluation

[

]

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be ? GetValue(baseReference).
Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let bv be ? RequireObjectCoercible(baseValue).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
If the code matched by this MemberExpression is strict mode code, let strict be true, else let strict be false.
Return a value of type Reference whose base value component is bv, whose referenced name component is propertyKey, and whose strict reference flag is strict.

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be ? GetValue(baseReference).
Let bv be ? RequireObjectCoercible(baseValue).
Let propertyNameString be StringValue of IdentifierName.
If the code matched by this MemberExpression is strict mode code, let strict be true, else let strict be false.
Return a value of type Reference whose base value component is bv, whose referenced name component is propertyNameString, and whose strict reference flag is strict.

[

]

Is evaluated in exactly the same manner as MemberExpression:MemberExpression[Expression] except that the contained CallExpression is evaluated in step 1.

Is evaluated in exactly the same manner as MemberExpression:MemberExpression.IdentifierName except that the contained CallExpression is evaluated in step 1.

12.3.3 The `new` Operator

12.3.3.1 Runtime Semantics: Evaluation

new

Return ? EvaluateNew(NewExpression, empty).

new

Return ? EvaluateNew(MemberExpression, Arguments).

12.3.3.1.1 Runtime Semantics: EvaluateNew ( `constructExpr`, `arguments` )

The abstract operation EvaluateNew with arguments constructExpr, and arguments performs the following steps:

Assert: constructExpr is either a NewExpression or a MemberExpression.
Assert: arguments is either empty or an Arguments.
Let ref be the result of evaluating constructExpr.
Let constructor be ? GetValue(ref).
If arguments is empty, let argList be a new empty List.
Else,
1. Let argList be ArgumentListEvaluation of arguments.
2. ReturnIfAbrupt(argList).
If IsConstructor(constructor) is false, throw a TypeError exception.
Return ? Construct(constructor, argList).

12.3.4 Function Calls

12.3.4.1 Runtime Semantics: Evaluation

Let expr be CoveredCallExpression of CoverCallExpressionAndAsyncArrowHead.
Let memberExpr be the MemberExpression of expr.
Let arguments be the Arguments of expr.
Let ref be the result of evaluating memberExpr.
Let func be ? GetValue(ref).
If Type(ref) is Reference and IsPropertyReference(ref) is false and GetReferencedName(ref) is "eval", then
1. If SameValue(func, %eval%) is true, then
  1. Let argList be ? ArgumentListEvaluation of arguments.
  2. If argList has no elements, return undefined.
  3. Let evalText be the first element of argList.
  4. If the source code matching this CallExpression is strict mode code, let strictCaller be true. Otherwise let strictCaller be false.
  5. Let evalRealm be the current Realm Record.
  6. Perform ? HostEnsureCanCompileStrings(evalRealm, evalRealm).
  7. Return ? PerformEval(evalText, evalRealm, strictCaller, true).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, arguments, tailCall).

A CallExpression evaluation that executes step 6.a.vii is a direct eval.

Let ref be the result of evaluating CallExpression.
Let func be ? GetValue(ref).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, Arguments, tailCall).

12.3.4.2 Runtime Semantics: EvaluateCall ( `func`, `ref`, `arguments`, `tailPosition` )

The abstract operation EvaluateCall takes as arguments a value func, a value ref, a Parse Node arguments, and a Boolean argument tailPosition. It performs the following steps:

If Type(ref) is Reference, then
1. If IsPropertyReference(ref) is true, then
  1. Let thisValue be GetThisValue(ref).
2. Else the base of ref is an Environment Record,
  1. Let refEnv be GetBase(ref).
  2. Let thisValue be refEnv.WithBaseObject().
Else Type(ref) is not Reference,
1. Let thisValue be undefined.
Let argList be ArgumentListEvaluation of arguments.
ReturnIfAbrupt(argList).
If Type(func) is not Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
If tailPosition is true, perform PrepareForTailCall().
Let result be Call(func, thisValue, argList).
Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
Assert: If result is not an abrupt completion, then Type(result) is an ECMAScript language type.
Return result.

12.3.5 The `super` Keyword

12.3.5.1 Runtime Semantics: Evaluation

SuperProperty

super

[

]

Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
If the code matched by this SuperProperty is strict mode code, let strict be true, else let strict be false.
Return ? MakeSuperPropertyReference(propertyKey, strict).

SuperProperty

super

Let propertyKey be StringValue of IdentifierName.
If the code matched by this SuperProperty is strict mode code, let strict be true, else let strict be false.
Return ? MakeSuperPropertyReference(propertyKey, strict).

SuperCall

super

Let newTarget be GetNewTarget().
Assert: Type(newTarget) is Object.
Let func be ? GetSuperConstructor().
Let argList be ArgumentListEvaluation of Arguments.
ReturnIfAbrupt(argList).
Let result be ? Construct(func, argList, newTarget).
Let thisER be GetThisEnvironment().
Return ? thisER.BindThisValue(result).

12.3.5.2 Runtime Semantics: GetSuperConstructor ( )

The abstract operation GetSuperConstructor performs the following steps:

Let envRec be GetThisEnvironment().
Assert: envRec is a function Environment Record.
Let activeFunction be envRec.[[FunctionObject]].
Assert: activeFunction is an ECMAScript function object.
Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
If IsConstructor(superConstructor) is false, throw a TypeError exception.
Return superConstructor.

12.3.5.3 Runtime Semantics: MakeSuperPropertyReference ( `propertyKey`, `strict` )

The abstract operation MakeSuperPropertyReference with arguments propertyKey and strict performs the following steps:

Let env be GetThisEnvironment().
Assert: env.HasSuperBinding() is true.
Let actualThis be ? env.GetThisBinding().
Let baseValue be ? env.GetSuperBase().
Let bv be ? RequireObjectCoercible(baseValue).
Return a value of type Reference that is a Super Reference whose base value component is bv, whose referenced name component is propertyKey, whose thisValue component is actualThis, and whose strict reference flag is strict.

12.3.6 Argument Lists

Note

The evaluation of an argument list produces a List of values.

12.3.6.1 Runtime Semantics: ArgumentListEvaluation

(

)

Return a new empty List.

Let ref be the result of evaluating AssignmentExpression.
Let arg be ? GetValue(ref).
Return a List whose sole item is arg.

...

Let list be a new empty List.
Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return list.
3. Let nextArg be ? IteratorValue(next).
4. Append nextArg as the last element of list.

Let precedingArgs be ArgumentListEvaluation of ArgumentList.
ReturnIfAbrupt(precedingArgs).
Let ref be the result of evaluating AssignmentExpression.
Let arg be ? GetValue(ref).
Append arg to the end of precedingArgs.
Return precedingArgs.

...

Let precedingArgs be ArgumentListEvaluation of ArgumentList.
ReturnIfAbrupt(precedingArgs).
Let spreadRef be the result of evaluating AssignmentExpression.
Let iteratorRecord be ? GetIterator(? GetValue(spreadRef)).
Repeat,
1. Let next be ? IteratorStep(iteratorRecord).
2. If next is false, return precedingArgs.
3. Let nextArg be ? IteratorValue(next).
4. Append nextArg as the last element of precedingArgs.

12.3.7 Tagged Templates

Note

A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (12.2.9). The actual arguments include a template object (12.2.9.4) and the values produced by evaluating the expressions embedded within the TemplateLiteral.

12.3.7.1 Runtime Semantics: Evaluation

Let tagRef be the result of evaluating MemberExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this MemberExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

Let tagRef be the result of evaluating CallExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

12.3.8 Meta Properties

12.3.8.1 Runtime Semantics: Evaluation

NewTarget

new

target

Return GetNewTarget().

12.4 Update Expressions

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[no LineTerminator here]

[?Yield, ?Await]

[?Yield, ?Await]

12.4.1 Static Semantics: Early Errors

It is an early Reference Error if IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

It is an early Reference Error if IsValidSimpleAssignmentTarget of UnaryExpression is false.

12.4.2 Static Semantics: IsFunctionDefinition

Return false.

12.4.3 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.4.4 Postfix Increment Operator

12.4.4.1 Runtime Semantics: Evaluation

Let lhs be the result of evaluating LeftHandSideExpression.
Let oldValue be ? ToNumber(? GetValue(lhs)).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 12.8.5).
Perform ? PutValue(lhs, newValue).
Return oldValue.

12.4.5 Postfix Decrement Operator

12.4.5.1 Runtime Semantics: Evaluation

Let lhs be the result of evaluating LeftHandSideExpression.
Let oldValue be ? ToNumber(? GetValue(lhs)).
Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the - operator (see 12.8.5).
Perform ? PutValue(lhs, newValue).
Return oldValue.

12.4.6 Prefix Increment Operator

12.4.6.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumber(? GetValue(expr)).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 12.8.5).
Perform ? PutValue(expr, newValue).
Return newValue.

12.4.7 Prefix Decrement Operator

12.4.7.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumber(? GetValue(expr)).
Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the - operator (see 12.8.5).
Perform ? PutValue(expr, newValue).
Return newValue.

12.5 Unary Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

delete

[?Yield, ?Await]

void

[?Yield, ?Await]

typeof

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+Await]

AwaitExpression

[?Yield]

12.5.1 Static Semantics: IsFunctionDefinition

delete

void

typeof

Return false.

12.5.2 Static Semantics: IsValidSimpleAssignmentTarget

delete

void

typeof

Return false.

12.5.3 The `delete` Operator

12.5.3.1 Static Semantics: Early Errors

delete

It is a Syntax Error if the UnaryExpression is contained in strict mode code and the derived UnaryExpression is PrimaryExpression:IdentifierReference .
It is a Syntax Error if the derived UnaryExpression is
PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
and CoverParenthesizedExpressionAndArrowParameterList ultimately derives a phrase that, if used in place of UnaryExpression, would produce a Syntax Error according to these rules. This rule is recursively applied.

Note

The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.

12.5.3.2 Runtime Semantics: Evaluation

delete

Let ref be the result of evaluating UnaryExpression.
ReturnIfAbrupt(ref).
If Type(ref) is not Reference, return true.
If IsUnresolvableReference(ref) is true, then
1. Assert: IsStrictReference(ref) is false.
2. Return true.
If IsPropertyReference(ref) is true, then
1. If IsSuperReference(ref) is true, throw a ReferenceError exception.
2. Let baseObj be ! ToObject(GetBase(ref)).
3. Let deleteStatus be ? baseObj.[[Delete]](GetReferencedName(ref)).
4. If deleteStatus is false and IsStrictReference(ref) is true, throw a TypeError exception.
5. Return deleteStatus.
Else ref is a Reference to an Environment Record binding,
1. Let bindings be GetBase(ref).
2. Return ? bindings.DeleteBinding(GetReferencedName(ref)).

Note

When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.

12.5.4 The `void` Operator

12.5.4.1 Runtime Semantics: Evaluation

void

Let expr be the result of evaluating UnaryExpression.
Perform ? GetValue(expr).
Return undefined.

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

12.5.5 The `typeof` Operator

12.5.5.1 Runtime Semantics: Evaluation

typeof

Let val be the result of evaluating UnaryExpression.
If Type(val) is Reference, then
1. If IsUnresolvableReference(val) is true, return "undefined".
Set val to ? GetValue(val).
Return a String according to Table 35.

Table 35: typeof Operator Results

Type of `val`	Result
Undefined	`"undefined"`
Null	`"object"`
Boolean	`"boolean"`
Number	`"number"`
String	`"string"`
Symbol	`"symbol"`
Object (ordinary and does not implement [[Call]])	`"object"`
Object (standard exotic and does not implement [[Call]])	`"object"`
Object (implements [[Call]])	`"function"`
Object (non-standard exotic and does not implement [[Call]])	Implementation-defined. Must not be `"undefined"`, `"boolean"`, `"function"`, `"number"`, `"symbol"`, or `"string"`.

Note

Implementations are discouraged from defining new typeof result values for non-standard exotic objects. If possible "object" should be used for such objects.

12.5.6 Unary `+` Operator

Note

The unary + operator converts its operand to Number type.

12.5.6.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Return ? ToNumber(? GetValue(expr)).

12.5.7 Unary `-` Operator

Note

The unary - operator converts its operand to Number type and then negates it. Negating +0 produces -0, and negating -0 produces +0.

12.5.7.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToNumber(? GetValue(expr)).
If oldValue is NaN, return NaN.
Return the result of negating oldValue; that is, compute a Number with the same magnitude but opposite sign.

12.5.8 Bitwise NOT Operator ( `~` )

12.5.8.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ? ToInt32(? GetValue(expr)).
Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit integer.

12.5.9 Logical NOT Operator ( `!` )

12.5.9.1 Runtime Semantics: Evaluation

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToBoolean(? GetValue(expr)).
If oldValue is true, return false.
Return true.

12.6 Exponentiation Operator

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.6.1 Static Semantics: IsFunctionDefinition

Return false.

12.6.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.6.3 Runtime Semantics: Evaluation

Let left be the result of evaluating UpdateExpression.
Let leftValue be ? GetValue(left).
Let right be the result of evaluating ExponentiationExpression.
Let rightValue be ? GetValue(right).
Let base be ? ToNumber(leftValue).
Let exponent be ? ToNumber(rightValue).
Return the result of Applying the ** operator with base and exponent as specified in 12.6.4.

12.6.4 Applying the `**` Operator

Returns an implementation-dependent approximation of the result of raising base to the power exponent.

If exponent is NaN, the result is NaN.
If exponent is +0, the result is 1, even if base is NaN.
If exponent is -0, the result is 1, even if base is NaN.
If base is NaN and exponent is nonzero, the result is NaN.
If abs(base) > 1 and exponent is +∞, the result is +∞.
If abs(base) > 1 and exponent is -∞, the result is +0.
If abs(base) is 1 and exponent is +∞, the result is NaN.
If abs(base) is 1 and exponent is -∞, the result is NaN.
If abs(base) < 1 and exponent is +∞, the result is +0.
If abs(base) < 1 and exponent is -∞, the result is +∞.
If base is +∞ and exponent > 0, the result is +∞.
If base is +∞ and exponent < 0, the result is +0.
If base is -∞ and exponent > 0 and exponent is an odd integer, the result is -∞.
If base is -∞ and exponent > 0 and exponent is not an odd integer, the result is +∞.
If base is -∞ and exponent < 0 and exponent is an odd integer, the result is -0.
If base is -∞ and exponent < 0 and exponent is not an odd integer, the result is +0.
If base is +0 and exponent > 0, the result is +0.
If base is +0 and exponent < 0, the result is +∞.
If base is -0 and exponent > 0 and exponent is an odd integer, the result is -0.
If base is -0 and exponent > 0 and exponent is not an odd integer, the result is +0.
If base is -0 and exponent < 0 and exponent is an odd integer, the result is -∞.
If base is -0 and exponent < 0 and exponent is not an odd integer, the result is +∞.
If base < 0 and base is finite and exponent is finite and exponent is not an integer, the result is NaN.

Note

The result of base ** exponent when base is 1 or -1 and exponent is +Infinity or -Infinity differs from IEEE 754-2008. The first edition of ECMAScript specified a result of NaN for this operation, whereas later versions of IEEE 754-2008 specified 1. The historical ECMAScript behaviour is preserved for compatibility reasons.

12.7 Multiplicative Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

one of

12.7.1 Static Semantics: IsFunctionDefinition

Return false.

12.7.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.7.3 Runtime Semantics: Evaluation

Let left be the result of evaluating MultiplicativeExpression.
Let leftValue be ? GetValue(left).
Let right be the result of evaluating ExponentiationExpression.
Let rightValue be ? GetValue(right).
Let lnum be ? ToNumber(leftValue).
Let rnum be ? ToNumber(rightValue).
Return the result of applying the MultiplicativeOperator (*, /, or %) to lnum and rnum as specified in 12.7.3.1, 12.7.3.2, or 12.7.3.3.

12.7.3.1 Applying the `*` Operator

The * MultiplicativeOperator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754-2008 binary double-precision arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Multiplication of an infinity by a zero results in NaN.
Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.
Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity nor NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.

12.7.3.2 Applying the `/` Operator

The / MultiplicativeOperator performs division, producing the quotient of its operands. The left operand is the dividend and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754-2008 arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in NaN.
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.
Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.

12.7.3.3 Applying the `%` Operator

The % MultiplicativeOperator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.

Note

In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2008. The IEEE 754-2008 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

If either operand is NaN, the result is NaN.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n - (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode.

12.8 Additive Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

12.8.1 Static Semantics: IsFunctionDefinition

Return false.

12.8.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.8.3 The Addition Operator ( `+` )

Note

The addition operator either performs string concatenation or numeric addition.

12.8.3.1 Runtime Semantics: Evaluation

Let lref be the result of evaluating AdditiveExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be ? GetValue(rref).
Let lprim be ? ToPrimitive(lval).
Let rprim be ? ToPrimitive(rval).
If Type(lprim) is String or Type(rprim) is String, then
1. Let lstr be ? ToString(lprim).
2. Let rstr be ? ToString(rprim).
3. Return the string-concatenation of lstr and rstr.
Let lnum be ? ToNumber(lprim).
Let rnum be ? ToNumber(rprim).
Return the result of applying the addition operation to lnum and rnum. See the Note below 12.8.5.

Note 1

No hint is provided in the calls to ToPrimitive in steps 5 and 6. All standard objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Exotic objects may handle the absence of a hint in some other manner.

Note 2

Step 7 differs from step 3 of the Abstract Relational Comparison algorithm, by using the logical-or operation instead of the logical-and operation.

12.8.4 The Subtraction Operator ( `-` )

12.8.4.1 Runtime Semantics: Evaluation

Let lref be the result of evaluating AdditiveExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be ? GetValue(rref).
Let lnum be ? ToNumber(lval).
Let rnum be ? ToNumber(rval).
Return the result of applying the subtraction operation to lnum and rnum. See the note below 12.8.5.

12.8.5 Applying the Additive Operators to Numbers

The + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The - operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754-2008 binary double-precision arithmetic:

If either operand is NaN, the result is NaN.
The sum of two infinities of opposite sign is NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is -0. The sum of two positive zeroes, or of two zeroes of opposite sign, is +0.
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is +0.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.

Note

The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that a-b produces the same result as a+(-b).

12.9 Bitwise Shift Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

>>>

[?Yield, ?Await]

12.9.1 Static Semantics: IsFunctionDefinition

>>>

Return false.

12.9.2 Static Semantics: IsValidSimpleAssignmentTarget

>>>

Return false.

12.9.3 The Left Shift Operator ( `<<` )

Note

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

12.9.3.1 Runtime Semantics: Evaluation

Let lref be the result of evaluating ShiftExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be ? GetValue(rref).
Let lnum be ? ToInt32(lval).
Let rnum be ? ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit integer.

12.9.4 The Signed Right Shift Operator ( `>>` )

Note

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

12.9.4.1 Runtime Semantics: Evaluation

Let lref be the result of evaluating ShiftExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be ? GetValue(rref).
Let lnum be ? ToInt32(lval).
Let rnum be ? ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.

12.9.5 The Unsigned Right Shift Operator ( `>>>` )

Note

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

12.9.5.1 Runtime Semantics: Evaluation

>>>

Let lref be the result of evaluating ShiftExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be ? GetValue(rref).
Let lnum be ? ToUint32(lval).
Let rnum be ? ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.

12.10 Relational Operators

Note 1

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

instanceof

[?Yield, ?Await]

[+In]

[+In, ?Yield, ?Await]

[?Yield, ?Await]

Note 2

The _[In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

12.10.1 Static Semantics: IsFunctionDefinition

instanceof

Return false.

12.10.2 Static Semantics: IsValidSimpleAssignmentTarget

instanceof

Return false.

12.10.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison lval < rval.
ReturnIfAbrupt(r).
If r is undefined, return false. Otherwise, return r.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison rval < lval with LeftFirst equal to false.
ReturnIfAbrupt(r).
If r is undefined, return false. Otherwise, return r.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison rval < lval with LeftFirst equal to false.
ReturnIfAbrupt(r).
If r is true or undefined, return false. Otherwise, return true.

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Relational Comparison lval < rval.
ReturnIfAbrupt(r).
If r is true or undefined, return false. Otherwise, return true.

instanceof

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
Return ? InstanceofOperator(lval, rval).

Let lref be the result of evaluating RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be ? GetValue(rref).
If Type(rval) is not Object, throw a TypeError exception.
Return ? HasProperty(rval, ToPropertyKey(lval)).

12.10.4 Runtime Semantics: InstanceofOperator ( `V`, `target` )

The abstract operation InstanceofOperator(V, target) implements the generic algorithm for determining if ECMAScript value V is an instance of object target either by consulting target's @@hasinstance method or, if absent, determining whether the value of target's prototype property is present in V's prototype chain. This abstract operation performs the following steps:

If Type(target) is not Object, throw a TypeError exception.
Let instOfHandler be ? GetMethod(target, @@hasInstance).
If instOfHandler is not undefined, then
1. Return ToBoolean(? Call(instOfHandler, target, « V »)).
If IsCallable(target) is false, throw a TypeError exception.
Return ? OrdinaryHasInstance(target, V).

Note

Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the default instanceof semantics.

12.11 Equality Operators

Note

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

===

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

!==

[?In, ?Yield, ?Await]

12.11.1 Static Semantics: IsFunctionDefinition

===

!==

Return false.

12.11.2 Static Semantics: IsValidSimpleAssignmentTarget

===

!==

Return false.

12.11.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Return the result of performing Abstract Equality Comparison rval == lval.

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Abstract Equality Comparison rval == lval.
If r is true, return false. Otherwise, return true.

===

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Return the result of performing Strict Equality Comparison rval === lval.

!==

Let lref be the result of evaluating EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be ? GetValue(rref).
Let r be the result of performing Strict Equality Comparison rval === lval.
If r is true, return false. Otherwise, return true.

Note 1

Given the above definition of equality:

String comparison can be forced by: "" + a == "" + b.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.

Note 2

The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.

Note 3

The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:

new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.

Note 4

Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.

12.12 Binary Bitwise Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

12.12.1 Static Semantics: IsFunctionDefinition

Return false.

12.12.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.12.3 Runtime Semantics: Evaluation

The production A:A@B , where @ is one of the bitwise operators in the productions above, is evaluated as follows:

Let lref be the result of evaluating A.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating B.
Let rval be ? GetValue(rref).
Let lnum be ? ToInt32(lval).
Let rnum be ? ToInt32(rval).
Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32-bit integer.

12.13 Binary Logical Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

Note

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

12.13.1 Static Semantics: IsFunctionDefinition

Return false.

12.13.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.13.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating LogicalANDExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is false, return lval.
Let rref be the result of evaluating BitwiseORExpression.
Return ? GetValue(rref).

Let lref be the result of evaluating LogicalORExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is true, return lval.
Let rref be the result of evaluating LogicalANDExpression.
Return ? GetValue(rref).

12.14 Conditional Operator ( `? :` )

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

Note

The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

12.14.1 Static Semantics: IsFunctionDefinition

Return false.

12.14.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.14.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating LogicalORExpression.
Let lval be ToBoolean(? GetValue(lref)).
If lval is true, then
1. Let trueRef be the result of evaluating the first AssignmentExpression.
2. Return ? GetValue(trueRef).
Else,
1. Let falseRef be the result of evaluating the second AssignmentExpression.
2. Return ? GetValue(falseRef).

12.15 Assignment Operators

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[+Yield]

[?In, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

one of

<<=

>>=

>>>=

**=

12.15.1 Static Semantics: Early Errors

It is a Syntax Error if LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and LeftHandSideExpression is not covering an AssignmentPattern.
It is an early Reference Error if LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral and IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

It is an early Reference Error if IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

12.15.2 Static Semantics: IsFunctionDefinition

Return true.

Return false.

12.15.3 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.15.4 Runtime Semantics: Evaluation

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating LeftHandSideExpression.
2. ReturnIfAbrupt(lref).
3. Let rref be the result of evaluating AssignmentExpression.
4. Let rval be ? GetValue(rref).
5. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
  1. Let hasNameProperty be ? HasOwnProperty(rval, "name").
  2. If hasNameProperty is false, perform SetFunctionName(rval, GetReferencedName(lref)).
6. Perform ? PutValue(lref, rval).
7. Return rval.
Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
Let rref be the result of evaluating AssignmentExpression.
Let rval be ? GetValue(rref).
Perform ? DestructuringAssignmentEvaluation of assignmentPattern using rval as the argument.
Return rval.

Let lref be the result of evaluating LeftHandSideExpression.
Let lval be ? GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Let rval be ? GetValue(rref).
Let op be the @ where AssignmentOperator is @=.
Let r be the result of applying op to lval and rval as if evaluating the expression lval op rval.
Perform ? PutValue(lref, r).
Return r.

Note

When an assignment occurs within strict mode code, it is a runtime error if lref in step 1.f of the first algorithm or step 7 of the second algorithm it is an unresolvable reference. If it is, a ReferenceError exception is thrown. The LeftHandSideExpression also may not be a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, nor to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.

12.15.5 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing an instance of the production AssignmentExpression:LeftHandSideExpression=AssignmentExpression the following grammar is used to refine the interpretation of LeftHandSideExpression.

AssignmentPattern

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

12.15.5.1 Static Semantics: Early Errors

opt

It is a Syntax Error if IsValidSimpleAssignmentTarget of IdentifierReference is false.

...

It is a Syntax Error if DestructuringAssignmentTarget is an ArrayLiteral or an ObjectLiteral.

It is a Syntax Error if LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if LeftHandSideExpression is not covering an AssignmentPattern.
It is a Syntax Error if LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral and IsValidSimpleAssignmentTarget(LeftHandSideExpression) is false.

12.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation

With parameter value.

{

}

Perform ? RequireObjectCoercible(value).
Return NormalCompletion(empty).

{

}

{

}

Perform ? RequireObjectCoercible(value).
Perform ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
Return NormalCompletion(empty).

[

]

Let iteratorRecord be ? GetIterator(value).
Return ? IteratorClose(iteratorRecord, NormalCompletion(empty)).

[

]

Let iteratorRecord be ? GetIterator(value).
Let result be the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

opt

]

Let iteratorRecord be ? GetIterator(value).
If Elision is present, then
1. Let status be the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
2. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return Completion(status).
Let result be the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

]

Let iteratorRecord be ? GetIterator(value).
Let result be the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElementList using iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

[

opt

opt

]

Let iteratorRecord be ? GetIterator(value).
Let status be the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElementList using iteratorRecord as the argument.
If status is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
2. Return Completion(status).
If Elision is present, then
1. Set status to the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
2. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return Completion(status).
If AssignmentRestElement is present, then
1. Set status to the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
Return Completion(status).

{

}

Let excludedNames be a new empty List.
Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with value and excludedNames as the arguments.

{

}

Let excludedNames be the result of performing ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with value and excludedNames as the arguments.

12.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation

With parameter value.

Note

The following operations collect a list of all destructured property names.

Let propertyNames be the result of performing ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
Let nextNames be the result of performing ? PropertyDestructuringAssignmentEvaluation for AssignmentProperty using value as the argument.
Append each item in nextNames to the end of propertyNames.
Return propertyNames.

opt

Let P be StringValue of IdentifierReference.
Let lref be ? ResolveBinding(P).
Let v be ? GetV(value, P).
If Initializeropt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
3. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Let hasNameProperty be ? HasOwnProperty(v, "name").
  2. If hasNameProperty is false, perform SetFunctionName(v, P).
Perform ? PutValue(lref, v).
Return a new List containing P.

Let name be the result of evaluating PropertyName.
ReturnIfAbrupt(name).
Perform ? KeyedDestructuringAssignmentEvaluation of AssignmentElement with value and name as the arguments.
Return a new List containing name.

12.15.5.4 Runtime Semantics: RestDestructuringAssignmentEvaluation

With parameters value and excludedNames.

...

Let lref be the result of evaluating DestructuringAssignmentTarget.
ReturnIfAbrupt(lref).
Let restObj be ObjectCreate(%ObjectPrototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
Return PutValue(lref, restObj).

12.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation

With parameter iteratorRecord.

Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement using iteratorRecord as the argument.

Perform ? IteratorDestructuringAssignmentEvaluation of AssignmentElementList using iteratorRecord as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement using iteratorRecord as the argument.

Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElement with iteratorRecord as the argument.

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of AssignmentElement with iteratorRecord as the argument.

If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return NormalCompletion(empty).

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return NormalCompletion(empty).

opt

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let value be IteratorValue(next).
  2. If value is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(value).
If iteratorRecord.[[Done]] is true, let value be undefined.
If Initializer is present and value is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Let v be ? GetValue(defaultValue).
Else, let v be value.
If DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral, then
1. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return the result of performing DestructuringAssignmentEvaluation of nestedAssignmentPattern with v as the argument.
If Initializer is present and value is undefined and IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
1. Let hasNameProperty be ? HasOwnProperty(v, "name").
2. If hasNameProperty is false, perform SetFunctionName(v, GetReferencedName(lref)).
Return ? PutValue(lref, v).

Note

Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.

...

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
Let A be ! ArrayCreate(0).
Let n be 0.
Repeat, while iteratorRecord.[[Done]] is false,
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let nextValue be IteratorValue(next).
  2. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(nextValue).
  4. Let status be CreateDataProperty(A, ! ToString(n), nextValue).
  5. Assert: status is true.
  6. Increment n by 1.
If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Return ? PutValue(lref, A).
Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
Return the result of performing DestructuringAssignmentEvaluation of nestedAssignmentPattern with A as the argument.

12.15.5.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation

With parameters value and propertyName.

opt

If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be the result of evaluating DestructuringAssignmentTarget.
2. ReturnIfAbrupt(lref).
Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Let rhsValue be ? GetValue(defaultValue).
Else, let rhsValue be v.
If DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral, then
1. Let assignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return the result of performing DestructuringAssignmentEvaluation of assignmentPattern with rhsValue as the argument.
If Initializer is present and v is undefined and IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
1. Let hasNameProperty be ? HasOwnProperty(rhsValue, "name").
2. If hasNameProperty is false, perform SetFunctionName(rhsValue, GetReferencedName(lref)).
Return ? PutValue(lref, rhsValue).

12.16 Comma Operator ( `,` )

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

12.16.1 Static Semantics: IsFunctionDefinition

Return false.

12.16.2 Static Semantics: IsValidSimpleAssignmentTarget

Return false.

12.16.3 Runtime Semantics: Evaluation

Let lref be the result of evaluating Expression.
Perform ? GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Return ? GetValue(rref).

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

13 ECMAScript Language: Statements and Declarations

Syntax

[Yield, Await, Return]

BlockStatement

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[+Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

DebuggerStatement

[Yield, Await]

[?Yield, ?Await, ~Default]

[?Yield, ?Await, ~Default]

[+In, ?Yield, ?Await]

[Yield, Await, Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[?Yield, ?Await, ?Default]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

13.1 Statement Semantics

13.1.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Return false.

13.1.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Return false.

13.1.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Return false.

Let newIterationSet be a copy of iterationSet with all the elements of labelSet appended.
Return ContainsUndefinedContinueTarget of IterationStatement with arguments newIterationSet and « ».

13.1.4 Static Semantics: DeclarationPart

Return FunctionDeclaration.

Return GeneratorDeclaration.

Return AsyncFunctionDeclaration.

Return AsyncGeneratorDeclaration.

Return ClassDeclaration.

Return LexicalDeclaration.

13.1.5 Static Semantics: VarDeclaredNames

Return a new empty List.

13.1.6 Static Semantics: VarScopedDeclarations

Return a new empty List.

13.1.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

Let stmtResult be the result of performing LabelledEvaluation of IterationStatement with argument labelSet.
If stmtResult.[[Type]] is break, then
1. If stmtResult.[[Target]] is empty, then
  1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
  2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

Let stmtResult be the result of evaluating SwitchStatement.
If stmtResult.[[Type]] is break, then
1. If stmtResult.[[Target]] is empty, then
  1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
  2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

Note

A BreakableStatement is one that can be exited via an unlabelled BreakStatement.

13.1.8 Runtime Semantics: Evaluation

Return NormalCompletion(empty).

Return the result of evaluating FunctionDeclaration.

Let newLabelSet be a new empty List.
Return the result of performing LabelledEvaluation of this BreakableStatement with argument newLabelSet.

13.2 Block

Syntax

BlockStatement

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

13.2.1 Static Semantics: Early Errors

{

}

It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

13.2.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

{

}

Return false.

Let hasDuplicates be ContainsDuplicateLabels of StatementList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of StatementListItem with argument labelSet.

Return false.

13.2.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

{

}

Return false.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of StatementList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of StatementListItem with argument labelSet.

Return false.

13.2.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

{

}

Return false.

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of StatementListItem with arguments iterationSet and « ».

Return false.

13.2.5 Static Semantics: LexicallyDeclaredNames

{

}

Return a new empty List.

Let names be LexicallyDeclaredNames of StatementList.
Append to names the elements of the LexicallyDeclaredNames of StatementListItem.
Return names.

If Statement is Statement:LabelledStatement , return LexicallyDeclaredNames of LabelledStatement.
Return a new empty List.

Return the BoundNames of Declaration.

13.2.6 Static Semantics: LexicallyScopedDeclarations

Let declarations be LexicallyScopedDeclarations of StatementList.
Append to declarations the elements of the LexicallyScopedDeclarations of StatementListItem.
Return declarations.

If Statement is Statement:LabelledStatement , return LexicallyScopedDeclarations of LabelledStatement.
Return a new empty List.

Return a new List containing DeclarationPart of Declaration.

13.2.7 Static Semantics: TopLevelLexicallyDeclaredNames

Let names be TopLevelLexicallyDeclaredNames of StatementList.
Append to names the elements of the TopLevelLexicallyDeclaredNames of StatementListItem.
Return names.

Return a new empty List.

If Declaration is Declaration:HoistableDeclaration , then
1. Return « ».
Return the BoundNames of Declaration.

Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

13.2.8 Static Semantics: TopLevelLexicallyScopedDeclarations

{

}

Return a new empty List.

Let declarations be TopLevelLexicallyScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelLexicallyScopedDeclarations of StatementListItem.
Return declarations.

Return a new empty List.

If Declaration is Declaration:HoistableDeclaration , then
1. Return « ».
Return a new List containing Declaration.

13.2.9 Static Semantics: TopLevelVarDeclaredNames

{

}

Return a new empty List.

Let names be TopLevelVarDeclaredNames of StatementList.
Append to names the elements of the TopLevelVarDeclaredNames of StatementListItem.
Return names.

If Declaration is Declaration:HoistableDeclaration , then
1. Return the BoundNames of HoistableDeclaration.
Return a new empty List.

If Statement is Statement:LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

Note

At the top level of a function or script, inner function declarations are treated like var declarations.

13.2.10 Static Semantics: TopLevelVarScopedDeclarations

{

}

Return a new empty List.

Let declarations be TopLevelVarScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelVarScopedDeclarations of StatementListItem.
Return declarations.

If Statement is Statement:LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

If Declaration is Declaration:HoistableDeclaration , then
1. Let declaration be DeclarationPart of HoistableDeclaration.
2. Return « declaration ».
Return a new empty List.

13.2.11 Static Semantics: VarDeclaredNames

{

}

Return a new empty List.

Let names be VarDeclaredNames of StatementList.
Append to names the elements of the VarDeclaredNames of StatementListItem.
Return names.

Return a new empty List.

13.2.12 Static Semantics: VarScopedDeclarations

{

}

Return a new empty List.

Let declarations be VarScopedDeclarations of StatementList.
Append to declarations the elements of the VarScopedDeclarations of StatementListItem.
Return declarations.

Return a new empty List.

13.2.13 Runtime Semantics: Evaluation

{

}

Return NormalCompletion(empty).

{

}

Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(StatementList, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let blockValue be the result of evaluating StatementList.
Set the running execution context's LexicalEnvironment to oldEnv.
Return blockValue.

Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

Let sl be the result of evaluating StatementList.
ReturnIfAbrupt(sl).
Let s be the result of evaluating StatementListItem.
Return Completion(UpdateEmpty(s, sl)).

Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

13.2.14 Runtime Semantics: BlockDeclarationInstantiation ( `code`, `env` )

Note

When a Block or CaseBlock is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

BlockDeclarationInstantiation is performed as follows using arguments code and env. code is the Parse Node corresponding to the body of the block. env is the Lexical Environment in which bindings are to be created.

Let envRec be env's EnvironmentRecord.
Assert: envRec is a declarative Environment Record.
Let declarations be the LexicallyScopedDeclarations of code.
For each element d in declarations, do
1. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! envRec.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! envRec.CreateMutableBinding(dn, false).
2. If d is a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
  1. Let fn be the sole element of the BoundNames of d.
  2. Let fo be the result of performing InstantiateFunctionObject for d with argument env.
  3. Perform envRec.InitializeBinding(fn, fo).

13.3 Declarations and the Variable Statement

13.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

[In, Yield, Await]

[?In, ?Yield, ?Await]

;

let

const

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

13.3.1.1 Static Semantics: Early Errors

;

It is a Syntax Error if the BoundNames of BindingList contains "let".
It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

opt

It is a Syntax Error if Initializer is not present and IsConstantDeclaration of the LexicalDeclaration containing this LexicalBinding is true.

13.3.1.2 Static Semantics: BoundNames

;

Return the BoundNames of BindingList.

Let names be the BoundNames of BindingList.
Append to names the elements of the BoundNames of LexicalBinding.
Return names.

opt

Return the BoundNames of BindingIdentifier.

Return the BoundNames of BindingPattern.

13.3.1.3 Static Semantics: IsConstantDeclaration

;

Return IsConstantDeclaration of LetOrConst.

let

Return false.

const

Return true.

13.3.1.4 Runtime Semantics: Evaluation

;

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return NormalCompletion(empty).

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return the result of evaluating LexicalBinding.

Let lhs be ResolveBinding(StringValue of BindingIdentifier).
Return InitializeReferencedBinding(lhs, undefined).

Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ResolveBinding(bindingId).
Let rhs be the result of evaluating Initializer.
Let value be ? GetValue(rhs).
If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let hasNameProperty be ? HasOwnProperty(value, "name").
2. If hasNameProperty is false, perform SetFunctionName(value, bindingId).
Return InitializeReferencedBinding(lhs, value).

Let rhs be the result of evaluating Initializer.
Let value be ? GetValue(rhs).
Let env be the running execution context's LexicalEnvironment.
Return the result of performing BindingInitialization for BindingPattern using value and env as the arguments.

13.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Lexical Environment is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

[Yield, Await]

var

[+In, ?Yield, ?Await]

;

[In, Yield, Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[?In, ?Yield, ?Await]

[In, Yield, Await]

[?Yield, ?Await]

[?In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?In, ?Yield, ?Await]

13.3.2.1 Static Semantics: BoundNames

Let names be BoundNames of VariableDeclarationList.
Append to names the elements of BoundNames of VariableDeclaration.
Return names.

opt

Return the BoundNames of BindingIdentifier.

Return the BoundNames of BindingPattern.

13.3.2.2 Static Semantics: VarDeclaredNames

var

;

Return BoundNames of VariableDeclarationList.

13.3.2.3 Static Semantics: VarScopedDeclarations

Return a new List containing VariableDeclaration.

Let declarations be VarScopedDeclarations of VariableDeclarationList.
Append VariableDeclaration to declarations.
Return declarations.

13.3.2.4 Runtime Semantics: Evaluation

var

;

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return NormalCompletion(empty).

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return the result of evaluating VariableDeclaration.

Return NormalCompletion(empty).

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId).
Let rhs be the result of evaluating Initializer.
Let value be ? GetValue(rhs).
If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let hasNameProperty be ? HasOwnProperty(value, "name").
2. If hasNameProperty is false, perform SetFunctionName(value, bindingId).
Return ? PutValue(lhs, value).

Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's object Environment Record, then step 6 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

Let rhs be the result of evaluating Initializer.
Let rval be ? GetValue(rhs).
Return the result of performing BindingInitialization for BindingPattern passing rval and undefined as arguments.

13.3.3 Destructuring Binding Patterns

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

}

{

[?Yield, ?Await]

[?Yield, ?Await]

opt

}

[Yield, Await]

[

opt

[?Yield, ?Await]

opt

]

[

[?Yield, ?Await]

]

[

[?Yield, ?Await]

opt

[?Yield, ?Await]

opt

]

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

[Yield, Await]

...

[?Yield, ?Await]

...

[?Yield, ?Await]

13.3.3.1 Static Semantics: BoundNames

{

}

Return a new empty List.

[

opt

]

Return a new empty List.

[

opt

]

Return the BoundNames of BindingRestElement.

[

opt

]

Return the BoundNames of BindingElementList.

[

opt

]

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingRestElement.
Return names.

Let names be BoundNames of BindingPropertyList.
Append to names the elements of BoundNames of BindingProperty.
Return names.

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingElisionElement.
Return names.

opt

Return BoundNames of BindingElement.

Return the BoundNames of BindingElement.

opt

Return the BoundNames of BindingIdentifier.

opt

Return the BoundNames of BindingPattern.

13.3.3.2 Static Semantics: ContainsExpression

{

}

Return false.

[

opt

]

Return false.

[

opt

]

Return ContainsExpression of BindingRestElement.

[

opt

]

Return ContainsExpression of BindingElementList.

[

opt

]

Let has be ContainsExpression of BindingElementList.
If has is true, return true.
Return ContainsExpression of BindingRestElement.

Let has be ContainsExpression of BindingPropertyList.
If has is true, return true.
Return ContainsExpression of BindingProperty.

Let has be ContainsExpression of BindingElementList.
If has is true, return true.
Return ContainsExpression of BindingElisionElement.

opt

Return ContainsExpression of BindingElement.

Let has be IsComputedPropertyKey of PropertyName.
If has is true, return true.
Return ContainsExpression of BindingElement.

Return true.

Return false.

Return true.

...

Return false.

...

Return ContainsExpression of BindingPattern.

13.3.3.3 Static Semantics: HasInitializer

Return false.

Return true.

Return false.

Return true.

13.3.3.4 Static Semantics: IsSimpleParameterList

Return false.

Return false.

Return true.

Return false.

13.3.3.5 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

Perform ? RequireObjectCoercible(value).
Return the result of performing BindingInitialization for ObjectBindingPattern using value and environment as arguments.

Let iteratorRecord be ? GetIterator(value).
Let result be IteratorBindingInitialization for ArrayBindingPattern using iteratorRecord and environment as arguments.
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

{

}

Return NormalCompletion(empty).

{

}

{

}

Perform ? PropertyBindingInitialization for BindingPropertyList using value and environment as the arguments.
Return NormalCompletion(empty).

{

}

Let excludedNames be a new empty List.
Return the result of performing RestBindingInitialization of BindingRestProperty with value, environment, and excludedNames as the arguments.

{

}

Let excludedNames be the result of performing ? PropertyBindingInitialization of BindingPropertyList using value and environment as arguments.
Return the result of performing RestBindingInitialization of BindingRestProperty with value, environment, and excludedNames as the arguments.

13.3.3.6 Runtime Semantics: PropertyBindingInitialization

With parameters value and environment.

Note

These collect a list of all bound property names rather than just empty completion.

Let boundNames be the result of performing ? PropertyBindingInitialization for BindingPropertyList using value and environment as arguments.
Let nextNames be the result of performing ? PropertyBindingInitialization for BindingProperty using value and environment as arguments.
Append each item in nextNames to the end of boundNames.
Return boundNames.

Let name be the string that is the only element of BoundNames of SingleNameBinding.
Perform ? KeyedBindingInitialization for SingleNameBinding using value, environment, and name as the arguments.
Return a new List containing name.

Let P be the result of evaluating PropertyName.
ReturnIfAbrupt(P).
Perform ? KeyedBindingInitialization of BindingElement with value, environment, and P as the arguments.
Return a new List containing P.

13.3.3.7 Runtime Semantics: RestBindingInitialization

With parameters value, environment, and excludedNames.

...

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let restObj be ObjectCreate(%ObjectPrototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
If environment is undefined, return PutValue(lhs, restObj).
Return InitializeReferencedBinding(lhs, restObj).

13.3.3.8 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

[

]

Return NormalCompletion(empty).

[

]

Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.

[

opt

]

If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.

[

]

Return the result of performing IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.

[

]

Return the result of performing IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.

[

]

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.

[

opt

]

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.

Return the result of performing IteratorBindingInitialization for BindingElisionElement with iteratorRecord and environment as arguments.

Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
Return the result of performing IteratorBindingInitialization for BindingElisionElement using iteratorRecord and environment as arguments.

Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the arguments.

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the arguments.

Return the result of performing IteratorBindingInitialization for SingleNameBinding with iteratorRecord and environment as the arguments.

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let v be IteratorValue(next).
  2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
3. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Let hasNameProperty be ? HasOwnProperty(v, "name").
  2. If hasNameProperty is false, perform SetFunctionName(v, bindingId).
If environment is undefined, return ? PutValue(lhs, v).
Return InitializeReferencedBinding(lhs, v).

opt

If iteratorRecord.[[Done]] is false, then
1. Let next be IteratorStep(iteratorRecord).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
5. Else,
  1. Let v be IteratorValue(next).
  2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
Return the result of performing BindingInitialization of BindingPattern with v and environment as the arguments.

...

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let A be ! ArrayCreate(0).
Let n be 0.
Repeat,
1. If iteratorRecord.[[Done]] is false, then
  1. Let next be IteratorStep(iteratorRecord).
  2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(next).
  4. If next is false, set iteratorRecord.[[Done]] to true.
2. If iteratorRecord.[[Done]] is true, then
  1. If environment is undefined, return ? PutValue(lhs, A).
  2. Return InitializeReferencedBinding(lhs, A).
3. Let nextValue be IteratorValue(next).
4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. ReturnIfAbrupt(nextValue).
6. Let status be CreateDataProperty(A, ! ToString(n), nextValue).
7. Assert: status is true.
8. Increment n by 1.

...

Let A be ! ArrayCreate(0).
Let n be 0.
Repeat,
1. If iteratorRecord.[[Done]] is false, then
  1. Let next be IteratorStep(iteratorRecord).
  2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  3. ReturnIfAbrupt(next).
  4. If next is false, set iteratorRecord.[[Done]] to true.
2. If iteratorRecord.[[Done]] is true, then
  1. Return the result of performing BindingInitialization of BindingPattern with A and environment as the arguments.
3. Let nextValue be IteratorValue(next).
4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. ReturnIfAbrupt(nextValue).
6. Let status be CreateDataProperty(A, ! ToString(n), nextValue).
7. Assert: status is true.
8. Increment n by 1.

13.3.3.9 Runtime Semantics: KeyedBindingInitialization

With parameters value, environment, and propertyName.

Note

opt

Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
Return the result of performing BindingInitialization for BindingPattern passing v and environment as arguments.

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
Let v be ? GetV(value, propertyName).
If Initializer is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initializer.
2. Set v to ? GetValue(defaultValue).
3. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Let hasNameProperty be ? HasOwnProperty(v, "name").
  2. If hasNameProperty is false, perform SetFunctionName(v, bindingId).
If environment is undefined, return ? PutValue(lhs, v).
Return InitializeReferencedBinding(lhs, v).

13.4 Empty Statement

Syntax

EmptyStatement

;

13.4.1 Runtime Semantics: Evaluation

EmptyStatement

;

Return NormalCompletion(empty).

13.5 Expression Statement

Syntax

[Yield, Await]

[lookahead ∉ { {, function, async

[no LineTerminator here]

function, class, let [ }]

[+In, ?Yield, ?Await]

;

Note

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

13.5.1 Runtime Semantics: Evaluation

;

Let exprRef be the result of evaluating Expression.
Return ? GetValue(exprRef).

13.6 The `if` Statement

Syntax

[Yield, Await, Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

else

[?Yield, ?Await, ?Return]

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

13.6.1 Static Semantics: Early Errors

(

)

else

(

)

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.6.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

(

)

else

Let hasDuplicate be ContainsDuplicateLabels of the first Statement with argument labelSet.
If hasDuplicate is true, return true.
Return ContainsDuplicateLabels of the second Statement with argument labelSet.

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.6.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

(

)

else

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first Statement with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of the second Statement with argument labelSet.

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.6.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

(

)

else

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first Statement with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of the second Statement with arguments iterationSet and « ».

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.6.5 Static Semantics: VarDeclaredNames

(

)

else

Let names be VarDeclaredNames of the first Statement.
Append to names the elements of the VarDeclaredNames of the second Statement.
Return names.

(

)

Return the VarDeclaredNames of Statement.

13.6.6 Static Semantics: VarScopedDeclarations

(

)

else

Let declarations be VarScopedDeclarations of the first Statement.
Append to declarations the elements of the VarScopedDeclarations of the second Statement.
Return declarations.

(

)

Return the VarScopedDeclarations of Statement.

13.6.7 Runtime Semantics: Evaluation

(

)

else

Let exprRef be the result of evaluating Expression.
Let exprValue be ToBoolean(? GetValue(exprRef)).
If exprValue is true, then
1. Let stmtCompletion be the result of evaluating the first Statement.
Else,
1. Let stmtCompletion be the result of evaluating the second Statement.
Return Completion(UpdateEmpty(stmtCompletion, undefined)).

(

)

Let exprRef be the result of evaluating Expression.
Let exprValue be ToBoolean(? GetValue(exprRef)).
If exprValue is false, then
1. Return NormalCompletion(undefined).
Else,
1. Let stmtCompletion be the result of evaluating Statement.
2. Return Completion(UpdateEmpty(stmtCompletion, undefined)).

13.7 Iteration Statements

Syntax

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

while

(

[+In, ?Yield, ?Await]

)

;

while

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ∉ { let [ }]

[~In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

var

[~In, ?Yield, ?Await]

;

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[~In, ?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

;

[+In, ?Yield, ?Await]

opt

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ∉ { let [ }]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

for

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[lookahead ≠ let]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

var

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[?Yield, ?Await]

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Note

This section is extended by Annex B.3.6.

13.7.1 Semantics

13.7.1.1 Static Semantics: Early Errors

while

(

)

;

while

(

)

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.7.1.2 Runtime Semantics: LoopContinues ( `completion`, `labelSet` )

The abstract operation LoopContinues with arguments completion and labelSet is defined by the following steps:

If completion.[[Type]] is normal, return true.
If completion.[[Type]] is not continue, return false.
If completion.[[Target]] is empty, return true.
If completion.[[Target]] is an element of labelSet, return true.
Return false.

Note

Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

13.7.2 The `do`-`while` Statement

13.7.2.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

while

(

)

;

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.2.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

while

(

)

;

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.2.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

while

(

)

;

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.2.4 Static Semantics: VarDeclaredNames

while

(

)

;

Return the VarDeclaredNames of Statement.

13.7.2.5 Static Semantics: VarScopedDeclarations

while

(

)

;

Return the VarScopedDeclarations of Statement.

13.7.2.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

while

(

)

;

Let V be undefined.
Repeat,
1. Let stmtResult be the result of evaluating Statement.
2. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
3. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].
4. Let exprRef be the result of evaluating Expression.
5. Let exprValue be ? GetValue(exprRef).
6. If ToBoolean(exprValue) is false, return NormalCompletion(V).

13.7.3 The `while` Statement

13.7.3.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

while

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.3.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

while

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.3.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

while

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.3.4 Static Semantics: VarDeclaredNames

while

(

)

Return the VarDeclaredNames of Statement.

13.7.3.5 Static Semantics: VarScopedDeclarations

while

(

)

Return the VarScopedDeclarations of Statement.

13.7.3.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

while

(

)

Let V be undefined.
Repeat,
1. Let exprRef be the result of evaluating Expression.
2. Let exprValue be ? GetValue(exprRef).
3. If ToBoolean(exprValue) is false, return NormalCompletion(V).
4. Let stmtResult be the result of evaluating Statement.
5. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

13.7.4 The `for` Statement

13.7.4.1 Static Semantics: Early Errors

for

(

opt

;

opt

)

It is a Syntax Error if any element of the BoundNames of LexicalDeclaration also occurs in the VarDeclaredNames of Statement.

13.7.4.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.4.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.4.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.4.5 Static Semantics: VarDeclaredNames

for

(

opt

;

opt

;

opt

)

Return the VarDeclaredNames of Statement.

for

(

var

;

opt

;

opt

)

Let names be BoundNames of VariableDeclarationList.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

for

(

opt

;

opt

)

Return the VarDeclaredNames of Statement.

13.7.4.6 Static Semantics: VarScopedDeclarations

for

(

opt

;

opt

;

opt

)

Return the VarScopedDeclarations of Statement.

for

(

var

;

opt

;

opt

)

Let declarations be VarScopedDeclarations of VariableDeclarationList.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

for

(

opt

;

opt

)

Return the VarScopedDeclarations of Statement.

13.7.4.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

for

(

opt

;

opt

;

opt

)

If the first Expression is present, then
1. Let exprRef be the result of evaluating the first Expression.
2. Perform ? GetValue(exprRef).
Return ? ForBodyEvaluation(the second Expression, the third Expression, Statement, « », labelSet).

for

(

var

;

opt

;

opt

)

Let varDcl be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(varDcl).
Return ? ForBodyEvaluation(the first Expression, the second Expression, Statement, « », labelSet).

for

(

opt

;

opt

)

Let oldEnv be the running execution context's LexicalEnvironment.
Let loopEnv be NewDeclarativeEnvironment(oldEnv).
Let loopEnvRec be loopEnv's EnvironmentRecord.
Let isConst be the result of performing IsConstantDeclaration of LexicalDeclaration.
Let boundNames be the BoundNames of LexicalDeclaration.
For each element dn of boundNames, do
1. If isConst is true, then
  1. Perform ! loopEnvRec.CreateImmutableBinding(dn, true).
2. Else,
  1. Perform ! loopEnvRec.CreateMutableBinding(dn, false).
Set the running execution context's LexicalEnvironment to loopEnv.
Let forDcl be the result of evaluating LexicalDeclaration.
If forDcl is an abrupt completion, then
1. Set the running execution context's LexicalEnvironment to oldEnv.
2. Return Completion(forDcl).
If isConst is false, let perIterationLets be boundNames; otherwise let perIterationLets be « ».
Let bodyResult be ForBodyEvaluation(the first Expression, the second Expression, Statement, perIterationLets, labelSet).
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(bodyResult).

13.7.4.8 Runtime Semantics: ForBodyEvaluation ( `test`, `increment`, `stmt`, `perIterationBindings`, `labelSet` )

The abstract operation ForBodyEvaluation with arguments test, increment, stmt, perIterationBindings, and labelSet is performed as follows:

Let V be undefined.
Perform ? CreatePerIterationEnvironment(perIterationBindings).
Repeat,
1. If test is not [empty], then
  1. Let testRef be the result of evaluating test.
  2. Let testValue be ? GetValue(testRef).
  3. If ToBoolean(testValue) is false, return NormalCompletion(V).
2. Let result be the result of evaluating stmt.
3. If LoopContinues(result, labelSet) is false, return Completion(UpdateEmpty(result, V)).
4. If result.[[Value]] is not empty, set V to result.[[Value]].
5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
6. If increment is not [empty], then
  1. Let incRef be the result of evaluating increment.
  2. Perform ? GetValue(incRef).

13.7.4.9 Runtime Semantics: CreatePerIterationEnvironment ( `perIterationBindings` )

The abstract operation CreatePerIterationEnvironment with argument perIterationBindings is performed as follows:

If perIterationBindings has any elements, then
1. Let lastIterationEnv be the running execution context's LexicalEnvironment.
2. Let lastIterationEnvRec be lastIterationEnv's EnvironmentRecord.
3. Let outer be lastIterationEnv's outer environment reference.
4. Assert: outer is not null.
5. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
6. Let thisIterationEnvRec be thisIterationEnv's EnvironmentRecord.
7. For each element bn of perIterationBindings, do
  1. Perform ! thisIterationEnvRec.CreateMutableBinding(bn, false).
  2. Let lastValue be ? lastIterationEnvRec.GetBindingValue(bn, true).
  3. Perform thisIterationEnvRec.InitializeBinding(bn, lastValue).
8. Set the running execution context's LexicalEnvironment to thisIterationEnv.
Return undefined.

13.7.5 The `for`-`in`, `for`-`of`, and `for`-`await`-`of` Statements

13.7.5.1 Static Semantics: Early Errors

for

(

)

for

(

)

for

await

(

)

It is a Syntax Error if LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if LeftHandSideExpression is not covering an AssignmentPattern.

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if LeftHandSideExpression is covering an AssignmentPattern then the following rules are not applied. Instead, the Early Error rules for AssignmentPattern are used.

It is a Syntax Error if IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.
It is a Syntax Error if the LeftHandSideExpression is CoverParenthesizedExpressionAndArrowParameterList:(Expression) and Expression derives a phrase that would produce a Syntax Error according to these rules if that phrase were substituted for LeftHandSideExpression. This rule is recursively applied.

Note

The last rule means that the other rules are applied even if parentheses surround Expression.

for

(

)

for

(

)

for

await

(

)

It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
It is a Syntax Error if any element of the BoundNames of ForDeclaration also occurs in the VarDeclaredNames of Statement.
It is a Syntax Error if the BoundNames of ForDeclaration contains any duplicate entries.

13.7.5.2 Static Semantics: BoundNames

Return the BoundNames of ForBinding.

13.7.5.3 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

Note

This section is extended by Annex B.3.6.

13.7.5.4 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

Note

This section is extended by Annex B.3.6.

13.7.5.5 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

for

(

)

for

(

var

)

for

(

)

for

(

)

for

(

var

)

for

(

)

for

await

(

)

for

await

(

var

)

for

await

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

Note

This section is extended by Annex B.3.6.

13.7.5.6 Static Semantics: IsDestructuring

Return IsDestructuring of ForBinding.

Return false.

Return true.

Note

This section is extended by Annex B.3.6.

13.7.5.7 Static Semantics: VarDeclaredNames

for

(

)

Return the VarDeclaredNames of Statement.

for

(

var

)

Let names be the BoundNames of ForBinding.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

for

(

)

Return the VarDeclaredNames of Statement.

for

(

)

for

await

(

)

Return the VarDeclaredNames of Statement.

for

(

var

)

for

await

(

var

)

Let names be the BoundNames of ForBinding.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

for

(

)

for

await

(

)

Return the VarDeclaredNames of Statement.

Note

This section is extended by Annex B.3.6.

13.7.5.8 Static Semantics: VarScopedDeclarations

for

(

)

Return the VarScopedDeclarations of Statement.

for

(

var

)

Let declarations be a List containing ForBinding.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

for

(

)

for

await

(

)

Return the VarScopedDeclarations of Statement.

for

(

)

for

await

(

var

)

Return the VarScopedDeclarations of Statement.

for

(

var

)

for

await

(

var

)

Let declarations be a List containing ForBinding.
Append to declarations the elements of the VarScopedDeclarations of Statement.
Return declarations.

for

(

)

for

await

(

)

Return the VarScopedDeclarations of Statement.

Note

This section is extended by Annex B.3.6.

13.7.5.9 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 9.2.15). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

Return the result of performing BindingInitialization for ForBinding passing value and environment as the arguments.

13.7.5.10 Runtime Semantics: BindingInstantiation

With parameter environment.

Let envRec be environment's EnvironmentRecord.
Assert: envRec is a declarative Environment Record.
For each element name of the BoundNames of ForBinding, do
1. If IsConstantDeclaration of LetOrConst is true, then
  1. Perform ! envRec.CreateImmutableBinding(name, true).
2. Else,
  1. Perform ! envRec.CreateMutableBinding(name, false).

13.7.5.11 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

for

(

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, enumerate, assignment, labelSet).

for

(

var

)

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, enumerate, varBinding, labelSet).

for

(

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, enumerate, lexicalBinding, labelSet).

for

(

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).

for

(

var

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet).

for

(

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet).

for

await

(

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet, async).

for

await

(

var

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet, async).

for

await

(

)

Let keyResult be the result of performing ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet, async).

Note

This section is extended by Annex B.3.6.

13.7.5.12 Runtime Semantics: ForIn/OfHeadEvaluation ( `TDZnames`, `expr`, `iterationKind` )

The abstract operation ForIn/OfHeadEvaluation is called with arguments TDZnames, expr, and iterationKind. The value of iterationKind is either enumerate, iterate, or async-iterate.

Let oldEnv be the running execution context's LexicalEnvironment.
If TDZnames is not an empty List, then
1. Assert: TDZnames has no duplicate entries.
2. Let TDZ be NewDeclarativeEnvironment(oldEnv).
3. Let TDZEnvRec be TDZ's EnvironmentRecord.
4. For each string name in TDZnames, do
  1. Perform ! TDZEnvRec.CreateMutableBinding(name, false).
5. Set the running execution context's LexicalEnvironment to TDZ.
Let exprRef be the result of evaluating expr.
Set the running execution context's LexicalEnvironment to oldEnv.
Let exprValue be ? GetValue(exprRef).
If iterationKind is enumerate, then
1. If exprValue is undefined or null, then
  1. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
2. Let obj be ! ToObject(exprValue).
3. Return ? EnumerateObjectProperties(obj).
Else,
1. Assert: iterationKind is iterate.
2. If iterationKind is async-iterate, let iteratorHint be async.
3. Else, let iteratorHint be sync.
4. Return ? GetIterator(exprValue, iteratorHint).

13.7.5.13 Runtime Semantics: ForIn/OfBodyEvaluation ( `lhs`, `stmt`, `iteratorRecord`, `iterationKind`, `lhsKind`, `labelSet` [ , `iteratorKind` ] )

The abstract operation ForIn/OfBodyEvaluation is called with arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet, and optional argument iteratorKind. The value of lhsKind is either assignment, varBinding or lexicalBinding. The value of iteratorKind is either sync or async.

If iteratorKind is not present, set iteratorKind to sync.
Let oldEnv be the running execution context's LexicalEnvironment.
Let V be undefined.
Let destructuring be IsDestructuring of lhs.
If destructuring is true and if lhsKind is assignment, then
1. Assert: lhs is a LeftHandSideExpression.
2. Let assignmentPattern be the AssignmentPattern that is covered by lhs.
Repeat,
1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « »).
2. If iteratorKind is async, then set nextResult to ? Await(nextResult).
3. If Type(nextResult) is not Object, throw a TypeError exception.
4. Let nextValue be ? IteratorValue(nextResult).
5. If lhsKind is either assignment or varBinding, then
  1. If destructuring is false, then
    1. Let lhsRef be the result of evaluating lhs. (It may be evaluated repeatedly.)
6. Else,
  1. Assert: lhsKind is lexicalBinding.
  2. Assert: lhs is a ForDeclaration.
  3. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
  4. Perform BindingInstantiation for lhs passing iterationEnv as the argument.
  5. Set the running execution context's LexicalEnvironment to iterationEnv.
  6. If destructuring is false, then
    1. Assert: lhs binds a single name.
    2. Let lhsName be the sole element of BoundNames of lhs.
    3. Let lhsRef be ! ResolveBinding(lhsName).
7. If destructuring is false, then
  1. If lhsRef is an abrupt completion, then
    1. Let status be lhsRef.
  2. Else if lhsKind is lexicalBinding, then
    1. Let status be InitializeReferencedBinding(lhsRef, nextValue).
  3. Else,
    1. Let status be PutValue(lhsRef, nextValue).
8. Else,
  1. If lhsKind is assignment, then
    1. Let status be the result of performing DestructuringAssignmentEvaluation of assignmentPattern using nextValue as the argument.
  2. Else if lhsKind is varBinding, then
    1. Assert: lhs is a ForBinding.
    2. Let status be the result of performing BindingInitialization for lhs passing nextValue and undefined as the arguments.
  3. Else,
    1. Assert: lhsKind is lexicalBinding.
    2. Assert: lhs is a ForDeclaration.
    3. Let status be the result of performing BindingInitialization for lhs passing nextValue and iterationEnv as arguments.
9. If status is an abrupt completion, then
  1. Set the running execution context's LexicalEnvironment to oldEnv.
  2. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
  3. If iterationKind is enumerate, then
    1. Return status.
  4. Else,
    1. Assert: iterationKind is iterate.
    2. Return ? IteratorClose(iteratorRecord, status).
10. Let result be the result of evaluating stmt.
11. Set the running execution context's LexicalEnvironment to oldEnv.
12. If LoopContinues(result, labelSet) is false, then
  1. If iterationKind is enumerate, then
    1. Return Completion(UpdateEmpty(result, V)).
  2. Else,
    1. Assert: iterationKind is iterate.
    2. Set status to UpdateEmpty(result, V).
    3. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
    4. Return ? IteratorClose(iteratorRecord, status).
13. If result.[[Value]] is not empty, set V to result.[[Value]].

13.7.5.14 Runtime Semantics: Evaluation

Let bindingId be StringValue of BindingIdentifier.
Return ? ResolveBinding(bindingId).

13.7.5.15 EnumerateObjectProperties ( `O` )

When the abstract operation EnumerateObjectProperties is called with argument O, the following steps are taken:

Assert: Type(O) is Object.
Return an Iterator object (25.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

Note

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}

13.8 The `continue` Statement

Syntax

[Yield, Await]

continue

;

continue

[no LineTerminator here]

[?Yield, ?Await]

;

13.8.1 Static Semantics: Early Errors

continue

;

continue

;

It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement.

13.8.2 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

continue

;

Return false.

continue

;

If the StringValue of LabelIdentifier is not an element of iterationSet, return true.
Return false.

13.8.3 Runtime Semantics: Evaluation

continue

;

Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: empty }.

continue

;

Let label be the StringValue of LabelIdentifier.
Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: label }.

13.9 The `break` Statement

Syntax

[Yield, Await]

break

;

break

[no LineTerminator here]

[?Yield, ?Await]

;

13.9.1 Static Semantics: Early Errors

break

;

It is a Syntax Error if this BreakStatement is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement or a SwitchStatement.

13.9.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

break

;

Return false.

break

;

If the StringValue of LabelIdentifier is not an element of labelSet, return true.
Return false.

13.9.3 Runtime Semantics: Evaluation

break

;

Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.

break

;

Let label be the StringValue of LabelIdentifier.
Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: label }.

13.10 The `return` Statement

Syntax

[Yield, Await]

return

;

return

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.

13.10.1 Runtime Semantics: Evaluation

return

;

Return Completion { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.

return

;

Let exprRef be the result of evaluating Expression.
Let exprValue be ? GetValue(exprRef).
If ! GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
Return Completion { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

13.11 The `with` Statement

Syntax

[Yield, Await, Return]

with

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

Note

The with statement adds an object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

13.11.1 Static Semantics: Early Errors

with

(

)

It is a Syntax Error if the code that matches this production is contained in strict mode code.
It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply the second rule if the extension specified in B.3.2 is implemented.

13.11.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

with

(

)

Return ContainsDuplicateLabels of Statement with argument labelSet.

13.11.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

with

(

)

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.11.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

with

(

)

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.11.5 Static Semantics: VarDeclaredNames

with

(

)

Return the VarDeclaredNames of Statement.

13.11.6 Static Semantics: VarScopedDeclarations

with

(

)

Return the VarScopedDeclarations of Statement.

13.11.7 Runtime Semantics: Evaluation

with

(

)

Let val be the result of evaluating Expression.
Let obj be ? ToObject(? GetValue(val)).
Let oldEnv be the running execution context's LexicalEnvironment.
Let newEnv be NewObjectEnvironment(obj, oldEnv).
Set the withEnvironment flag of newEnv's EnvironmentRecord to true.
Set the running execution context's LexicalEnvironment to newEnv.
Let C be the result of evaluating Statement.
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(UpdateEmpty(C, undefined)).

Note

No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

13.12 The `switch` Statement

Syntax

[Yield, Await, Return]

switch

(

[+In, ?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

{

[?Yield, ?Await, ?Return]

opt

}

{

[?Yield, ?Await, ?Return]

opt

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

opt

}

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

case

[+In, ?Yield, ?Await]

[?Yield, ?Await, ?Return]

opt

[Yield, Await, Return]

default

[?Yield, ?Await, ?Return]

opt

13.12.1 Static Semantics: Early Errors

switch

(

)

It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of CaseBlock also occurs in the VarDeclaredNames of CaseBlock.

13.12.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

switch

(

)

Return ContainsDuplicateLabels of CaseBlock with argument labelSet.

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasDuplicates be ContainsDuplicateLabels of the first CaseClauses with argument labelSet.
2. If hasDuplicates is true, return true.
Let hasDuplicates be ContainsDuplicateLabels of DefaultClause with argument labelSet.
If hasDuplicates is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsDuplicateLabels of the second CaseClauses with argument labelSet.

Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of CaseClause with argument labelSet.

case

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

default

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

13.12.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

switch

(

)

Return ContainsUndefinedBreakTarget of CaseBlock with argument labelSet.

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first CaseClauses with argument labelSet.
2. If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedBreakTarget of DefaultClause with argument labelSet.
If hasUndefinedLabels is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedBreakTarget of the second CaseClauses with argument labelSet.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of CaseClause with argument labelSet.

case

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

default

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

13.12.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

switch

(

)

Return ContainsUndefinedContinueTarget of CaseBlock with arguments iterationSet and « ».

{

}

Return false.

{

opt

opt

}

If the first CaseClauses is present, then
1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first CaseClauses with arguments iterationSet and « ».
2. If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedContinueTarget of DefaultClause with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedContinueTarget of the second CaseClauses with arguments iterationSet and « ».

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of CaseClauses with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of CaseClause with arguments iterationSet and « ».

case

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

default

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

13.12.5 Static Semantics: LexicallyDeclaredNames

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names be the LexicallyDeclaredNames of the first CaseClauses.
Else, let names be a new empty List.
Append to names the elements of the LexicallyDeclaredNames of the DefaultClause.
If the second CaseClauses is not present, return names.
Return the result of appending to names the elements of the LexicallyDeclaredNames of the second CaseClauses.

Let names be LexicallyDeclaredNames of CaseClauses.
Append to names the elements of the LexicallyDeclaredNames of CaseClause.
Return names.

case

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

13.12.6 Static Semantics: LexicallyScopedDeclarations

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations be the LexicallyScopedDeclarations of the first CaseClauses.
Else, let declarations be a new empty List.
Append to declarations the elements of the LexicallyScopedDeclarations of the DefaultClause.
If the second CaseClauses is not present, return declarations.
Return the result of appending to declarations the elements of the LexicallyScopedDeclarations of the second CaseClauses.

Let declarations be LexicallyScopedDeclarations of CaseClauses.
Append to declarations the elements of the LexicallyScopedDeclarations of CaseClause.
Return declarations.

case

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

13.12.7 Static Semantics: VarDeclaredNames

switch

(

)

Return the VarDeclaredNames of CaseBlock.

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names be the VarDeclaredNames of the first CaseClauses.
Else, let names be a new empty List.
Append to names the elements of the VarDeclaredNames of the DefaultClause.
If the second CaseClauses is not present, return names.
Return the result of appending to names the elements of the VarDeclaredNames of the second CaseClauses.

Let names be VarDeclaredNames of CaseClauses.
Append to names the elements of the VarDeclaredNames of CaseClause.
Return names.

case

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

13.12.8 Static Semantics: VarScopedDeclarations

switch

(

)

Return the VarScopedDeclarations of CaseBlock.

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations be the VarScopedDeclarations of the first CaseClauses.
Else, let declarations be a new empty List.
Append to declarations the elements of the VarScopedDeclarations of the DefaultClause.
If the second CaseClauses is not present, return declarations.
Return the result of appending to declarations the elements of the VarScopedDeclarations of the second CaseClauses.

Let declarations be VarScopedDeclarations of CaseClauses.
Append to declarations the elements of the VarScopedDeclarations of CaseClause.
Return declarations.

case

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

default

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

13.12.9 Runtime Semantics: CaseBlockEvaluation

With parameter input.

{

}

Return NormalCompletion(undefined).

{

}

Let V be undefined.
Let A be the List of CaseClause items in CaseClauses, in source text order.
Let found be false.
For each CaseClause C in A, do
1. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. If found is true, then
  1. Let R be the result of evaluating C.
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Return NormalCompletion(V).

{

opt

opt

}

Let V be undefined.
If the first CaseClauses is present, then
1. Let A be the List of CaseClause items in the first CaseClauses, in source text order.
Else,
1. Let A be « ».
Let found be false.
For each CaseClause C in A, do
1. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. If found is true, then
  1. Let R be the result of evaluating C.
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Let foundInB be false.
If the second CaseClauses is present, then
1. Let B be the List of CaseClause items in the second CaseClauses, in source text order.
Else,
1. Let B be « ».
If found is false, then
1. For each CaseClause C in B, do
  1. If foundInB is false, then
    1. Set foundInB to ? CaseClauseIsSelected(C, input).
  2. If foundInB is true, then
    1. Let R be the result of evaluating CaseClause C.
    2. If R.[[Value]] is not empty, set V to R.[[Value]].
    3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
If foundInB is true, return NormalCompletion(V).
Let R be the result of evaluating DefaultClause.
If R.[[Value]] is not empty, set V to R.[[Value]].
If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
For each CaseClause C in B (NOTE: this is another complete iteration of the second CaseClauses), do
1. Let R be the result of evaluating CaseClause C.
2. If R.[[Value]] is not empty, set V to R.[[Value]].
3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
Return NormalCompletion(V).

13.12.10 Runtime Semantics: CaseClauseIsSelected ( `C`, `input` )

The abstract operation CaseClauseIsSelected, given CaseClause C and value input, determines whether C matches input.

Assert: C is an instance of the production CaseClause:caseExpression:StatementListopt .
Let exprRef be the result of evaluating the Expression of C.
Let clauseSelector be ? GetValue(exprRef).
Return the result of performing Strict Equality Comparison input === clauseSelector.

Note

This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.

13.12.11 Runtime Semantics: Evaluation

switch

(

)

Let exprRef be the result of evaluating Expression.
Let switchValue be ? GetValue(exprRef).
Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let R be the result of performing CaseBlockEvaluation of CaseBlock with argument switchValue.
Set the running execution context's LexicalEnvironment to oldEnv.
Return R.

Note

No matter how control leaves the SwitchStatement the LexicalEnvironment is always restored to its former state.

case

Return NormalCompletion(empty).

case

Return the result of evaluating StatementList.

default

Return NormalCompletion(empty).

default

Return the result of evaluating StatementList.

13.13 Labelled Statements

Syntax

[Yield, Await, Return]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ~Default]

Note

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

13.13.1 Static Semantics: Early Errors

It is a Syntax Error if any source text matches this rule.

Note

An alternative definition for this rule is provided in B.3.2.

13.13.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
If label is an element of labelSet, return true.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsDuplicateLabels of LabelledItem with argument newLabelSet.

Return false.

13.13.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsUndefinedBreakTarget of LabelledItem with argument newLabelSet.

Return false.

13.13.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be a copy of labelSet with label appended.
Return ContainsUndefinedContinueTarget of LabelledItem with arguments iterationSet and newLabelSet.

Return false.

13.13.5 Static Semantics: IsLabelledFunction ( `stmt` )

The abstract operation IsLabelledFunction with argument stmt performs the following steps:

If stmt is not a LabelledStatement, return false.
Let item be the LabelledItem of stmt.
If item is LabelledItem:FunctionDeclaration , return true.
Let subStmt be the Statement of item.
Return IsLabelledFunction(subStmt).

13.13.6 Static Semantics: LexicallyDeclaredNames

Return the LexicallyDeclaredNames of LabelledItem.

Return a new empty List.

Return BoundNames of FunctionDeclaration.

13.13.7 Static Semantics: LexicallyScopedDeclarations

Return the LexicallyScopedDeclarations of LabelledItem.

Return a new empty List.

Return a new List containing FunctionDeclaration.

13.13.8 Static Semantics: TopLevelLexicallyDeclaredNames

Return a new empty List.

13.13.9 Static Semantics: TopLevelLexicallyScopedDeclarations

Return a new empty List.

13.13.10 Static Semantics: TopLevelVarDeclaredNames

Return the TopLevelVarDeclaredNames of LabelledItem.

If Statement is Statement:LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

Return BoundNames of FunctionDeclaration.

13.13.11 Static Semantics: TopLevelVarScopedDeclarations

Return the TopLevelVarScopedDeclarations of LabelledItem.

If Statement is Statement:LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

Return a new List containing FunctionDeclaration.

13.13.12 Static Semantics: VarDeclaredNames

Return the VarDeclaredNames of LabelledItem.

Return a new empty List.

13.13.13 Static Semantics: VarScopedDeclarations

Return the VarScopedDeclarations of LabelledItem.

Return a new empty List.

13.13.14 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

Let label be the StringValue of LabelIdentifier.
Append label as an element of labelSet.
Let stmtResult be LabelledEvaluation of LabelledItem with argument labelSet.
If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]], label) is true, then
1. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
Return Completion(stmtResult).

If Statement is either a LabelledStatement or a BreakableStatement, then
1. Return LabelledEvaluation of Statement with argument labelSet.
Else,
1. Return the result of evaluating Statement.

Return the result of evaluating FunctionDeclaration.

13.13.15 Runtime Semantics: Evaluation

Let newLabelSet be a new empty List.
Return LabelledEvaluation of this LabelledStatement with argument newLabelSet.

13.14 The `throw` Statement

Syntax

[Yield, Await]

throw

[no LineTerminator here]

[+In, ?Yield, ?Await]

;

13.14.1 Runtime Semantics: Evaluation

throw

;

Let exprRef be the result of evaluating Expression.
Let exprValue be ? GetValue(exprRef).
Return ThrowCompletion(exprValue).

13.15 The `try` Statement

Syntax

[Yield, Await, Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

try

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

catch

(

[?Yield, ?Await]

)

[?Yield, ?Await, ?Return]

[Yield, Await, Return]

finally

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.

13.15.1 Static Semantics: Early Errors

catch

(

)

It is a Syntax Error if BoundNames of CatchParameter contains any duplicate elements.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the LexicallyDeclaredNames of Block.
It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block.

Note

An alternative static semantics for this production is given in B.3.5.

13.15.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Catch with argument labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

try

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Let hasDuplicates be ContainsDuplicateLabels of Catch with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

catch

(

)

Return ContainsDuplicateLabels of Block with argument labelSet.

13.15.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Catch with argument labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Catch with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

catch

(

)

Return ContainsUndefinedBreakTarget of Block with argument labelSet.

13.15.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

try

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

catch

(

)

Return ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».

13.15.5 Static Semantics: VarDeclaredNames

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Catch.
Return names.

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Finally.
Return names.

try

Let names be VarDeclaredNames of Block.
Append to names the elements of the VarDeclaredNames of Catch.
Append to names the elements of the VarDeclaredNames of Finally.
Return names.

catch

(

)

Return the VarDeclaredNames of Block.

13.15.6 Static Semantics: VarScopedDeclarations

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Catch.
Return declarations.

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Finally.
Return declarations.

try

Let declarations be VarScopedDeclarations of Block.
Append to declarations the elements of the VarScopedDeclarations of Catch.
Append to declarations the elements of the VarScopedDeclarations of Finally.
Return declarations.

catch

(

)

Return the VarScopedDeclarations of Block.

13.15.7 Runtime Semantics: CatchClauseEvaluation

With parameter thrownValue.

catch

(

)

Let oldEnv be the running execution context's LexicalEnvironment.
Let catchEnv be NewDeclarativeEnvironment(oldEnv).
Let catchEnvRec be catchEnv's EnvironmentRecord.
For each element argName of the BoundNames of CatchParameter, do
1. Perform ! catchEnvRec.CreateMutableBinding(argName, false).
Set the running execution context's LexicalEnvironment to catchEnv.
Let status be the result of performing BindingInitialization for CatchParameter passing thrownValue and catchEnv as arguments.
If status is an abrupt completion, then
1. Set the running execution context's LexicalEnvironment to oldEnv.
2. Return Completion(status).
Let B be the result of evaluating Block.
Set the running execution context's LexicalEnvironment to oldEnv.
Return Completion(B).

Note

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

13.15.8 Runtime Semantics: Evaluation

try

Let B be the result of evaluating Block.
If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
Else, let C be B.
Return Completion(UpdateEmpty(C, undefined)).

try

Let B be the result of evaluating Block.
Let F be the result of evaluating Finally.
If F.[[Type]] is normal, set F to B.
Return Completion(UpdateEmpty(F, undefined)).

try

Let B be the result of evaluating Block.
If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
Else, let C be B.
Let F be the result of evaluating Finally.
If F.[[Type]] is normal, set F to C.
Return Completion(UpdateEmpty(F, undefined)).

13.16 The `debugger` Statement

Syntax

DebuggerStatement

debugger

;

13.16.1 Runtime Semantics: Evaluation

Note

Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

DebuggerStatement

debugger

;

If an implementation-defined debugging facility is available and enabled, then
1. Perform an implementation-defined debugging action.
2. Let result be an implementation-defined Completion value.
Else,
1. Let result be NormalCompletion(empty).
Return result.

14 ECMAScript Language: Functions and Classes

Note

Various ECMAScript language elements cause the creation of ECMAScript function objects (9.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (9.2.1).

14.1 Function Definitions

Syntax

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[+Default]

function

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

function

[~Yield, ~Await]

opt

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[empty]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await, +Return]

opt

14.1.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatements occurring as the initial StatementListItems or ModuleItems of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact code unit sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

Note

The ExpressionStatements of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatements which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.

14.1.2 Static Semantics: Early Errors

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

If the source code matching this production is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If the source code matching this production is strict mode code, it is a Syntax Error if BindingIdentifier is present and the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if FunctionBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if FunctionBody Contains SuperCall is true.

Note 1

The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound using var or function declarations.

It is a Syntax Error if BoundNames of FormalParameters contains any duplicate elements.

It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false and BoundNames of FormalParameterList contains any duplicate elements.

Note 2

Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions which have simple parameter lists and which are not defined in strict mode code.

It is a Syntax Error if the LexicallyDeclaredNames of FunctionStatementList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of FunctionStatementList also occurs in the VarDeclaredNames of FunctionStatementList.
It is a Syntax Error if ContainsDuplicateLabels of FunctionStatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of FunctionStatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of FunctionStatementList with arguments « » and « » is true.

14.1.3 Static Semantics: BoundNames

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

[empty]

Return a new empty List.

Let names be BoundNames of FormalParameterList.
Append to names the BoundNames of FunctionRestParameter.
Return names.

Let names be BoundNames of FormalParameterList.
Append to names the BoundNames of FormalParameter.
Return names.

14.1.4 Static Semantics: Contains

With parameter symbol.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.1.5 Static Semantics: ContainsExpression

[empty]

Return false.

If ContainsExpression of FormalParameterList is true, return true.
Return ContainsExpression of FunctionRestParameter.

If ContainsExpression of FormalParameterList is true, return true.
Return ContainsExpression of FormalParameter.

14.1.6 Static Semantics: ContainsUseStrict

If the Directive Prologue of FunctionStatementList contains a Use Strict Directive, return true; otherwise, return false.

14.1.7 Static Semantics: ExpectedArgumentCount

[empty]

Return 0.

Return ExpectedArgumentCount of FormalParameterList.

Note

The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.

Let count be ExpectedArgumentCount of FormalParameterList.
If HasInitializer of FormalParameterList is true or HasInitializer of FormalParameter is true, return count.
Return count + 1.

14.1.8 Static Semantics: HasInitializer

If HasInitializer of FormalParameterList is true, return true.
Return HasInitializer of FormalParameter.

14.1.9 Static Semantics: HasName

function

(

)

{

}

Return false.

function

(

)

{

}

Return true.

14.1.10 Static Semantics: IsAnonymousFunctionDefinition ( `expr` )

The abstract operation IsAnonymousFunctionDefinition determines if its argument is a function definition that does not bind a name. The argument expr is the result of parsing an AssignmentExpression or Initializer. The following steps are taken:

If IsFunctionDefinition of expr is false, return false.
Let hasName be the result of HasName of expr.
If hasName is true, return false.
Return true.

14.1.11 Static Semantics: IsConstantDeclaration

function

(

)

{

}

function

(

)

{

}

Return false.

14.1.12 Static Semantics: IsFunctionDefinition

function

opt

(

)

{

}

Return true.

14.1.13 Static Semantics: IsSimpleParameterList

[empty]

Return true.

Return false.

If IsSimpleParameterList of FormalParameterList is false, return false.
Return IsSimpleParameterList of FormalParameter.

Return IsSimpleParameterList of BindingElement.

14.1.14 Static Semantics: LexicallyDeclaredNames

[empty]

Return a new empty List.

Return TopLevelLexicallyDeclaredNames of StatementList.

14.1.15 Static Semantics: LexicallyScopedDeclarations

[empty]

Return a new empty List.

Return the TopLevelLexicallyScopedDeclarations of StatementList.

14.1.16 Static Semantics: VarDeclaredNames

[empty]

Return a new empty List.

Return TopLevelVarDeclaredNames of StatementList.

14.1.17 Static Semantics: VarScopedDeclarations

[empty]

Return a new empty List.

Return the TopLevelVarScopedDeclarations of StatementList.

14.1.18 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Return the result of evaluating FunctionStatementList.

14.1.19 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note 1

[empty]

Return NormalCompletion(empty).

Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
Return the result of performing IteratorBindingInitialization for FunctionRestParameter using iteratorRecord and environment as the arguments.

Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
Return the result of performing IteratorBindingInitialization for FormalParameter using iteratorRecord and environment as the arguments.

If ContainsExpression of BindingElement is false, return the result of performing IteratorBindingInitialization for BindingElement using iteratorRecord and environment as the arguments.
Let currentContext be the running execution context.
Let originalEnv be the VariableEnvironment of currentContext.
Assert: The VariableEnvironment and LexicalEnvironment of currentContext are the same.
Assert: environment and originalEnv are the same.
Let paramVarEnv be NewDeclarativeEnvironment(originalEnv).
Set the VariableEnvironment of currentContext to paramVarEnv.
Set the LexicalEnvironment of currentContext to paramVarEnv.
Let result be the result of performing IteratorBindingInitialization for BindingElement using iteratorRecord and environment as the arguments.
Set the VariableEnvironment of currentContext to originalEnv.
Set the LexicalEnvironment of currentContext to originalEnv.
Return result.

Note 2

The new Environment Record created in step 6 is only used if the BindingElement contains a direct eval.

If ContainsExpression of BindingRestElement is false, return the result of performing IteratorBindingInitialization for BindingRestElement using iteratorRecord and environment as the arguments.
Let currentContext be the running execution context.
Let originalEnv be the VariableEnvironment of currentContext.
Assert: The VariableEnvironment and LexicalEnvironment of currentContext are the same.
Assert: environment and originalEnv are the same.
Let paramVarEnv be NewDeclarativeEnvironment(originalEnv).
Set the VariableEnvironment of currentContext to paramVarEnv.
Set the LexicalEnvironment of currentContext to paramVarEnv.
Let result be the result of performing IteratorBindingInitialization for BindingRestElement using iteratorRecord and environment as the arguments.
Set the VariableEnvironment of currentContext to originalEnv.
Set the LexicalEnvironment of currentContext to originalEnv.
Return result.

Note 3

The new Environment Record created in step 6 is only used if the BindingRestElement contains a direct eval.

14.1.20 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

function

(

)

{

}

If the function code for FunctionDeclaration is strict mode code, let strict be true. Otherwise let strict be false.
Let name be StringValue of BindingIdentifier.
Let F be FunctionCreate(Normal, FormalParameters, FunctionBody, scope, strict).
Perform MakeConstructor(F).
Perform SetFunctionName(F, name).
Return F.

function

(

)

{

}

Let F be FunctionCreate(Normal, FormalParameters, FunctionBody, scope, true).
Perform MakeConstructor(F).
Perform SetFunctionName(F, "default").
Return F.

Note

An anonymous FunctionDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

14.1.21 Runtime Semantics: Evaluation

function

(

)

{

}

Return NormalCompletion(empty).

Note 1

An alternative semantics is provided in B.3.3.

function

(

)

{

}

Return NormalCompletion(empty).

function

(

)

{

}

If the function code for FunctionExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let closure be FunctionCreate(Normal, FormalParameters, FunctionBody, scope, strict).
Perform MakeConstructor(closure).
Return closure.

function

(

)

{

}

If the function code for FunctionExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be NewDeclarativeEnvironment(scope).
Let envRec be funcEnv's EnvironmentRecord.
Let name be StringValue of BindingIdentifier.
Perform envRec.CreateImmutableBinding(name, false).
Let closure be FunctionCreate(Normal, FormalParameters, FunctionBody, funcEnv, strict).
Perform MakeConstructor(closure).
Perform SetFunctionName(closure, name).
Perform envRec.InitializeBinding(name, closure).
Return closure.

Note 2

The BindingIdentifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

Note 3

A prototype property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.

[empty]

Return NormalCompletion(undefined).

14.2 Arrow Function Definitions

Syntax

[In, Yield, Await]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[In]

[lookahead ≠ {]

[?In, ~Yield, ~Await]

{

[~Yield, ~Await]

}

Supplemental Syntax

When the production
ArrowParameters[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
is recognized the following grammar is used to refine the interpretation of CoverParenthesizedExpressionAndArrowParameterList:

ArrowFormalParameters

[Yield, Await]

(

[?Yield, ?Await]

)

14.2.1 Static Semantics: Early Errors

It is a Syntax Error if ArrowParameters Contains YieldExpression is true.
It is a Syntax Error if ArrowParameters Contains AwaitExpression is true.
It is a Syntax Error if ContainsUseStrict of ConciseBody is true and IsSimpleParameterList of ArrowParameters is false.
It is a Syntax Error if any element of the BoundNames of ArrowParameters also occurs in the LexicallyDeclaredNames of ConciseBody.

It is a Syntax Error if CoverParenthesizedExpressionAndArrowParameterList is not covering an ArrowFormalParameters.
All early error rules for ArrowFormalParameters and its derived productions also apply to CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.

14.2.2 Static Semantics: BoundNames

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return the BoundNames of formals.

14.2.3 Static Semantics: Contains

With parameter symbol.

If symbol is not one of NewTarget, SuperProperty, SuperCall, super or this, return false.
If ArrowParameters Contains symbol is true, return true.
Return ConciseBody Contains symbol.

Note

Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an ArrowFunction.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return formals Contains symbol.

14.2.4 Static Semantics: ContainsExpression

Return false.

14.2.5 Static Semantics: ContainsUseStrict

Return false.

14.2.6 Static Semantics: ExpectedArgumentCount

Return 1.

14.2.7 Static Semantics: HasName

Return false.

14.2.8 Static Semantics: IsSimpleParameterList

Return true.

Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
Return IsSimpleParameterList of formals.

14.2.9 Static Semantics: CoveredFormalsList

Return this ArrowParameters.

(

)

(

)

(

...

)

(

...

)

(

...

)

(

...

)

Return the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.

14.2.10 Static Semantics: LexicallyDeclaredNames

Return a new empty List.

14.2.11 Static Semantics: LexicallyScopedDeclarations

Return a new empty List.

14.2.12 Static Semantics: VarDeclaredNames

Return a new empty List.

14.2.13 Static Semantics: VarScopedDeclarations

Return a new empty List.

14.2.14 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

Assert: iteratorRecord.[[Done]] is false.
Let next be IteratorStep(iteratorRecord).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
ReturnIfAbrupt(next).
If next is false, set iteratorRecord.[[Done]] to true.
Else,
1. Let v be IteratorValue(next).
2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.

14.2.15 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Let exprRef be the result of evaluating AssignmentExpression.
Let exprValue be ? GetValue(exprRef).
Return Completion { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

14.2.16 Runtime Semantics: Evaluation

If the function code for this ArrowFunction is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let parameters be CoveredFormalsList of ArrowParameters.
Let closure be FunctionCreate(Arrow, parameters, ConciseBody, scope, strict).
Return closure.

Note

An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction.

14.3 Method Definitions

Syntax

[Yield, Await]

[?Yield, ?Await]

(

[~Yield, ~Await]

)

{

[~Yield, ~Await]

}

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

get

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

set

[?Yield, ?Await]

(

)

{

[~Yield, ~Await]

}

[~Yield, ~Await]

14.3.1 Static Semantics: Early Errors

(

)

{

}

It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.

set

(

)

{

}

It is a Syntax Error if BoundNames of PropertySetParameterList contains any duplicate elements.
It is a Syntax Error if ContainsUseStrict of FunctionBody is true and IsSimpleParameterList of PropertySetParameterList is false.
It is a Syntax Error if any element of the BoundNames of PropertySetParameterList also occurs in the LexicallyDeclaredNames of FunctionBody.

14.3.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

(

)

{

}

get

(

)

{

}

set

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.3.3 Static Semantics: ExpectedArgumentCount

If HasInitializer of FormalParameter is true, return 0.
Return 1.

14.3.4 Static Semantics: HasDirectSuper

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return FunctionBody Contains SuperCall.

get

(

)

{

}

Return FunctionBody Contains SuperCall.

set

(

)

{

}

If PropertySetParameterList Contains SuperCall is true, return true.
Return FunctionBody Contains SuperCall.

14.3.5 Static Semantics: PropName

(

)

{

}

get

(

)

{

}

set

(

)

{

}

Return PropName of PropertyName.

14.3.6 Static Semantics: SpecialMethod

(

)

{

}

Return false.

get

(

)

{

}

set

(

)

{

}

Return true.

14.3.7 Runtime Semantics: DefineMethod

With parameters object and optional parameter functionPrototype.

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this MethodDefinition is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
If functionPrototype is present as a parameter, then
1. Let kind be Normal.
2. Let prototype be functionPrototype.
Else,
1. Let kind be Method.
2. Let prototype be the intrinsic object %FunctionPrototype%.
Let closure be FunctionCreate(kind, UniqueFormalParameters, FunctionBody, scope, strict, prototype).
Perform MakeMethod(closure, object).
Return the Record { [[Key]]: propKey, [[Closure]]: closure }.

14.3.8 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

(

)

{

}

Let methodDef be DefineMethod of MethodDefinition with argument object.
ReturnIfAbrupt(methodDef).
Perform SetFunctionName(methodDef.[[Closure]], methodDef.[[Key]]).
Let desc be the PropertyDescriptor { [[Value]]: methodDef.[[Closure]], [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, methodDef.[[Key]], desc).

get

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this MethodDefinition is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let formalParameterList be an instance of the production FormalParameters:[empty] .
Let closure be FunctionCreate(Method, formalParameterList, FunctionBody, scope, strict).
Perform MakeMethod(closure, object).
Perform SetFunctionName(closure, propKey, "get").
Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

set

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this MethodDefinition is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let closure be FunctionCreate(Method, PropertySetParameterList, FunctionBody, scope, strict).
Perform MakeMethod(closure, object).
Perform SetFunctionName(closure, propKey, "set").
Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.4 Generator Function Definitions

Syntax

[Yield, Await]

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[Yield, Await, Default]

function

[?Yield, ?Await]

(

[+Yield, ~Await]

)

{

}

[+Default]

function

(

[+Yield, ~Await]

)

{

}

function

[+Yield, ~Await]

opt

(

[+Yield, ~Await]

)

{

}

[+Yield, ~Await]

[In, Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

[?In, +Yield, ?Await]

Note 1

The syntactic context immediately following yield requires use of the InputElementRegExpOrTemplateTail lexical goal.

Note 2

YieldExpression cannot be used within the FormalParameters of a generator function because any expressions that are part of FormalParameters are evaluated before the resulting generator object is in a resumable state.

Note 3

Abstract operations relating to generator objects are defined in 25.4.3.

14.4.1 Static Semantics: Early Errors

(

)

{

}

It is a Syntax Error if HasDirectSuper of GeneratorMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
It is a Syntax Error if ContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

If the source code matching this production is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If the source code matching this production is strict mode code, it is a Syntax Error if BindingIdentifier is present and the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if ContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.
It is a Syntax Error if FormalParameters Contains YieldExpression is true.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if GeneratorBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if GeneratorBody Contains SuperCall is true.

14.4.2 Static Semantics: BoundNames

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.4.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.4.4 Static Semantics: Contains

With parameter symbol.

function

(

)

{

}

function

(

)

{

}

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.4.5 Static Semantics: HasDirectSuper

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return GeneratorBody Contains SuperCall.

14.4.6 Static Semantics: HasName

function

(

)

{

}

Return false.

function

(

)

{

}

Return true.

14.4.7 Static Semantics: IsConstantDeclaration

function

(

)

{

}

function

(

)

{

}

Return false.

14.4.8 Static Semantics: IsFunctionDefinition

function

opt

(

)

{

}

Return true.

14.4.9 Static Semantics: PropName

(

)

{

}

Return PropName of PropertyName.

14.4.10 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Let G be ? OrdinaryCreateFromConstructor(functionObject, "%GeneratorPrototype%", « [[GeneratorState]], [[GeneratorContext]] »).
Perform GeneratorStart(G, FunctionBody).
Return Completion { [[Type]]: return, [[Value]]: G, [[Target]]: empty }.

14.4.11 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

function

(

)

{

}

If the function code for GeneratorDeclaration is strict mode code, let strict be true. Otherwise let strict be false.
Let name be StringValue of BindingIdentifier.
Let F be GeneratorFunctionCreate(Normal, FormalParameters, GeneratorBody, scope, strict).
Let prototype be ObjectCreate(%GeneratorPrototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetFunctionName(F, name).
Return F.

function

(

)

{

}

Let F be GeneratorFunctionCreate(Normal, FormalParameters, GeneratorBody, scope, true).
Let prototype be ObjectCreate(%GeneratorPrototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetFunctionName(F, "default").
Return F.

Note

An anonymous GeneratorDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

14.4.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this GeneratorMethod is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let closure be GeneratorFunctionCreate(Method, UniqueFormalParameters, GeneratorBody, scope, strict).
Perform MakeMethod(closure, object).
Let prototype be ObjectCreate(%GeneratorPrototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetFunctionName(closure, propKey).
Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.4.13 Runtime Semantics: Evaluation

function

(

)

{

}

If the function code for this GeneratorExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let closure be GeneratorFunctionCreate(Normal, FormalParameters, GeneratorBody, scope, strict).
Let prototype be ObjectCreate(%GeneratorPrototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return closure.

function

(

)

{

}

If the function code for this GeneratorExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be NewDeclarativeEnvironment(scope).
Let envRec be funcEnv's EnvironmentRecord.
Let name be StringValue of BindingIdentifier.
Perform envRec.CreateImmutableBinding(name, false).
Let closure be GeneratorFunctionCreate(Normal, FormalParameters, GeneratorBody, funcEnv, strict).
Let prototype be ObjectCreate(%GeneratorPrototype%).
Perform DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetFunctionName(closure, name).
Perform envRec.InitializeBinding(name, closure).
Return closure.

Note

The BindingIdentifier in a GeneratorExpression can be referenced from inside the GeneratorExpression's FunctionBody to allow the generator code to call itself recursively. However, unlike in a GeneratorDeclaration, the BindingIdentifier in a GeneratorExpression cannot be referenced from and does not affect the scope enclosing the GeneratorExpression.

yield

Let generatorKind be ! GetGeneratorKind().
If generatorKind is async, then return ? AsyncGeneratorYield(undefined).
Otherwise, return ? GeneratorYield(CreateIterResultObject(undefined, false)).

yield

Let generatorKind be ! GetGeneratorKind().
Let exprRef be the result of evaluating AssignmentExpression.
Let value be ? GetValue(exprRef).
If generatorKind is async, then return ? AsyncGeneratorYield(value).
Otherwise, return ? GeneratorYield(CreateIterResultObject(value, false)).

yield

Let generatorKind be ! GetGeneratorKind().
Let exprRef be the result of evaluating AssignmentExpression.
Let value be ? GetValue(exprRef).
Let iteratorRecord be ? GetIterator(value, generatorKind).
Let iterator be iteratorRecord.[[Iterator]].
Let received be NormalCompletion(undefined).
Repeat,
1. If received.[[Type]] is normal, then
  1. Let innerResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »).
  2. If generatorKind is async, then set innerResult to ? Await(innerResult).
  3. If Type(innerResult) is not Object, throw a TypeError exception.
  4. Let done be ? IteratorComplete(innerResult).
  5. If done is true, then
    1. Return ? IteratorValue(innerResult).
  6. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerResult)).
  7. Else, set received to GeneratorYield(innerResult).
2. Else if received.[[Type]] is throw, then
  1. Let throw be ? GetMethod(iterator, "throw").
  2. If throw is not undefined, then
    1. Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
    2. If generatorKind is async, then set innerResult to ? Await(innerResult).
    3. NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
    4. If Type(innerResult) is not Object, throw a TypeError exception.
    5. Let done be ? IteratorComplete(innerResult).
    6. If done is true, then
      1. Return ? IteratorValue(innerResult).
    7. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerResult)).
    8. Else, set received to GeneratorYield(innerResult).
  3. Else,
    1. NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
    2. Let closeCompletion be Completion { [[Type]]: normal, [[Value]]: empty, [[Target]]: empty }.
    3. If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
    4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
    5. NOTE: The next step throws a TypeError to indicate that there was a yield* protocol violation: iterator does not have a throw method.
    6. Throw a TypeError exception.
3. Else,
  1. Assert: received.[[Type]] is return.
  2. Let return be ? GetMethod(iterator, "return").
  3. If return is undefined, then
    1. If generatorKind is async, then set received.[[Value]] to ? Await(received.[[Value]]).
    2. Return Completion(received).
  4. Let innerReturnResult be ? Call(return, iterator, « received.[[Value]] »).
  5. If generatorKind is async, then set innerReturnResult to ? Await(innerReturnResult).
  6. If Type(innerReturnResult) is not Object, throw a TypeError exception.
  7. Let done be ? IteratorComplete(innerReturnResult).
  8. If done is true, then
    1. Let value be ? IteratorValue(innerReturnResult).
    2. Return Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
  9. If generatorKind is async, then set received to AsyncGeneratorYield(? IteratorValue(innerReturnResult)).
  10. Else, set received to GeneratorYield(innerReturnResult).

14.5 Async Generator Function Definitions

Syntax

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[+Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[+Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[+Yield, +Await]

opt

(

[+Yield, +Await]

)

{

}

[+Yield, +Await]

Note 1

YieldExpression and AwaitExpression cannot be used within the FormalParameters of an async generator function because any expressions that are part of FormalParameters are evaluated before the resulting async generator object is in a resumable state.

Note 2

Abstract operations relating to async generator objects are defined in 25.5.3.

14.5.1 Static Semantics: Early Errors

async

(

)

{

}

It is a Syntax Error if HasDirectSuper of AsyncGeneratorMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is true.
It is a Syntax Error if ContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

opt

(

)

{

}

If the source code matching this production is strict mode code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If the source code matching this production is strict mode code, it is a Syntax Error if BindingIdentifier is the IdentifierName eval or the IdentifierName arguments.
It is a Syntax Error if ContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.
It is a Syntax Error if FormalParameters Contains YieldExpression is true.
It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if AsyncGeneratorBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if AsyncGeneratorBody Contains SuperCall is true.

14.5.2 Static Semantics: BoundNames

async

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

async

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.5.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

async

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.5.4 Static Semantics: Contains

With parameter symbol.

async

function

(

)

{

}

async

function

(

)

{

}

async

function

opt

(

)

{

}

Return false.

Note

Static semantic rules that depend upon substructure generally do not look into function definitions.

14.5.5 Static Semantics: HasDirectSuper

async

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return AsyncGeneratorBody Contains SuperCall.

14.5.6 Static Semantics: HasName

async

function

(

)

{

}

Return false.

async

function

(

)

{

}

Return true.

14.5.7 Static Semantics: IsConstantDeclaration

async

function

(

)

{

}

async

function

(

)

{

}

Return false.

14.5.8 Static Semantics: IsFunctionDefinition

async

function

opt

(

)

{

}

Return true.

14.5.9 Static Semantics: PropName

async

(

)

{

}

Return PropName of PropertyName.

14.5.10 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
Let generator be ? OrdinaryCreateFromConstructor(functionObject, "%AsyncGeneratorPrototype%", « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]] »).
Perform ! AsyncGeneratorStart(generator, FunctionBody).
Return Completion { [[Type]]: return, [[Value]]: generator, [[Target]]: empty }.

14.5.11 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncGeneratorDeclaration is strict mode code, let strict be true. Otherwise let strict be false.
Let name be StringValue of BindingIdentifier.
Let F be ! AsyncGeneratorFunctionCreate(Normal, FormalParameters, AsyncGeneratorBody, scope, strict).
Let prototype be ! ObjectCreate(%AsyncGeneratorPrototype%).
Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform ! SetFunctionName(F, name).
Return F.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncGeneratorDeclaration is strict mode code, let strict be true. Otherwise let strict be false.
Let F be AsyncGeneratorFunctionCreate(Normal, FormalParameters, AsyncGeneratorBody, scope, strict).
Let prototype be ObjectCreate(%AsyncGeneratorPrototype%).
Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform SetFunctionName(F, "default").
Return F.

Note

An anonymous AsyncGeneratorDeclaration can only occur as part of an export default declaration.

14.5.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameter object and enumerable.

async

[no LineTerminator here]

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this AsyncGeneratorMethod is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let closure be ! AsyncGeneratorFunctionCreate(Method, UniqueFormalParameters, AsyncGeneratorBody, scope, strict).
Perform ! MakeMethod(closure, object).
Let prototype be ! ObjectCreate(%AsyncGeneratorPrototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform ! SetFunctionName(closure, propKey).
Let desc be PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.5.13 Runtime Semantics: Evaluation

async

[no LineTerminator here]

function

(

)

{

}

If the function code for this AsyncGeneratorExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let closure be ! AsyncGeneratorFunctionCreate(Normal, FormalParameters, AsyncGeneratorBody, scope, strict).
Let prototype be ! ObjectCreate(%AsyncGeneratorPrototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return closure.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for this AsyncGeneratorExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the running execution context's LexicalEnvironment.
Let funcEnv be ! NewDeclarativeEnvironment(scope).
Let envRec be funcEnv's EnvironmentRecord.
Let name be StringValue of BindingIdentifier.
Perform ! envRec.CreateImmutableBinding(name).
Let closure be ! AsyncGeneratorFunctionCreate(Normal, FormalParameters, AsyncGeneratorBody, funcEnv, strict).
Let prototype be ! ObjectCreate(%AsyncGeneratorPrototype%).
Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Perform ! SetFunctionName(closure, name).
Perform ! envRec.InitializeBinding(name, closure).
Return closure.

Note

The BindingIdentifier in an AsyncGeneratorExpression can be referenced from inside the AsyncGeneratorExpression's AsyncGeneratorBody to allow the generator code to call itself recursively. However, unlike in an AsyncGeneratorDeclaration, the BindingIdentifier in an AsyncGeneratorExpression cannot be referenced from and does not affect the scope enclosing the AsyncGeneratorExpression.

14.6 Class Definitions

Syntax

[Yield, Await, Default]

class

[?Yield, ?Await]

[?Yield, ?Await]

[+Default]

class

[?Yield, ?Await]

[Yield, Await]

class

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

opt

{

[?Yield, ?Await]

opt

}

[Yield, Await]

extends

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

static

[?Yield, ?Await]

;

Note

A class definition is always strict mode code.

14.6.1 Static Semantics: Early Errors

opt

{

}

It is a Syntax Error if ClassHeritage is not present and the following algorithm evaluates to true:
1. Let constructor be ConstructorMethod of ClassBody.
2. If constructor is empty, return false.
3. Return HasDirectSuper of constructor.

It is a Syntax Error if PrototypePropertyNameList of ClassElementList contains more than one occurrence of "constructor".

It is a Syntax Error if PropName of MethodDefinition is not "constructor" and HasDirectSuper of MethodDefinition is true.
It is a Syntax Error if PropName of MethodDefinition is "constructor" and SpecialMethod of MethodDefinition is true.

static

It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
It is a Syntax Error if PropName of MethodDefinition is "prototype".

14.6.2 Static Semantics: BoundNames

class

Return the BoundNames of BindingIdentifier.

class

Return « "*default*" ».

14.6.3 Static Semantics: ConstructorMethod

If ClassElement is ClassElement:; , return empty.
If IsStatic of ClassElement is true, return empty.
If PropName of ClassElement is not "constructor", return empty.
Return ClassElement.

Let head be ConstructorMethod of ClassElementList.
If head is not empty, return head.
If ClassElement is ClassElement:; , return empty.
If IsStatic of ClassElement is true, return empty.
If PropName of ClassElement is not "constructor", return empty.
Return ClassElement.

Note

Early Error rules ensure that there is only one method definition named "constructor" and that it is not an accessor property or generator definition.

14.6.4 Static Semantics: Contains

With parameter symbol.

opt

{

}

If symbol is ClassBody, return true.
If symbol is ClassHeritage, then
1. If ClassHeritage is present, return true; otherwise return false.
Let inHeritage be ClassHeritage Contains symbol.
If inHeritage is true, return true.
Return the result of ComputedPropertyContains for ClassBody with argument symbol.

Note

Static semantic rules that depend upon substructure generally do not look into class bodies except for PropertyNames.

14.6.5 Static Semantics: ComputedPropertyContains

With parameter symbol.

Let inList be the result of ComputedPropertyContains for ClassElementList with argument symbol.
If inList is true, return true.
Return the result of ComputedPropertyContains for ClassElement with argument symbol.

Return the result of ComputedPropertyContains for MethodDefinition with argument symbol.

static

Return the result of ComputedPropertyContains for MethodDefinition with argument symbol.

;

Return false.

14.6.6 Static Semantics: HasName

class

Return false.

class

Return true.

14.6.7 Static Semantics: IsConstantDeclaration

class

class

Return false.

14.6.8 Static Semantics: IsFunctionDefinition

class

opt

Return true.

14.6.9 Static Semantics: IsStatic

Return false.

static

Return true.

;

Return false.

14.6.10 Static Semantics: NonConstructorMethodDefinitions

If ClassElement is ClassElement:; , return a new empty List.
If IsStatic of ClassElement is false and PropName of ClassElement is "constructor", return a new empty List.
Return a List containing ClassElement.

Let list be NonConstructorMethodDefinitions of ClassElementList.
If ClassElement is ClassElement:; , return list.
If IsStatic of ClassElement is false and PropName of ClassElement is "constructor", return list.
Append ClassElement to the end of list.
Return list.

14.6.11 Static Semantics: PrototypePropertyNameList

If PropName of ClassElement is empty, return a new empty List.
If IsStatic of ClassElement is true, return a new empty List.
Return a List containing PropName of ClassElement.

Let list be PrototypePropertyNameList of ClassElementList.
If PropName of ClassElement is empty, return list.
If IsStatic of ClassElement is true, return list.
Append PropName of ClassElement to the end of list.
Return list.

14.6.12 Static Semantics: PropName

;

Return empty.

14.6.13 Runtime Semantics: ClassDefinitionEvaluation

With parameter className.

opt

{

opt

}

Let lex be the LexicalEnvironment of the running execution context.
Let classScope be NewDeclarativeEnvironment(lex).
Let classScopeEnvRec be classScope's EnvironmentRecord.
If className is not undefined, then
1. Perform classScopeEnvRec.CreateImmutableBinding(className, true).
If ClassHeritageopt is not present, then
1. Let protoParent be the intrinsic object %ObjectPrototype%.
2. Let constructorParent be the intrinsic object %FunctionPrototype%.
Else,
1. Set the running execution context's LexicalEnvironment to classScope.
2. Let superclass be the result of evaluating ClassHeritage.
3. Set the running execution context's LexicalEnvironment to lex.
4. ReturnIfAbrupt(superclass).
5. If superclass is null, then
  1. Let protoParent be null.
  2. Let constructorParent be the intrinsic object %FunctionPrototype%.
6. Else if IsConstructor(superclass) is false, throw a TypeError exception.
7. Else,
  1. Let protoParent be ? Get(superclass, "prototype").
  2. If Type(protoParent) is neither Object nor Null, throw a TypeError exception.
  3. Let constructorParent be superclass.
Let proto be ObjectCreate(protoParent).
If ClassBodyopt is not present, let constructor be empty.
Else, let constructor be ConstructorMethod of ClassBody.
If constructor is empty, then
1. If ClassHeritageopt is present, then
  1. Set constructor to the result of parsing the source text
```
constructor(... args){ super (...args);}
```
    using the syntactic grammar with the goal symbol MethodDefinition[~Yield, ~Await].
2. Else,
  1. Set constructor to the result of parsing the source text
```
constructor(){ }
```
    using the syntactic grammar with the goal symbol MethodDefinition[~Yield, ~Await].
Set the running execution context's LexicalEnvironment to classScope.
Let constructorInfo be the result of performing DefineMethod for constructor with arguments proto and constructorParent as the optional functionPrototype argument.
Assert: constructorInfo is not an abrupt completion.
Let F be constructorInfo.[[Closure]].
If ClassHeritageopt is present, set F.[[ConstructorKind]] to "derived".
Perform MakeConstructor(F, false, proto).
Perform MakeClassConstructor(F).
Perform CreateMethodProperty(proto, "constructor", F).
If ClassBodyopt is not present, let methods be a new empty List.
Else, let methods be NonConstructorMethodDefinitions of ClassBody.
For each ClassElement m in order from methods, do
1. If IsStatic of m is false, then
  1. Let status be the result of performing PropertyDefinitionEvaluation for m with arguments proto and false.
2. Else,
  1. Let status be the result of performing PropertyDefinitionEvaluation for m with arguments F and false.
3. If status is an abrupt completion, then
  1. Set the running execution context's LexicalEnvironment to lex.
  2. Return Completion(status).
Set the running execution context's LexicalEnvironment to lex.
If className is not undefined, then
1. Perform classScopeEnvRec.InitializeBinding(className, F).
Return F.

14.6.14 Runtime Semantics: BindingClassDeclarationEvaluation

class

Let className be StringValue of BindingIdentifier.
Let value be the result of ClassDefinitionEvaluation of ClassTail with argument className.
ReturnIfAbrupt(value).
Let hasNameProperty be ? HasOwnProperty(value, "name").
If hasNameProperty is false, perform SetFunctionName(value, className).
Let env be the running execution context's LexicalEnvironment.
Perform ? InitializeBoundName(className, value, env).
Return value.

class

Return the result of ClassDefinitionEvaluation of ClassTail with argument undefined.

Note

ClassDeclaration:classClassTail only occurs as part of an ExportDeclaration and the setting of a name property and establishing its binding are handled as part of the evaluation action for that production. See 15.2.3.11.

14.6.15 Runtime Semantics: Evaluation

class

Perform ? BindingClassDeclarationEvaluation of this ClassDeclaration.
Return NormalCompletion(empty).

Note 1

ClassDeclaration:classClassTail only occurs as part of an ExportDeclaration and is never directly evaluated.

class

opt

If BindingIdentifieropt is not present, let className be undefined.
Else, let className be StringValue of BindingIdentifier.
Let value be the result of ClassDefinitionEvaluation of ClassTail with argument className.
ReturnIfAbrupt(value).
If className is not undefined, then
1. Let hasNameProperty be ? HasOwnProperty(value, "name").
2. If hasNameProperty is false, then
  1. Perform SetFunctionName(value, className).
Return NormalCompletion(value).

Note 2

If the class definition included a name static method then that method is not over-written with a name data property for the class name.

14.7 Async Function Definitions

Syntax

[Yield, Await, Default]

async

[no LineTerminator here]

function

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[+Default]

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

(

[~Yield, +Await]

)

{

}

async

[no LineTerminator here]

function

[~Yield, +Await]

(

[~Yield, +Await]

)

{

}

[Yield, Await]

async

[no LineTerminator here]

[?Yield, ?Await]

(

[~Yield, +Await]

)

{

}

[~Yield, +Await]

[Yield]

await

[?Yield, +Await]

Note 1

await is parsed as an AwaitExpression when the _[Await] parameter is present. The _[Await] parameter is present in the following contexts:

In an AsyncFunctionBody.
In the FormalParameters of an AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, or AsyncGeneratorExpression. AwaitExpression in this position is a Syntax error via static semantics.

When Module is the syntactic goal symbol and the _[Await] parameter is absent, await is parsed as a keyword and will be a Syntax error. When Script is the syntactic goal symbol, await may be parsed as an identifier when the _[Await] parameter is absent. This includes the following contexts:

Anywhere outside of an AsyncFunctionBody or FormalParameters of an AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, or AsyncGeneratorExpression.
In the BindingIdentifier of a FunctionExpression, GeneratorExpression, or AsyncGeneratorExpression.

Note 2

Unlike YieldExpression, it is a Syntax Error to omit the operand of an AwaitExpression. You must await something.

14.7.1 Static Semantics: Early Errors

async

[no LineTerminator here]

(

)

{

}

It is a Syntax Error if ContainsUseStrict of AsyncFunctionBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
It is a Syntax Error if HasDirectSuper of AsyncMethod is true.
It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is true.
It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of AsyncFunctionBody.

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

It is a Syntax Error if ContainsUseStrict of AsyncFunctionBody is true and IsSimpleParameterList of FormalParameters is false.
It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
If the source code matching this production is strict code, the Early Error rules for UniqueFormalParameters:FormalParameters are applied.
If the source code matching this production is strict code, it is a Syntax Error if BindingIdentifier is present and the StringValue of BindingIdentifier is "eval" or "arguments".
It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of AsyncFunctionBody.
It is a Syntax Error if FormalParameters Contains SuperProperty is true.
It is a Syntax Error if AsyncFunctionBody Contains SuperProperty is true.
It is a Syntax Error if FormalParameters Contains SuperCall is true.
It is a Syntax Error if AsyncFunctionBody Contains SuperCall is true.

14.7.2 Static Semantics: BoundNames

async

[no LineTerminator here]

function

(

)

{

}

Return the BoundNames of BindingIdentifier.

async

[no LineTerminator here]

function

(

)

{

}

Return « "*default*" ».

Note

"*default*" is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

14.7.3 Static Semantics: ComputedPropertyContains

With parameter symbol.

async

[no LineTerminator here]

(

)

{

}

Return the result of ComputedPropertyContains for PropertyName with argument symbol.

14.7.4 Static Semantics: Contains

With parameter symbol.

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

Return false.

14.7.5 Static Semantics: HasDirectSuper

async

[no LineTerminator here]

(

)

{

}

If UniqueFormalParameters Contains SuperCall is true, return true.
Return AsyncFunctionBody Contains SuperCall.

14.7.6 Static Semantics: HasName

async

[no LineTerminator here]

function

(

)

{

}

Return false.

async

[no LineTerminator here]

function

(

)

{

}

Return true.

14.7.7 Static Semantics: IsConstantDeclaration

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

Return false.

14.7.8 Static Semantics: IsFunctionDefinition

async

[no LineTerminator here]

function

(

)

{

}

async

[no LineTerminator here]

function

(

)

{

}

Return true.

14.7.9 Static Semantics: PropName

async

[no LineTerminator here]

(

)

{

}

Return PropName of PropertyName.

14.7.10 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncFunctionDeclaration is strict mode code, let strict be true. Otherwise, let strict be false.
Let name be StringValue of BindingIdentifier.
Let F be ! AsyncFunctionCreate(Normal, FormalParameters, AsyncFunctionBody, scope, strict).
Perform ! SetFunctionName(F, name).
Return F.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncFunctionDeclaration is strict mode code, let strict be true. Otherwise, let strict be false.
Let F be ! AsyncFunctionCreate(Normal, FormalParameters, AsyncFunctionBody, scope, strict).
Perform ! SetFunctionName(F, "default").
Return F.

14.7.11 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let declResult be FunctionDeclarationInstantiation(functionObject, argumentsList).
If declResult is not an abrupt completion, then
1. Perform ! AsyncFunctionStart(promiseCapability, FunctionBody).
Else declResult is an abrupt completion,
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

14.7.12 Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

async

[no LineTerminator here]

(

)

{

}

Let propKey be the result of evaluating PropertyName.
ReturnIfAbrupt(propKey).
If the function code for this AsyncMethod is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let closure be ! AsyncFunctionCreate(Method, UniqueFormalParameters, AsyncFunctionBody, scope, strict).
Perform ! MakeMethod(closure, object).
Perform ! SetFunctionName(closure, propKey).
Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
Return ? DefinePropertyOrThrow(object, propKey, desc).

14.7.13 Runtime Semantics: Evaluation

async

[no LineTerminator here]

function

(

)

{

}

Return NormalCompletion(empty).

async

[no LineTerminator here]

function

(

)

{

}

Return NormalCompletion(empty).

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncFunctionExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let closure be ! AsyncFunctionCreate(Normal, FormalParameters, AsyncFunctionBody, scope, strict).
Return closure.

async

[no LineTerminator here]

function

(

)

{

}

If the function code for AsyncFunctionExpression is strict mode code, let strict be true. Otherwise let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let funcEnv be ! NewDeclarativeEnvironment(scope).
Let envRec be funcEnv's EnvironmentRecord.
Let name be StringValue of BindingIdentifier.
Perform ! envRec.CreateImmutableBinding(name).
Let closure be ! AsyncFunctionCreate(Normal, FormalParameters, AsyncFunctionBody, funcEnv, strict).
Perform ! SetFunctionName(closure, name).
Perform ! envRec.InitializeBinding(name, closure).
Return closure.

AwaitExpression

await

Let exprRef be the result of evaluating UnaryExpression.
Let value be ? GetValue(exprRef).
Return ? Await(value).

14.8 Async Arrow Function Definitions

Syntax

[In, Yield, Await]

async

[no LineTerminator here]

[?Yield]

[no LineTerminator here]

[?In]

[?Yield, ?Await]

[no LineTerminator here]

[?In]

[In]

[lookahead ≠ {]

[?In, ~Yield, +Await]

{

}

[Yield]

[?Yield, +Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production AsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead[no LineTerminator here]=>AsyncConciseBody the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

AsyncArrowHead

async

[no LineTerminator here]

ArrowFormalParameters

[~Yield, +Await]

14.8.1 Static Semantics: Early Errors

async

[no LineTerminator here]

[no LineTerminator here]

It is a Syntax Error if any element of the BoundNames of AsyncArrowBindingIdentifier also occurs in the LexicallyDeclaredNames of AsyncConciseBody.

[no LineTerminator here]

It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains YieldExpression is true.
It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains AwaitExpression is true.
It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead is not covering an AsyncArrowHead.
It is a Syntax Error if any element of the BoundNames of CoverCallExpressionAndAsyncArrowHead also occurs in the LexicallyDeclaredNames of AsyncConciseBody.
It is a Syntax Error if ContainsUseStrict of AsyncConciseBody is true and IsSimpleParameterList of CoverCallExpressionAndAsyncArrowHead is false.
All Early Error rules for AsyncArrowHead and its derived productions apply to CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.

14.8.2 Static Semantics: CoveredAsyncArrowHead

Return the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.

14.8.3 Static Semantics: BoundNames

Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Return the BoundNames of head.

14.8.4 Static Semantics: Contains

With parameter symbol.

async

[no LineTerminator here]

[no LineTerminator here]

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Return AsyncConciseBody Contains symbol.

[no LineTerminator here]

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
If head Contains symbol is true, return true.
Return AsyncConciseBody Contains symbol.

Note

Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an AsyncArrowFunction.

14.8.5 Static Semantics: ContainsExpression

Return false.

14.8.6 Static Semantics: ExpectedArgumentCount

Return 1.

14.8.7 Static Semantics: HasName

async

[no LineTerminator here]

[no LineTerminator here]

[no LineTerminator here]

Return false.

14.8.8 Static Semantics: IsSimpleParameterList

[Yield]

[?Yield, +Await]

Return true.

Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Return IsSimpleParameterList of head.

14.8.9 Static Semantics: LexicallyDeclaredNames

[lookahead ≠ {]

Return a new empty List.

14.8.10 Static Semantics: LexicallyScopedDeclarations

[lookahead ≠ {]

Return a new empty List.

14.8.11 Static Semantics: VarDeclaredNames

[lookahead ≠ {]

Return a new empty List.

14.8.12 Static Semantics: VarScopedDeclarations

[lookahead ≠ {]

Return a new empty List.

14.8.13 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Assert: iteratorRecord.[[Done]] is false.
Let next be ? IteratorStep(iteratorRecord).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
ReturnIfAbrupt(next).
If next is false, set iteratorRecord.[[Done]] to true.
Else,
1. Let v be ? IteratorValue(next).
2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(v).
If iteratorRecord.[[Done]] is true, let v be undefined.
Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.

14.8.14 Runtime Semantics: EvaluateBody

With parameters functionObject and List argumentsList.

[lookahead ≠ {]

Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let declResult be FunctionDeclarationInstantiation(functionObject, argumentsList).
If declResult is not an abrupt completion, then
1. Perform ! AsyncFunctionStart(promiseCapability, AssignmentExpression).
Else declResult is an abrupt completion,
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

{

}

Return the result of EvaluateBody of AsyncFunctionBody passing functionObject and argumentsList as the arguments.

14.8.15 Runtime Semantics: Evaluation

async

[no LineTerminator here]

[no LineTerminator here]

If the function code for this AsyncArrowFunction is strict mode code, let strict be true. Otherwise, let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let parameters be AsyncArrowBindingIdentifier.
Let closure be ! AsyncFunctionCreate(Arrow, parameters, AsyncConciseBody, scope, strict).
Return closure.

[no LineTerminator here]

If the function code for this AsyncArrowFunction is strict mode code, let strict be true. Otherwise, let strict be false.
Let scope be the LexicalEnvironment of the running execution context.
Let head be CoveredAsyncArrowHead of CoverCallExpressionAndAsyncArrowHead.
Let parameters be the ArrowFormalParameters of head.
Let closure be ! AsyncFunctionCreate(Arrow, parameters, AsyncConciseBody, scope, strict).
Return closure.

14.9 Tail Position Calls

14.9.1 Static Semantics: IsInTailPosition ( `call` )

The abstract operation IsInTailPosition with argument call performs the following steps:

Assert: call is a Parse Node.
If the source code matching call is non-strict code, return false.
If call is not contained within a FunctionBody, ConciseBody, or AsyncConciseBody, return false.
Let body be the FunctionBody, ConciseBody, or AsyncConciseBody that most closely contains call.
If body is the FunctionBody of a GeneratorBody, return false.
If body is the FunctionBody of an AsyncFunctionBody, return false.
If body is the FunctionBody of an AsyncGeneratorBody, return false.
If body is an AsyncConciseBody, return false.
Return the result of HasCallInTailPosition of body with argument call.

Note

Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 9.2.9) that enables observation of the chain of caller contexts.

14.9.2 Static Semantics: HasCallInTailPosition

With parameter call.

Note

call is a Parse Node that represents a specific range of source text. When the following algorithms compare call to another Parse Node, it is a test of whether they represent the same source text.

14.9.2.1 Statement Rules

Return HasCallInTailPosition of AssignmentExpression with argument call.

Let has be HasCallInTailPosition of StatementList with argument call.
If has is true, return true.
Return HasCallInTailPosition of StatementListItem with argument call.

[empty]

{

}

return

;

for

(

)

for

(

var

)

for

(

)

{

}

Return false.

(

)

else

Let has be HasCallInTailPosition of the first Statement with argument call.
If has is true, return true.
Return HasCallInTailPosition of the second Statement with argument call.

(

)

while

(

)

;

while

(

)

for

(

opt

;

opt

;

opt

)

for

(

var

;

opt

;

opt

)

for

(

opt

;

opt

)

for

(

)

for

(

var

)

for

(

)

with

(

)

Return HasCallInTailPosition of Statement with argument call.

Return HasCallInTailPosition of LabelledItem with argument call.

return

;

Return HasCallInTailPosition of Expression with argument call.

switch

(

)

Return HasCallInTailPosition of CaseBlock with argument call.

{

opt

opt

}

Let has be false.
If the first CaseClauses is present, let has be HasCallInTailPosition of the first CaseClauses with argument call.
If has is true, return true.
Let has be HasCallInTailPosition of the DefaultClause with argument call.
If has is true, return true.
If the second CaseClauses is present, let has be HasCallInTailPosition of the second CaseClauses with argument call.
Return has.

Let has be HasCallInTailPosition of CaseClauses with argument call.
If has is true, return true.
Return HasCallInTailPosition of CaseClause with argument call.

case

opt

default

opt

If StatementList is present, return HasCallInTailPosition of StatementList with argument call.
Return false.

try

Return HasCallInTailPosition of Catch with argument call.

try

try

Return HasCallInTailPosition of Finally with argument call.

catch

(

)

Return HasCallInTailPosition of Block with argument call.

14.9.2.2 Expression Rules

Note

A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a GetValue operation will always return the same value as the actual function call result.

===

!==

instanceof

>>>

delete

void

typeof

[

]

new

[

]

new

this

Return false.

Return HasCallInTailPosition of AssignmentExpression with argument call.

Let has be HasCallInTailPosition of the first AssignmentExpression with argument call.
If has is true, return true.
Return HasCallInTailPosition of the second AssignmentExpression with argument call.

Return HasCallInTailPosition of BitwiseORExpression with argument call.

Return HasCallInTailPosition of LogicalANDExpression with argument call.

If this CallExpression is call, return true.
Return false.

If this MemberExpression is call, return true.
Return false.

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return HasCallInTailPosition of expr with argument call.

(

)

Return HasCallInTailPosition of Expression with argument call.

14.9.3 Runtime Semantics: PrepareForTailCall ( )

The abstract operation PrepareForTailCall performs the following steps:

Let leafContext be the running execution context.
Suspend leafContext.
Pop leafContext from the execution context stack. The execution context now on the top of the stack becomes the running execution context.
Assert: leafContext has no further use. It will never be activated as the running execution context.

A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.

Note

For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.

15 ECMAScript Language: Scripts and Modules

15.1 Scripts

Syntax

opt

[~Yield, ~Await, ~Return]

15.1.1 Static Semantics: Early Errors

Script

It is a Syntax Error if the LexicallyDeclaredNames of ScriptBody contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ScriptBody also occurs in the VarDeclaredNames of ScriptBody.

It is a Syntax Error if StatementList Contains super unless the source code containing super is eval code that is being processed by a direct eval. Additional early error rules for super within direct eval are defined in 18.2.1.1.
It is a Syntax Error if StatementList Contains NewTarget unless the source code containing NewTarget is eval code that is being processed by a direct eval. Additional early error rules for NewTarget in direct eval are defined in 18.2.1.1.
It is a Syntax Error if ContainsDuplicateLabels of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of StatementList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of StatementList with arguments « » and « » is true.

15.1.2 Static Semantics: IsStrict

If the Directive Prologue of StatementList contains a Use Strict Directive, return true; otherwise, return false.

15.1.3 Static Semantics: LexicallyDeclaredNames

Return TopLevelLexicallyDeclaredNames of StatementList.

Note

At the top level of a Script, function declarations are treated like var declarations rather than like lexical declarations.

15.1.4 Static Semantics: LexicallyScopedDeclarations

Return TopLevelLexicallyScopedDeclarations of StatementList.

15.1.5 Static Semantics: VarDeclaredNames

Return TopLevelVarDeclaredNames of StatementList.

15.1.6 Static Semantics: VarScopedDeclarations

Return TopLevelVarScopedDeclarations of StatementList.

15.1.7 Runtime Semantics: Evaluation

Script

[empty]

Return NormalCompletion(undefined).

15.1.8 Script Records

A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed in Table 36.

Table 36: Script Record Fields

Field Name	Value Type	Meaning
[[Realm]]	Realm Record \| undefined	The realm within which this script was created. undefined if not yet assigned.
[[Environment]]	Lexical Environment \| undefined	The Lexical Environment containing the top level bindings for this script. This field is set when the script is instantiated.
[[ECMAScriptCode]]	a Parse Node	The result of parsing the source text of this module using Script as the goal symbol.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a script.

15.1.9 ParseScript ( `sourceText`, `realm`, `hostDefined` )

The abstract operation ParseScript with arguments sourceText, realm, and hostDefined creates a Script Record based upon the result of parsing sourceText as a Script. ParseScript performs the following steps:

Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Script as the goal symbol and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation-dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If body is a List of errors, return body.
Return Script Record { [[Realm]]: realm, [[Environment]]: undefined, [[ECMAScriptCode]]: body, [[HostDefined]]: hostDefined }.

Note

An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.

15.1.10 ScriptEvaluation ( `scriptRecord` )

Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
Let scriptCxt be a new ECMAScript code execution context.
Set the Function of scriptCxt to null.
Set the Realm of scriptCxt to scriptRecord.[[Realm]].
Set the ScriptOrModule of scriptCxt to scriptRecord.
Set the VariableEnvironment of scriptCxt to globalEnv.
Set the LexicalEnvironment of scriptCxt to globalEnv.
Suspend the currently running execution context.
Push scriptCxt on to the execution context stack; scriptCxt is now the running execution context.
Let scriptBody be scriptRecord.[[ECMAScriptCode]].
Let result be GlobalDeclarationInstantiation(scriptBody, globalEnv).
If result.[[Type]] is normal, then
1. Set result to the result of evaluating scriptBody.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Set result to NormalCompletion(undefined).
Suspend scriptCxt and remove it from the execution context stack.
Assert: The execution context stack is not empty.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

15.1.11 Runtime Semantics: GlobalDeclarationInstantiation ( `script`, `env` )

Note 1

When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.

GlobalDeclarationInstantiation is performed as follows using arguments script and env. script is the ScriptBody for which the execution context is being established. env is the global lexical environment in which bindings are to be created.

Let envRec be env's EnvironmentRecord.
Assert: envRec is a global Environment Record.
Let lexNames be the LexicallyDeclaredNames of script.
Let varNames be the VarDeclaredNames of script.
For each name in lexNames, do
1. If envRec.HasVarDeclaration(name) is true, throw a SyntaxError exception.
2. If envRec.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
3. Let hasRestrictedGlobal be ? envRec.HasRestrictedGlobalProperty(name).
4. If hasRestrictedGlobal is true, throw a SyntaxError exception.
For each name in varNames, do
1. If envRec.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
Let varDeclarations be the VarScopedDeclarations of script.
Let functionsToInitialize be a new empty List.
Let declaredFunctionNames be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
  3. Let fn be the sole element of the BoundNames of d.
  4. If fn is not an element of declaredFunctionNames, then
    1. Let fnDefinable be ? envRec.CanDeclareGlobalFunction(fn).
    2. If fnDefinable is false, throw a TypeError exception.
    3. Append fn to declaredFunctionNames.
    4. Insert d as the first element of functionsToInitialize.
Let declaredVarNames be a new empty List.
For each d in varDeclarations, do
1. If d is a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
  1. For each String vn in the BoundNames of d, do
    1. If vn is not an element of declaredFunctionNames, then
      1. Let vnDefinable be ? envRec.CanDeclareGlobalVar(vn).
      2. If vnDefinable is false, throw a TypeError exception.
      3. If vn is not an element of declaredVarNames, then
        Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.3.2 adds additional steps at this point.
Let lexDeclarations be the LexicallyScopedDeclarations of script.
For each element d in lexDeclarations, do
1. NOTE: Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ? envRec.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ? envRec.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be the result of performing InstantiateFunctionObject for f with argument env.
3. Perform ? envRec.CreateGlobalFunctionBinding(fn, fo, false).
For each String vn in declaredVarNames, in list order, do
1. Perform ? envRec.CreateGlobalVarBinding(vn, false).
Return NormalCompletion(empty).

Note 2

Early errors specified in 15.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.

Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.

15.1.12 Runtime Semantics: ScriptEvaluationJob ( `sourceText`, `hostDefined` )

The job ScriptEvaluationJob with parameters sourceText and hostDefined parses, validates, and evaluates sourceText as a Script.

Assert: sourceText is an ECMAScript source text (see clause 10).
Let realm be the current Realm Record.
Let s be ParseScript(sourceText, realm, hostDefined).
If s is a List of errors, then
1. Perform HostReportErrors(s).
2. Return NormalCompletion(undefined).
Return ? ScriptEvaluation(s).

15.2 Modules

Syntax

opt

[~Yield, ~Await, ~Return]

15.2.1 Module Semantics

15.2.1.1 Static Semantics: Early Errors

ModuleBody

It is a Syntax Error if the LexicallyDeclaredNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ModuleItemList also occurs in the VarDeclaredNames of ModuleItemList.
It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of ModuleItemList does not also occur in either the VarDeclaredNames of ModuleItemList, or the LexicallyDeclaredNames of ModuleItemList.
It is a Syntax Error if ModuleItemList Contains super.
It is a Syntax Error if ModuleItemList Contains NewTarget.
It is a Syntax Error if ContainsDuplicateLabels of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedBreakTarget of ModuleItemList with argument « » is true.
It is a Syntax Error if ContainsUndefinedContinueTarget of ModuleItemList with arguments « » and « » is true.

Note

The duplicate ExportedNames rule implies that multiple export default ExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.

15.2.1.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Let hasDuplicates be ContainsDuplicateLabels of ModuleItemList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of ModuleItem with argument labelSet.

Return false.

15.2.1.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of ModuleItemList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of ModuleItem with argument labelSet.

Return false.

15.2.1.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of ModuleItemList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of ModuleItem with arguments iterationSet and « ».

Return false.

15.2.1.5 Static Semantics: ExportedBindings

Note

ExportedBindings are the locally bound names that are explicitly associated with a Module's ExportedNames.

Let names be ExportedBindings of ModuleItemList.
Append to names the elements of the ExportedBindings of ModuleItem.
Return names.

Return a new empty List.

15.2.1.6 Static Semantics: ExportedNames

Note

ExportedNames are the externally visible names that a Module explicitly maps to one of its local name bindings.

Let names be ExportedNames of ModuleItemList.
Append to names the elements of the ExportedNames of ModuleItem.
Return names.

Return the ExportedNames of ExportDeclaration.

Return a new empty List.

15.2.1.7 Static Semantics: ExportEntries

[empty]

Return a new empty List.

Let entries be ExportEntries of ModuleItemList.
Append to entries the elements of the ExportEntries of ModuleItem.
Return entries.

Return a new empty List.

15.2.1.8 Static Semantics: ImportEntries

[empty]

Return a new empty List.

Let entries be ImportEntries of ModuleItemList.
Append to entries the elements of the ImportEntries of ModuleItem.
Return entries.

Return a new empty List.

15.2.1.9 Static Semantics: ImportedLocalNames ( `importEntries` )

The abstract operation ImportedLocalNames with argument importEntries creates a List of all of the local name bindings defined by a List of ImportEntry Records (see Table 40). ImportedLocalNames performs the following steps:

Let localNames be a new empty List.
For each ImportEntry Record i in importEntries, do
1. Append i.[[LocalName]] to localNames.
Return localNames.

15.2.1.10 Static Semantics: ModuleRequests

[empty]

Return a new empty List.

Return ModuleRequests of ModuleItem.

Let moduleNames be ModuleRequests of ModuleItemList.
Let additionalNames be ModuleRequests of ModuleItem.
Append to moduleNames each element of additionalNames that is not already an element of moduleNames.
Return moduleNames.

Return a new empty List.

15.2.1.11 Static Semantics: LexicallyDeclaredNames

Note 1

The LexicallyDeclaredNames of a Module includes the names of all of its imported bindings.

Let names be LexicallyDeclaredNames of ModuleItemList.
Append to names the elements of the LexicallyDeclaredNames of ModuleItem.
Return names.

Return the BoundNames of ImportDeclaration.

If ExportDeclaration is export VariableStatement, return a new empty List.
Return the BoundNames of ExportDeclaration.

Return LexicallyDeclaredNames of StatementListItem.

Note 2

At the top level of a Module, function declarations are treated like lexical declarations rather than like var declarations.

15.2.1.12 Static Semantics: LexicallyScopedDeclarations

[empty]

Return a new empty List.

Let declarations be LexicallyScopedDeclarations of ModuleItemList.
Append to declarations the elements of the LexicallyScopedDeclarations of ModuleItem.
Return declarations.

Return a new empty List.

15.2.1.13 Static Semantics: VarDeclaredNames

[empty]

Return a new empty List.

Let names be VarDeclaredNames of ModuleItemList.
Append to names the elements of the VarDeclaredNames of ModuleItem.
Return names.

Return a new empty List.

If ExportDeclaration is export VariableStatement, return BoundNames of ExportDeclaration.
Return a new empty List.

15.2.1.14 Static Semantics: VarScopedDeclarations

[empty]

Return a new empty List.

Let declarations be VarScopedDeclarations of ModuleItemList.
Append to declarations the elements of the VarScopedDeclarations of ModuleItem.
Return declarations.

Return a new empty List.

If ExportDeclaration is export VariableStatement, return VarScopedDeclarations of VariableStatement.
Return a new empty List.

15.2.1.15 Abstract Module Records

A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.

For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with concrete subclasses. This specification only defines a single Module Record concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.

Module Record defines the fields listed in Table 37. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 38. All Module definition subclasses must provide concrete implementations of these abstract methods.

Table 37: Module Record Fields

Field Name	Value Type	Meaning
[[Realm]]	Realm Record \| undefined	The Realm within which this module was created. undefined if not yet assigned.
[[Environment]]	Lexical Environment \| undefined	The Lexical Environment containing the top level bindings for this module. This field is set when the module is instantiated.
[[Namespace]]	Object \| undefined	The Module Namespace Object (26.3) if one has been created for this module. Otherwise undefined.
[[HostDefined]]	Any, default value is undefined.	Field reserved for use by host environments that need to associate additional information with a module.

Table 38: Abstract Methods of Module Records

Method	Purpose
GetExportedNames(`exportStarSet`)	Return a list of all names that are either directly or indirectly exported from this module.
ResolveExport(`exportName`, `resolveSet`)	Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]: Module Record, [[BindingName]]: String }. Return null if the name cannot be resolved, or `"ambiguous"` if multiple bindings were found. This operation must be idempotent if it completes normally. Each time it is called with a specific `exportName`, `resolveSet` pair as arguments it must return the same result.
Instantiate()	Prepare the module for evaluation by transitively resolving all module dependencies and creating a module Environment Record.
Evaluate()	If this module has already been evaluated successfully, return undefined; if it has already been evaluated unsuccessfully, throw the exception that was produced. Otherwise, transitively evaluate all module dependencies of this module and then evaluate this module. Instantiate must have completed successfully prior to invoking this method.

15.2.1.16 Source Text Module Records

A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (10) that was parsed using the goal symbol Module. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, instantiate, and evaluate the module.

A Source Text Module Record can exist in a module graph with other subclasses of the abstract Module Record type. However, non-source text Module Records must not participate in dependency cycles with Source Text Module Records.

In addition to the fields, defined in Table 37, Source Text Module Records have the additional fields listed in Table 39. Each of these fields is initially set in ParseModule.

Table 39: Additional Fields of Source Text Module Records

Field Name	Value Type	Meaning
[[ECMAScriptCode]]	a Parse Node	The result of parsing the source text of this module using Module as the goal symbol.
[[RequestedModules]]	List of String	A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered.
[[ImportEntries]]	List of ImportEntry Records	A List of ImportEntry records derived from the code of this module.
[[LocalExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
[[IndirectExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module.
[[StarExportEntries]]	List of ExportEntry Records	A List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module.
[[Status]]	String	Initially `"uninstantiated"`. Transitions to `"instantiating"`, `"instantiated"`, `"evaluating"`, `"evaluated"` (in that order) as the module progresses throughout its lifecycle.
[[EvaluationError]]	An abrupt completion \| undefined	A completion of type throw representing the exception that occurred during evaluation. undefined if no exception occurred or if [[Status]] is not `"evaluated"`.
[[DFSIndex]]	Integer \| undefined	Auxiliary field used during Instantiate and Evaluate only. If [[Status]] is `"instantiating"` or `"evaluating"`, this non-negative number records the point at which the module was first visited during the ongoing depth-first traversal of the dependency graph.
[[DFSAncestorIndex]]	Integer \| undefined	Auxiliary field used during Instantiate and Evaluate only. If [[Status]] is `"instantiating"` or `"evaluating"`, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.

An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 40:

Table 40: ImportEntry Record Fields

Field Name	Value Type	Meaning
[[ModuleRequest]]	String	String value of the ModuleSpecifier of the ImportDeclaration.
[[ImportName]]	String	The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value `"*"` indicates that the import request is for the target module's namespace object.
[[LocalName]]	String	The name that is used to locally access the imported value from within the importing module.

Note 1

Table 41 gives examples of ImportEntry records fields used to represent the syntactic import forms:

Table 41 (Informative): Import Forms Mappings to ImportEntry Records

Import Statement Form	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]
`import v from "mod";`	`"mod"`	`"default"`	`"v"`
`import * as ns from "mod";`	`"mod"`	`"*"`	`"ns"`
`import {x} from "mod";`	`"mod"`	`"x"`	`"x"`
`import {x as v} from "mod";`	`"mod"`	`"x"`	`"v"`
`import "mod";`	An ImportEntry Record is not created.

An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 42:

Table 42: ExportEntry Record Fields

Field Name	Value Type	Meaning
[[ExportName]]	String	The name used to export this binding by this module.
[[ModuleRequest]]	String \| null	The String value of the ModuleSpecifier of the ExportDeclaration. null if the ExportDeclaration does not have a ModuleSpecifier.
[[ImportName]]	String \| null	The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. `"*"` indicates that the export request is for all exported bindings.
[[LocalName]]	String \| null	The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.

Note 2

Table 43 gives examples of the ExportEntry record fields used to represent the syntactic export forms:

Table 43 (Informative): Export Forms Mappings to ExportEntry Records

Export Statement Form	[[ExportName]]	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]
`export var v;`	`"v"`	null	null	`"v"`
`export default function f(){}`	`"default"`	null	null	`"f"`
`export default function(){}`	`"default"`	null	null	`"default"`
`export default 42;`	`"default"`	null	null	`"default"`
`export {x};`	`"x"`	null	null	`"x"`
`export {v as x};`	`"x"`	null	null	`"v"`
`export {x} from "mod";`	`"x"`	`"mod"`	`"x"`	null
`export {v as x} from "mod";`	`"x"`	`"mod"`	`"v"`	null
`export * from "mod";`	null	`"mod"`	`"*"`	null

The following definitions specify the required concrete methods and other abstract operations for Source Text Module Records

15.2.1.16.1 ParseModule ( `sourceText`, `realm`, `hostDefined` )

The abstract operation ParseModule with arguments sourceText, realm, and hostDefined creates a Source Text Module Record based upon the result of parsing sourceText as a Module. ParseModule performs the following steps:

Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Module as the goal symbol and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation-dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If body is a List of errors, return body.
Let requestedModules be the ModuleRequests of body.
Let importEntries be ImportEntries of body.
Let importedBoundNames be ImportedLocalNames(importEntries).
Let indirectExportEntries be a new empty List.
Let localExportEntries be a new empty List.
Let starExportEntries be a new empty List.
Let exportEntries be ExportEntries of body.
For each ExportEntry Record ee in exportEntries, do
1. If ee.[[ModuleRequest]] is null, then
  1. If ee.[[LocalName]] is not an element of importedBoundNames, then
    1. Append ee to localExportEntries.
  2. Else,
    1. Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
    2. If ie.[[ImportName]] is "*", then
      1. Assert: This is a re-export of an imported module namespace object.
      2. Append ee to localExportEntries.
    3. Else this is a re-export of a single name,
      1. Append the ExportEntry Record { [[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
2. Else if ee.[[ImportName]] is "*", then
  1. Append ee to starExportEntries.
3. Else,
  1. Append ee to indirectExportEntries.
Return Source Text Module Record { [[Realm]]: realm, [[Environment]]: undefined, [[Namespace]]: undefined, [[Status]]: "uninstantiated", [[EvaluationError]]: undefined, [[HostDefined]]: hostDefined, [[ECMAScriptCode]]: body, [[RequestedModules]]: requestedModules, [[ImportEntries]]: importEntries, [[LocalExportEntries]]: localExportEntries, [[IndirectExportEntries]]: indirectExportEntries, [[StarExportEntries]]: starExportEntries, [[DFSIndex]]: undefined, [[DFSAncestorIndex]]: undefined }.

Note

An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.

15.2.1.16.2 GetExportedNames ( `exportStarSet` ) Concrete Method

The GetExportedNames concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

It performs the following steps:

Let module be this Source Text Module Record.
If exportStarSet contains module, then
1. Assert: We've reached the starting point of an import * circularity.
2. Return a new empty List.
Append module to exportStarSet.
Let exportedNames be a new empty List.
For each ExportEntry Record e in module.[[LocalExportEntries]], do
1. Assert: module provides the direct binding for this export.
2. Append e.[[ExportName]] to exportedNames.
For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. Assert: module imports a specific binding for this export.
2. Append e.[[ExportName]] to exportedNames.
For each ExportEntry Record e in module.[[StarExportEntries]], do
1. Let requestedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
2. Let starNames be ? requestedModule.GetExportedNames(exportStarSet).
3. For each element n of starNames, do
  1. If SameValue(n, "default") is false, then
    1. If n is not an element of exportedNames, then
      1. Append n to exportedNames.
Return exportedNames.

Note

GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.

15.2.1.16.3 ResolveExport ( `exportName`, `resolveSet` ) Concrete Method

The ResolveExport concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.

If a defining module is found, a ResolvedBinding Record { [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export. If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, the string "ambiguous" is returned.

This abstract method performs the following steps:

Let module be this Source Text Module Record.
For each Record { [[Module]], [[ExportName]] } r in resolveSet, do
1. If module and r.[[Module]] are the same Module Record and SameValue(exportName, r.[[ExportName]]) is true, then
  1. Assert: This is a circular import request.
  2. Return null.
Append the Record { [[Module]]: module, [[ExportName]]: exportName } to resolveSet.
For each ExportEntry Record e in module.[[LocalExportEntries]], do
1. If SameValue(exportName, e.[[ExportName]]) is true, then
  1. Assert: module provides the direct binding for this export.
  2. Return ResolvedBinding Record { [[Module]]: module, [[BindingName]]: e.[[LocalName]] }.
For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. If SameValue(exportName, e.[[ExportName]]) is true, then
  1. Assert: module imports a specific binding for this export.
  2. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
  3. Return importedModule.ResolveExport(e.[[ImportName]], resolveSet).
If SameValue(exportName, "default") is true, then
1. Assert: A default export was not explicitly defined by this module.
2. Return null.
3. NOTE: A default export cannot be provided by an export *.
Let starResolution be null.
For each ExportEntry Record e in module.[[StarExportEntries]], do
1. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
2. Let resolution be ? importedModule.ResolveExport(exportName, resolveSet).
3. If resolution is "ambiguous", return "ambiguous".
4. If resolution is not null, then
  1. Assert: resolution is a ResolvedBinding Record.
  2. If starResolution is null, set starResolution to resolution.
  3. Else,
    1. Assert: There is more than one * import that includes the requested name.
    2. If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record or SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return "ambiguous".
Return starResolution.

15.2.1.16.4 Instantiate ( ) Concrete Method

The Instantiate concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

On success, Instantiate transitions this module's [[Status]] from "uninstantiated" to "instantiated". On failure, an exception is thrown and this module's [[Status]] remains "uninstantiated".

This abstract method performs the following steps (most of the work is done by the auxiliary function InnerModuleInstantiation):

Let module be this Source Text Module Record.
Assert: module.[[Status]] is not "instantiating" or "evaluating".
Let stack be a new empty List.
Let result be InnerModuleInstantiation(module, stack, 0).
If result is an abrupt completion, then
1. For each module m in stack, do
  1. Assert: m.[[Status]] is "instantiating".
  2. Set m.[[Status]] to "uninstantiated".
  3. Set m.[[Environment]] to undefined.
  4. Set m.[[DFSIndex]] to undefined.
  5. Set m.[[DFSAncestorIndex]] to undefined.
2. Assert: module.[[Status]] is "uninstantiated".
3. Return result.
Assert: module.[[Status]] is "instantiated" or "evaluated".
Assert: stack is empty.
Return undefined.

15.2.1.16.4.1 InnerModuleInstantiation ( `module`, `stack`, `index` )

The InnerModuleInstantiation abstract operation is used by Instantiate to perform the actual instantiation process for the Source Text Module Record module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition to "instantiated" together.

This abstract operation performs the following steps:

If module is not a Source Text Module Record, then
1. Perform ? module.Instantiate().
2. Return index.
If module.[[Status]] is "instantiating", "instantiated", or "evaluated", then
1. Return index.
Assert: module.[[Status]] is "uninstantiated".
Set module.[[Status]] to "instantiating".
Set module.[[DFSIndex]] to index.
Set module.[[DFSAncestorIndex]] to index.
Set index to index + 1.
Append module to stack.
For each String required that is an element of module.[[RequestedModules]], do
1. Let requiredModule be ? HostResolveImportedModule(module, required).
2. Set index to ? InnerModuleInstantiation(requiredModule, stack, index).
3. Assert: requiredModule.[[Status]] is either "instantiating", "instantiated", or "evaluated".
4. Assert: requiredModule.[[Status]] is "instantiating" if and only if requiredModule is in stack.
5. If requiredModule.[[Status]] is "instantiating", then
  1. Assert: requiredModule is a Source Text Module Record.
  2. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
Perform ? ModuleDeclarationEnvironmentSetup(module).
Assert: module occurs exactly once in stack.
Assert: module.[[DFSAncestorIndex]] is less than or equal to module.[[DFSIndex]].
If module.[[DFSAncestorIndex]] equals module.[[DFSIndex]], then
1. Let done be false.
2. Repeat, while done is false,
  1. Let requiredModule be the last element in stack.
  2. Remove the last element of stack.
  3. Set requiredModule.[[Status]] to "instantiated".
  4. If requiredModule and module are the same Module Record, set done to true.
Return index.

15.2.1.16.4.2 ModuleDeclarationEnvironmentSetup ( `module` )

The ModuleDeclarationEnvironmentSetup abstract operation is used by InnerModuleInstantiation to initialize the Lexical Environment of the module, including resolving all imported bindings.

This abstract operation performs the following steps:

For each ExportEntry Record e in module.[[IndirectExportEntries]], do
1. Let resolution be ? module.ResolveExport(e.[[ExportName]], « »).
2. If resolution is null or "ambiguous", throw a SyntaxError exception.
3. Assert: resolution is a ResolvedBinding Record.
Assert: All named exports from module are resolvable.
Let realm be module.[[Realm]].
Assert: realm is not undefined.
Let env be NewModuleEnvironment(realm.[[GlobalEnv]]).
Set module.[[Environment]] to env.
Let envRec be env's EnvironmentRecord.
For each ImportEntry Record in in module.[[ImportEntries]], do
1. Let importedModule be ! HostResolveImportedModule(module, in.[[ModuleRequest]]).
2. NOTE: The above call cannot fail because imported module requests are a subset of module.[[RequestedModules]], and these have been resolved earlier in this algorithm.
3. If in.[[ImportName]] is "*", then
  1. Let namespace be ? GetModuleNamespace(importedModule).
  2. Perform ! envRec.CreateImmutableBinding(in.[[LocalName]], true).
  3. Call envRec.InitializeBinding(in.[[LocalName]], namespace).
4. Else,
  1. Let resolution be ? importedModule.ResolveExport(in.[[ImportName]], « »).
  2. If resolution is null or "ambiguous", throw a SyntaxError exception.
  3. Call envRec.CreateImportBinding(in.[[LocalName]], resolution.[[Module]], resolution.[[BindingName]]).
Let code be module.[[ECMAScriptCode]].
Let varDeclarations be the VarScopedDeclarations of code.
Let declaredVarNames be a new empty List.
For each element d in varDeclarations, do
1. For each element dn of the BoundNames of d, do
  1. If dn is not an element of declaredVarNames, then
    1. Perform ! envRec.CreateMutableBinding(dn, false).
    2. Call envRec.InitializeBinding(dn, undefined).
    3. Append dn to declaredVarNames.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations, do
1. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ! envRec.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ! envRec.CreateMutableBinding(dn, false).
  3. If d is a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
    1. Let fo be the result of performing InstantiateFunctionObject for d with argument env.
    2. Call envRec.InitializeBinding(dn, fo).

15.2.1.16.5 Evaluate ( ) Concrete Method

The Evaluate concrete method of a Source Text Module Record implements the corresponding Module Record abstract method.

Evaluate transitions this module's [[Status]] from "instantiated" to "evaluated".

If execution results in an exception, that exception is recorded in the [[EvaluationError]] field and rethrown by future invocations of Evaluate.

This abstract method performs the following steps (most of the work is done by the auxiliary function InnerModuleEvaluation):

Let module be this Source Text Module Record.
Assert: module.[[Status]] is "instantiated" or "evaluated".
Let stack be a new empty List.
Let result be InnerModuleEvaluation(module, stack, 0).
If result is an abrupt completion, then
1. For each module m in stack, do
  1. Assert: m.[[Status]] is "evaluating".
  2. Set m.[[Status]] to "evaluated".
  3. Set m.[[EvaluationError]] to result.
2. Assert: module.[[Status]] is "evaluated" and module.[[EvaluationError]] is result.
3. Return result.
Assert: module.[[Status]] is "evaluated" and module.[[EvaluationError]] is undefined.
Assert: stack is empty.
Return undefined.

15.2.1.16.5.1 InnerModuleEvaluation ( `module`, `stack`, `index` )

The InnerModuleEvaluation abstract operation is used by Evaluate to perform the actual evaluation process for the Source Text Module Record module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestoreIndex]] fields, are used the same way as in InnerModuleInstantiation.

This abstract operation performs the following steps:

If module is not a Source Text Module Record, then
1. Perform ? module.Evaluate().
2. Return index.
If module.[[Status]] is "evaluated", then
1. If module.[[EvaluationError]] is undefined, return index.
2. Otherwise return module.[[EvaluationError]].
If module.[[Status]] is "evaluating", return index.
Assert: module.[[Status]] is "instantiated".
Set module.[[Status]] to "evaluating".
Set module.[[DFSIndex]] to index.
Set module.[[DFSAncestorIndex]] to index.
Set index to index + 1.
Append module to stack.
For each String required that is an element of module.[[RequestedModules]], do
1. Let requiredModule be ! HostResolveImportedModule(module, required).
2. NOTE: Instantiate must be completed successfully prior to invoking this method, so every requested module is guaranteed to resolve successfully.
3. Set index to ? InnerModuleEvaluation(requiredModule, stack, index).
4. Assert: requiredModule.[[Status]] is either "evaluating" or "evaluated".
5. Assert: requiredModule.[[Status]] is "evaluating" if and only if requiredModule is in stack.
6. If requiredModule.[[Status]] is "evaluating", then
  1. Assert: requiredModule is a Source Text Module Record.
  2. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
Perform ? ModuleExecution(module).
Assert: module occurs exactly once in stack.
Assert: module.[[DFSAncestorIndex]] is less than or equal to module.[[DFSIndex]].
If module.[[DFSAncestorIndex]] equals module.[[DFSIndex]], then
1. Let done be false.
2. Repeat, while done is false,
  1. Let requiredModule be the last element in stack.
  2. Remove the last element of stack.
  3. Set requiredModule.[[Status]] to "evaluated".
  4. If requiredModule and module are the same Module Record, set done to true.
Return index.

15.2.1.16.5.2 ModuleExecution ( `module` )

The ModuleExecution abstract operation is used by InnerModuleEvaluation to initialize the execution context of the module and evaluate the module's code within it.

This abstract operation performs the following steps:

Let moduleCxt be a new ECMAScript code execution context.
Set the Function of moduleCxt to null.
Assert: module.[[Realm]] is not undefined.
Set the Realm of moduleCxt to module.[[Realm]].
Set the ScriptOrModule of moduleCxt to module.
Assert: module has been linked and declarations in its module environment have been instantiated.
Set the VariableEnvironment of moduleCxt to module.[[Environment]].
Set the LexicalEnvironment of moduleCxt to module.[[Environment]].
Suspend the currently running execution context.
Push moduleCxt on to the execution context stack; moduleCxt is now the running execution context.
Let result be the result of evaluating module.[[ECMAScriptCode]].
Suspend moduleCxt and remove it from the execution context stack.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

15.2.1.16.6 Example Source Text Module Record Graphs

This non-normative section gives a series of examples of the instantiation and evaluation of a few common module graphs, with a specific focus on how errors can occur.

First consider the following simple module graph:

A module graph in which module A depends on module B — Figure 2: A simple module graph

Let's first assume that there are no error conditions. When a host first calls A.Instantiate(), this will complete successfully by assumption, and recursively instantiate modules B and C as well, such that A.[[Status]] = B.[[Status]] = C.[[Status]] = "instantiated". This preparatory step can be performed at any time. Later, when the host is ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will be "evaluated".

Consider then cases involving instantiation errors. If InnerModuleInstantiation of C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Instantiate() will fail, and both A and B's [[Status]] remain "uninstantiated". C's [[Status]] has become "instantiated", though.

Finally, consider a case involving evaluation errors. If InnerModuleEvaluation of C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will become "evaluated". C will also become "evaluated" but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time a host tries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Source Text Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)

The difference here between instantiation and evaluation errors is due to how evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.) Instantiation, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.

Now consider a different type of error condition:

A module graph in which module A depends on a missing (unresolvable) module, represented by ??? — Figure 3: A module graph with an unresolvable module

In this scenario, module A declares a dependency on some other module, but no Module Record exists for that module, i.e. HostResolveImportedModule throws an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing but ParseModule throwing an exception when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the exception they throw from HostResolveImportedModule. In any case, this exception causes an instantiation failure, which as before results in A's [[Status]] remaining "uninstantiated".

Lastly, consider a module graph with a cycle:

A module graph in which module A depends on module B and C, but module B also depends on module A — Figure 4: A cyclic module graph

Here we assume that the entry point is module A, so that the host proceeds by calling A.Instantiate(), which performs InnerModuleInstantiation on A. This in turn calls InnerModuleInstantiation on B. Because of the cycle, this again triggers InnerModuleInstantiation on A, but at this point it is a no-op since A.[[Status]] is already "instantiating". B.[[Status]] itself remains "instantiating" when control gets back to A and InnerModuleInstantiation is triggered on C. After this returns with C.[[Status]] being "instantiated" , both A and B transition from "instantiating" to "instantiated" together; this is by design, since they form a strongly connected component.

An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.

Now consider a case where A has an instantiation error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call to InnerModuleInstantiation on A. However, once we unwind back to the original InnerModuleInstantiation on A, it fails during ModuleDeclarationEnvironmentSetup, namely right after C.ResolveExport(). The thrown SyntaxError exception propagates up to A.Instantiate, which resets all modules that are currently on its stack (these are always exactly the modules that are still "instantiating"). Hence both A and B become "uninstantiated". Note that C is left as "instantiated".

Finally, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call to InnerModuleEvaluation on A. However, once we unwind back to the original InnerModuleEvaluation on A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are still "evaluating"). Hence both A and B become "evaluated" and the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left as "evaluated" with no [[EvaluationError]].

15.2.1.17 Runtime Semantics: HostResolveImportedModule ( `referencingModule`, `specifier` )

HostResolveImportedModule is an implementation-defined abstract operation that provides the concrete Module Record subclass instance that corresponds to the ModuleSpecifier String, specifier, occurring within the context of the module represented by the Module Record referencingModule.

The implementation of HostResolveImportedModule must conform to the following requirements:

The normal return value must be an instance of a concrete subclass of Module Record.
If a Module Record corresponding to the pair referencingModule, specifier does not exist or cannot be created, an exception must be thrown.
This operation must be idempotent if it completes normally. Each time it is called with a specific referencingModule, specifier pair as arguments it must return the same Module Record instance.

Multiple different referencingModule, specifier pairs may map to the same Module Record instance. The actual mapping semantic is implementation-defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.

15.2.1.18 Runtime Semantics: GetModuleNamespace ( `module` )

The GetModuleNamespace abstract operation retrieves the Module Namespace Exotic object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval.

This abstract operation performs the following steps:

Assert: module is an instance of a concrete subclass of Module Record.
Assert: module.[[Status]] is not "uninstantiated".
Assert: If module.[[Status]] is "evaluated", module.[[EvaluationError]] is undefined.
Let namespace be module.[[Namespace]].
If namespace is undefined, then
1. Let exportedNames be ? module.GetExportedNames(« »).
2. Let unambiguousNames be a new empty List.
3. For each name that is an element of exportedNames, do
  1. Let resolution be ? module.ResolveExport(name, « »).
  2. If resolution is a ResolvedBinding Record, append name to unambiguousNames.
4. Set namespace to ModuleNamespaceCreate(module, unambiguousNames).
Return namespace.

Note

The only way GetModuleNamespace can throw is via one of the triggered HostResolveImportedModule calls. Unresolvable names are simply excluded from the namespace at this point. They will lead to a real instantiation error later unless they are all ambiguous star exports that are not explicitly requested anywhere.

15.2.1.19 Runtime Semantics: TopLevelModuleEvaluationJob ( `sourceText`, `hostDefined` )

A TopLevelModuleEvaluationJob with parameters sourceText and hostDefined is a job that parses, validates, and evaluates sourceText as a Module.

Assert: sourceText is an ECMAScript source text (see clause 10).
Let realm be the current Realm Record.
Let m be ParseModule(sourceText, realm, hostDefined).
If m is a List of errors, then
1. Perform HostReportErrors(m).
2. Return NormalCompletion(undefined).
Perform ? m.Instantiate().
Assert: All dependencies of m have been transitively resolved and m is ready for evaluation.
Return ? m.Evaluate().

Note

An implementation may parse a sourceText as a Module, analyse it for Early Error conditions, and instantiate it prior to the execution of the TopLevelModuleEvaluationJob for that sourceText. An implementation may also resolve, pre-parse and pre-analyse, and pre-instantiate module dependencies of sourceText. However, the reporting of any errors detected by these actions must be deferred until the TopLevelModuleEvaluationJob is actually executed.

15.2.1.20 Runtime Semantics: Evaluation

[empty]

Return NormalCompletion(undefined).

ModuleBody

Let result be the result of evaluating ModuleItemList.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Return NormalCompletion(undefined).
Return Completion(result).

Let sl be the result of evaluating ModuleItemList.
ReturnIfAbrupt(sl).
Let s be the result of evaluating ModuleItem.
Return Completion(UpdateEmpty(s, sl.[[Value]])).

Note

The value of a ModuleItemList is the value of the last value-producing item in the ModuleItemList.

Return NormalCompletion(empty).

15.2.2 Imports

Syntax

import

;

import

;

{

}

{

ImportsList

}

{

}

from

[~Yield, ~Await]

15.2.2.1 Static Semantics: Early Errors

It is a Syntax Error if the BoundNames of ImportDeclaration contains any duplicate entries.

15.2.2.2 Static Semantics: BoundNames

import

;

Return the BoundNames of ImportClause.

import

;

Return a new empty List.

Let names be the BoundNames of ImportedDefaultBinding.
Append to names the elements of the BoundNames of NameSpaceImport.
Return names.

Let names be the BoundNames of ImportedDefaultBinding.
Append to names the elements of the BoundNames of NamedImports.
Return names.

{

}

Return a new empty List.

ImportsList

Let names be the BoundNames of ImportsList.
Append to names the elements of the BoundNames of ImportSpecifier.
Return names.

Return the BoundNames of ImportedBinding.

15.2.2.3 Static Semantics: ImportEntries

import

;

Let module be the sole element of ModuleRequests of FromClause.
Return ImportEntriesForModule of ImportClause with argument module.

import

;

Return a new empty List.

15.2.2.4 Static Semantics: ImportEntriesForModule

With parameter module.

Let entries be ImportEntriesForModule of ImportedDefaultBinding with argument module.
Append to entries the elements of the ImportEntriesForModule of NameSpaceImport with argument module.
Return entries.

Let entries be ImportEntriesForModule of ImportedDefaultBinding with argument module.
Append to entries the elements of the ImportEntriesForModule of NamedImports with argument module.
Return entries.

Let localName be the sole element of BoundNames of ImportedBinding.
Let defaultEntry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "default", [[LocalName]]: localName }.
Return a new List containing defaultEntry.

Let localName be the StringValue of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "*", [[LocalName]]: localName }.
Return a new List containing entry.

{

}

Return a new empty List.

ImportsList

Let specs be the ImportEntriesForModule of ImportsList with argument module.
Append to specs the elements of the ImportEntriesForModule of ImportSpecifier with argument module.
Return specs.

Let localName be the sole element of BoundNames of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: localName, [[LocalName]]: localName }.
Return a new List containing entry.

Let importName be the StringValue of IdentifierName.
Let localName be the StringValue of ImportedBinding.
Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName }.
Return a new List containing entry.

15.2.2.5 Static Semantics: ModuleRequests

import

;

Return ModuleRequests of FromClause.

Return a List containing the StringValue of StringLiteral.

15.2.3 Exports

Syntax

export

;

export

;

export

;

export

[~Yield, ~Await]

export

[~Yield, ~Await]

export

default

[~Yield, ~Await, +Default]

export

default

[~Yield, ~Await, +Default]

export

default

[lookahead ∉ { function, async

[no LineTerminator here]

function, class }]

[+In, ~Yield, ~Await]

;

{

}

{

}

{

}

15.2.3.1 Static Semantics: Early Errors

export

;

For each IdentifierName n in ReferencedBindings of ExportClause: It is a Syntax Error if StringValue of n is a ReservedWord or if the StringValue of n is one of: "implements", "interface", "let", "package", "private", "protected", "public", or "static".

Note

The above rule means that each ReferencedBindings of ExportClause is treated as an IdentifierReference.

15.2.3.2 Static Semantics: BoundNames

export

;

export

;

export

;

Return a new empty List.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

Let declarationNames be the BoundNames of HoistableDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

export

default

Let declarationNames be the BoundNames of ClassDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

export

default

;

Return « "*default*" ».

15.2.3.3 Static Semantics: ExportedBindings

export

;

export

;

Return a new empty List.

export

;

Return the ExportedBindings of ExportClause.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

export

default

export

default

;

Return the BoundNames of this ExportDeclaration.

{

}

Return a new empty List.

Let names be the ExportedBindings of ExportsList.
Append to names the elements of the ExportedBindings of ExportSpecifier.
Return names.

Return a List containing the StringValue of IdentifierName.

Return a List containing the StringValue of the first IdentifierName.

15.2.3.4 Static Semantics: ExportedNames

export

;

Return a new empty List.

export

;

export

;

Return the ExportedNames of ExportClause.

export

Return the BoundNames of VariableStatement.

export

Return the BoundNames of Declaration.

export

default

export

default

export

default

;

Return « "default" ».

{

}

Return a new empty List.

Let names be the ExportedNames of ExportsList.
Append to names the elements of the ExportedNames of ExportSpecifier.
Return names.

Return a List containing the StringValue of IdentifierName.

Return a List containing the StringValue of the second IdentifierName.

15.2.3.5 Static Semantics: ExportEntries

export

;

Let module be the sole element of ModuleRequests of FromClause.
Let entry be the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "*", [[LocalName]]: null, [[ExportName]]: null }.
Return a new List containing entry.

export

;

Let module be the sole element of ModuleRequests of FromClause.
Return ExportEntriesForModule of ExportClause with argument module.

export

;

Return ExportEntriesForModule of ExportClause with argument null.

export

Let entries be a new empty List.
Let names be the BoundNames of VariableStatement.
For each name in names, do
1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
Return entries.

export

Let entries be a new empty List.
Let names be the BoundNames of Declaration.
For each name in names, do
1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
Return entries.

export

default

Let names be BoundNames of HoistableDeclaration.
Let localName be the sole element of names.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.

export

default

Let names be BoundNames of ClassDeclaration.
Let localName be the sole element of names.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.

export

default

;

Let entry be the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: "*default*", [[ExportName]]: "default" }.
Return a new List containing entry.

Note

"*default*" is used within this specification as a synthetic name for anonymous default export values.

15.2.3.6 Static Semantics: ExportEntriesForModule

With parameter module.

{

}

Return a new empty List.

Let specs be the ExportEntriesForModule of ExportsList with argument module.
Append to specs the elements of the ExportEntriesForModule of ExportSpecifier with argument module.
Return specs.

Let sourceName be the StringValue of IdentifierName.
If module is null, then
1. Let localName be sourceName.
2. Let importName be null.
Else,
1. Let localName be null.
2. Let importName be sourceName.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.

Let sourceName be the StringValue of the first IdentifierName.
Let exportName be the StringValue of the second IdentifierName.
If module is null, then
1. Let localName be sourceName.
2. Let importName be null.
Else,
1. Let localName be null.
2. Let importName be sourceName.
Return a new List containing the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.

15.2.3.7 Static Semantics: IsConstantDeclaration

export

;

export

;

export

;

export

default

;

Return false.

Note

It is not necessary to treat export default AssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.

15.2.3.8 Static Semantics: LexicallyScopedDeclarations

export

;

export

;

export

;

export

Return a new empty List.

export

Return a new List containing DeclarationPart of Declaration.

export

default

Return a new List containing DeclarationPart of HoistableDeclaration.

export

default

Return a new List containing ClassDeclaration.

export

default

;

Return a new List containing this ExportDeclaration.

15.2.3.9 Static Semantics: ModuleRequests

export

;

export

;

Return the ModuleRequests of FromClause.

export

;

export

export

export

default

export

default

export

default

;

Return a new empty List.

15.2.3.10 Static Semantics: ReferencedBindings

{

}

Return a new empty List.

Let names be the ReferencedBindings of ExportsList.
Append to names the elements of the ReferencedBindings of ExportSpecifier.
Return names.

Return a List containing the IdentifierName.

Return a List containing the first IdentifierName.

15.2.3.11 Runtime Semantics: Evaluation

export

;

export

;

export

;

Return NormalCompletion(empty).

export

Return the result of evaluating VariableStatement.

export

Return the result of evaluating Declaration.

export

default

Return the result of evaluating HoistableDeclaration.

export

default

Let value be the result of BindingClassDeclarationEvaluation of ClassDeclaration.
ReturnIfAbrupt(value).
Let className be the sole element of BoundNames of ClassDeclaration.
If className is "*default*", then
1. Let hasNameProperty be ? HasOwnProperty(value, "name").
2. If hasNameProperty is false, perform SetFunctionName(value, "default").
3. Let env be the running execution context's LexicalEnvironment.
4. Perform ? InitializeBoundName("*default*", value, env).
Return NormalCompletion(empty).

export

default

;

Let rhs be the result of evaluating AssignmentExpression.
Let value be ? GetValue(rhs).
If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. Let hasNameProperty be ? HasOwnProperty(value, "name").
2. If hasNameProperty is false, perform SetFunctionName(value, "default").
Let env be the running execution context's LexicalEnvironment.
Perform ? InitializeBoundName("*default*", value, env).
Return NormalCompletion(empty).

16 Error Handling and Language Extensions

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.

An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

Except as restricted in 16.2, an implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in 16.2, an implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).

16.1 HostReportErrors ( `errorList` )

HostReportErrors is an implementation-defined abstract operation that allows host environments to report parsing errors, early errors, and runtime errors.

An implementation of HostReportErrors must complete normally in all cases. The default implementation of HostReportErrors is to unconditionally return an empty normal completion.

Note

errorList will be a List of ECMAScript language values. If the errors are parsing errors or early errors, these will always be SyntaxError or ReferenceError objects. Runtime errors, however, can be any ECMAScript value.

16.2 Forbidden Extensions

An implementation must not extend this specification in the following ways:

ECMAScript function objects defined using syntactic constructors in strict mode code must not be created with own properties named "caller" or "arguments". Such own properties also must not be created for function objects defined using an ArrowFunction, MethodDefinition, GeneratorDeclaration, GeneratorExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, ClassDeclaration, ClassExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, or AsyncArrowFunction regardless of whether the definition is contained in strict mode code. Built-in functions, strict functions created using the Function constructor, generator functions created using the Generator constructor, async functions created using the AsyncFunction constructor, and functions created using the bind method also must not be created with such own properties.
If an implementation extends any function object with an own property named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never return a strict function when called.
Neither mapped nor unmapped arguments objects may be created with an own property named "caller".
The behaviour of the following methods must not be extended except as specified in ECMA-402: Object.prototype.toLocaleString, Array.prototype.toLocaleString, Number.prototype.toLocaleString, Date.prototype.toLocaleDateString, Date.prototype.toLocaleString, Date.prototype.toLocaleTimeString, String.prototype.localeCompare, %TypedArray%.prototype.toLocaleString.
The RegExp pattern grammars in 21.2.1 and B.1.4 must not be extended to recognize any of the source characters A-Z or a-z as IdentityEscape[+U] when the _[U] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately follow source text that matches the BindingIdentifier nonterminal symbol.
When processing strict mode code, the syntax of NumericLiteral must not be extended to include LegacyOctalIntegerLiteral and the syntax of DecimalIntegerLiteral must not be extended to include NonOctalDecimalIntegerLiteral as described in B.1.1.
TemplateCharacter must not be extended to include LegacyOctalEscapeSequence as defined in B.1.2.
When processing strict mode code, the extensions defined in B.3.2, B.3.3, B.3.4, and B.3.6 must not be supported.
When parsing for the Module goal symbol, the lexical grammar extensions defined in B.1.3 must not be supported.

17 ECMAScript Standard Built-in Objects

There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.

Unless specified otherwise, a built-in object that is callable as a function is a built-in function object with the characteristics described in 9.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 9.3.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.

Note 1

Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (19.2.3), as the value of its [[Prototype]] internal slot.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (19.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation (9.3.3).

Every built-in function object, including constructors, has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description. Optional parameters (which are indicated with brackets: [ ]) or rest parameters (which are shown using the form «...name») are not included in the default argument count.

Note 2

For example, the function object that is the initial value of the map property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the length property of that function object is 1.

Unless otherwise specified, the length property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every built-in function object, including constructors, that is not identified as an anonymous function has a name property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get " or "set " prepended to the property name string. The value of the name property is explicitly specified for each built-in functions whose property key is a Symbol value.

Unless otherwise specified, the name property of a built-in function object, if it exists, has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every other data property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.

Every accessor property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.

18 The Global Object

The global object:

is created before control enters any execution context.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is implementation-dependent.
may have host defined properties in addition to the properties defined in this specification. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.

18.1 Value Properties of the Global Object

18.1.1 Infinity

The value of Infinity is +∞ (see 6.1.6). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.1.2 NaN

The value of NaN is NaN (see 6.1.6). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.1.3 undefined

The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

18.2 Function Properties of the Global Object

18.2.1 eval ( `x` )

The eval function is the %eval% intrinsic object. When the eval function is called with one argument x, the following steps are taken:

Assert: The execution context stack has at least two elements.
Let callerContext be the second to top element of the execution context stack.
Let callerRealm be callerContext's Realm.
Let calleeRealm be the current Realm Record.
Perform ? HostEnsureCanCompileStrings(callerRealm, calleeRealm).
Return ? PerformEval(x, calleeRealm, false, false).

18.2.1.1 Runtime Semantics: PerformEval ( `x`, `evalRealm`, `strictCaller`, `direct` )

The abstract operation PerformEval with arguments x, evalRealm, strictCaller, and direct performs the following steps:

Assert: If direct is false, then strictCaller is also false.
If Type(x) is not String, return x.
Let thisEnvRec be ! GetThisEnvironment().
If thisEnvRec is a function Environment Record, then
1. Let F be thisEnvRec.[[FunctionObject]].
2. Let inFunction be true.
3. Let inMethod be thisEnvRec.HasSuperBinding().
4. If F.[[ConstructorKind]] is "derived", let inDerivedConstructor be true; otherwise, let inDerivedConstructor be false.
Else,
1. Let inFunction be false.
2. Let inMethod be false.
3. Let inDerivedConstructor be false.
Let script be the ECMAScript code that is the result of parsing x, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, for the goal symbol Script. If inFunction is false, additional early error rules from 18.2.1.1.1 are applied. If inMethod is false, additional early error rules from 18.2.1.1.2 are applied. If inDerivedConstructor is false, additional early error rules from 18.2.1.1.3 are applied. If the parse fails, throw a SyntaxError exception. If any early errors are detected, throw a SyntaxError or a ReferenceError exception, depending on the type of the error (but see also clause 16). Parsing and early error detection may be interweaved in an implementation-dependent manner.
If script Contains ScriptBody is false, return undefined.
Let body be the ScriptBody of script.
If strictCaller is true, let strictEval be true.
Else, let strictEval be IsStrict of script.
Let ctx be the running execution context.
NOTE: If direct is true, ctx will be the execution context that performed the direct eval. If direct is false, ctx will be the execution context for the invocation of the eval function.
If direct is true, then
1. Let lexEnv be NewDeclarativeEnvironment(ctx's LexicalEnvironment).
2. Let varEnv be ctx's VariableEnvironment.
Else,
1. Let lexEnv be NewDeclarativeEnvironment(evalRealm.[[GlobalEnv]]).
2. Let varEnv be evalRealm.[[GlobalEnv]].
If strictEval is true, set varEnv to lexEnv.
If ctx is not already suspended, suspend ctx.
Let evalCxt be a new ECMAScript code execution context.
Set the evalCxt's Function to null.
Set the evalCxt's Realm to evalRealm.
Set the evalCxt's ScriptOrModule to ctx's ScriptOrModule.
Set the evalCxt's VariableEnvironment to varEnv.
Set the evalCxt's LexicalEnvironment to lexEnv.
Push evalCxt on to the execution context stack; evalCxt is now the running execution context.
Let result be EvalDeclarationInstantiation(body, varEnv, lexEnv, strictEval).
If result.[[Type]] is normal, then
1. Set result to the result of evaluating body.
If result.[[Type]] is normal and result.[[Value]] is empty, then
1. Set result to NormalCompletion(undefined).
Suspend evalCxt and remove it from the execution context stack.
Resume the context that is now on the top of the execution context stack as the running execution context.
Return Completion(result).

Note

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling context or the eval code is strict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.

18.2.1.1.1 Additional Early Error Rules for Eval Outside Functions

These static semantics are applied by PerformEval when a direct eval call occurs outside of any function.

It is a Syntax Error if StatementList Contains NewTarget.

18.2.1.1.2 Additional Early Error Rules for Eval Outside Methods

These static semantics are applied by PerformEval when a direct eval call occurs outside of a MethodDefinition.

It is a Syntax Error if StatementList Contains SuperProperty.

18.2.1.1.3 Additional Early Error Rules for Eval Outside Constructor Methods

These static semantics are applied by PerformEval when a direct eval call occurs outside of the constructor method of a ClassDeclaration or ClassExpression.

It is a Syntax Error if StatementList Contains SuperCall.

18.2.1.2 HostEnsureCanCompileStrings ( `callerRealm`, `calleeRealm` )

HostEnsureCanCompileStrings is an implementation-defined abstract operation that allows host environments to block certain ECMAScript functions which allow developers to compile strings into ECMAScript code.

An implementation of HostEnsureCanCompileStrings may complete normally or abruptly. Any abrupt completions will be propagated to its callers. The default implementation of HostEnsureCanCompileStrings is to unconditionally return an empty normal completion.

18.2.1.3 Runtime Semantics: EvalDeclarationInstantiation ( `body`, `varEnv`, `lexEnv`, `strict` )

When the abstract operation EvalDeclarationInstantiation is called with arguments body, varEnv, lexEnv, and strict, the following steps are taken:

Let varNames be the VarDeclaredNames of body.
Let varDeclarations be the VarScopedDeclarations of body.
Let lexEnvRec be lexEnv's EnvironmentRecord.
Let varEnvRec be varEnv's EnvironmentRecord.
If strict is false, then
1. If varEnvRec is a global Environment Record, then
  1. For each name in varNames, do
    1. If varEnvRec.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
    2. NOTE: eval will not create a global var declaration that would be shadowed by a global lexical declaration.
2. Let thisLex be lexEnv.
3. Assert: The following loop will terminate.
4. Repeat, while thisLex is not the same as varEnv,
  1. Let thisEnvRec be thisLex's EnvironmentRecord.
  2. If thisEnvRec is not an object Environment Record, then
    1. NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
    2. For each name in varNames, do
      1. If thisEnvRec.HasBinding(name) is true, then
        Throw a SyntaxError exception.
        NOTE: Annex B.3.5 defines alternate semantics for the above step.
      2. NOTE: A direct eval will not hoist var declaration over a like-named lexical declaration.
  3. Set thisLex to thisLex's outer environment reference.
Let functionsToInitialize be a new empty List.
Let declaredFunctionNames be a new empty List.
For each d in varDeclarations, in reverse list order, do
1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
  3. Let fn be the sole element of the BoundNames of d.
  4. If fn is not an element of declaredFunctionNames, then
    1. If varEnvRec is a global Environment Record, then
      1. Let fnDefinable be ? varEnvRec.CanDeclareGlobalFunction(fn).
      2. If fnDefinable is false, throw a TypeError exception.
    2. Append fn to declaredFunctionNames.
    3. Insert d as the first element of functionsToInitialize.
NOTE: Annex B.3.3.3 adds additional steps at this point.
Let declaredVarNames be a new empty List.
For each d in varDeclarations, do
1. If d is a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
  1. For each String vn in the BoundNames of d, do
    1. If vn is not an element of declaredFunctionNames, then
      1. If varEnvRec is a global Environment Record, then
        Let vnDefinable be ? varEnvRec.CanDeclareGlobalVar(vn).
        If vnDefinable is false, throw a TypeError exception.
      2. If vn is not an element of declaredVarNames, then
        Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step unless varEnvRec is a global Environment Record and the global object is a Proxy exotic object.
Let lexDeclarations be the LexicallyScopedDeclarations of body.
For each element d in lexDeclarations, do
1. NOTE: Lexically declared names are only instantiated here but not initialized.
2. For each element dn of the BoundNames of d, do
  1. If IsConstantDeclaration of d is true, then
    1. Perform ? lexEnvRec.CreateImmutableBinding(dn, true).
  2. Else,
    1. Perform ? lexEnvRec.CreateMutableBinding(dn, false).
For each Parse Node f in functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be the result of performing InstantiateFunctionObject for f with argument lexEnv.
3. If varEnvRec is a global Environment Record, then
  1. Perform ? varEnvRec.CreateGlobalFunctionBinding(fn, fo, true).
4. Else,
  1. Let bindingExists be varEnvRec.HasBinding(fn).
  2. If bindingExists is false, then
    1. Let status be ! varEnvRec.CreateMutableBinding(fn, true).
    2. Assert: status is not an abrupt completion because of validation preceding step 12.
    3. Perform ! varEnvRec.InitializeBinding(fn, fo).
  3. Else,
    1. Perform ! varEnvRec.SetMutableBinding(fn, fo, false).
For each String vn in declaredVarNames, in list order, do
1. If varEnvRec is a global Environment Record, then
  1. Perform ? varEnvRec.CreateGlobalVarBinding(vn, true).
2. Else,
  1. Let bindingExists be varEnvRec.HasBinding(vn).
  2. If bindingExists is false, then
    1. Let status be ! varEnvRec.CreateMutableBinding(vn, true).
    2. Assert: status is not an abrupt completion because of validation preceding step 12.
    3. Perform ! varEnvRec.InitializeBinding(vn, undefined).
Return NormalCompletion(empty).

Note

An alternative version of this algorithm is described in B.3.5.

18.2.2 isFinite ( `number` )

The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:

Let num be ? ToNumber(number).
If num is NaN, +∞, or -∞, return false.
Otherwise, return true.

18.2.3 isNaN ( `number` )

The isNaN function is the %isNaN% intrinsic object. When the isNaN function is called with one argument number, the following steps are taken:

Let num be ? ToNumber(number).
If num is NaN, return true.
Otherwise, return false.

Note

A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.

18.2.4 parseFloat ( `string` )

The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.

The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:

Let inputString be ? ToString(string).
Let trimmedString be a substring of inputString consisting of the leftmost code unit that is not a StrWhiteSpaceChar and all code units to the right of that code unit. (In other words, remove leading white space.) If inputString does not contain any such code units, let trimmedString be the empty string.
If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 7.1.3.1), return NaN.
Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
Let mathFloat be MV of numberString.
If mathFloat=0, then
1. If the first code unit of trimmedString is the code unit 0x002D (HYPHEN-MINUS), return -0.
2. Return +0.
Return the Number value for mathFloat.

Note

parseFloat may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.

18.2.5 parseInt ( `string`, `radix` )

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.

The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:

Let inputString be ? ToString(string).
Let S be a newly created substring of inputString consisting of the first code unit that is not a StrWhiteSpaceChar and all code units following that code unit. (In other words, remove leading white space.) If inputString does not contain any such code unit, let S be the empty string.
Let sign be 1.
If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), let sign be -1.
If S is not empty and the first code unit of S is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from S.
Let R be ? ToInt32(radix).
Let stripPrefix be true.
If R ≠ 0, then
1. If R < 2 or R > 36, return NaN.
2. If R ≠ 16, let stripPrefix be false.
Else R = 0,
1. Let R be 10.
If stripPrefix is true, then
1. If the length of S is at least 2 and the first two code units of S are either "0x" or "0X", remove the first two code units from S and let R be 16.
If S contains a code unit that is not a radix-R digit, let Z be the substring of S consisting of all code units before the first such code unit; otherwise, let Z be S.
If Z is empty, return NaN.
Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
If mathInt = 0, then
1. If sign = -1, return -0.
2. Return +0.
Let number be the Number value for mathInt.
Return sign × number.

Note

parseInt may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.

18.2.6 URI Handling Functions

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 18.2.6.2, 18.2.6.3, 18.2.6.4 and 18.2.6.5

Note

Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

18.2.6.1 URI Syntax and Semantics

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme

:

First

/

Second

;

Third

?

Fourth

where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

:::

opt

:::

opt

:::

:::

one of

;

:::

:::

:::

one of

uriMark

:::

one of

(

)

Note

The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

Runtime Semantics

When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0, 127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form "%xx".

18.2.6.1.1 Runtime Semantics: Encode ( `string`, `unescapedSet` )

The encoding and escaping process is described by the abstract operation Encode taking two String arguments string and unescapedSet.

Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat,
1. If k equals strLen, return R.
2. Let C be the code unit at index k within string.
3. If C is in unescapedSet, then
  1. Let S be the String value containing only the code unit C.
  2. Set R to the string-concatenation of the previous value of R and S.
4. Else C is not in unescapedSet,
  1. If C is a 尾代理, throw a URIError exception.
  2. If C is not a 首代理, then
    1. Let V be the code point with the same numeric value as code unit C.
  3. Else,
    1. Increase k by 1.
    2. If k equals strLen, throw a URIError exception.
    3. Let kChar be the code unit at index k within string.
    4. If kChar is not a 尾代理, throw a URIError exception.
    5. Let V be UTF16Decode(C, kChar).
  4. Let Octets be the List of octets resulting by applying the UTF-8 transformation to V.
  5. For each element octet of Octets in List order, do
    1. Let S be the string-concatenation of:
      - "%"
      - the String representation of octet, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
    2. Set R to the string-concatenation of the previous value of R and S.
5. Increase k by 1.

18.2.6.1.2 Runtime Semantics: Decode ( `string`, `reservedSet` )

The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.

Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat,
1. If k equals strLen, return R.
2. Let C be the code unit at index k within string.
3. If C is not the code unit 0x0025 (PERCENT SIGN), then
  1. Let S be the String value containing only the code unit C.
4. Else C is the code unit 0x0025 (PERCENT SIGN),
  1. Let start be k.
  2. If k + 2 is greater than or equal to strLen, throw a URIError exception.
  3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
  4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
  5. Increment k by 2.
  6. If the most significant bit in B is 0, then
    1. Let C be the code unit whose value is B.
    2. If C is not in reservedSet, then
      1. Let S be the String value containing only the code unit C.
    3. Else C is in reservedSet,
      1. Let S be the substring of string from index start to index k inclusive.
  7. Else the most significant bit in B is 1,
    1. Let n be the smallest nonnegative integer such that (B << n) & 0x80 is equal to 0.
    2. If n equals 1 or n is greater than 4, throw a URIError exception.
    3. Let Octets be a List of 8-bit integers of size n.
    4. Set Octets[0] to B.
    5. If k + (3 × (n - 1)) is greater than or equal to strLen, throw a URIError exception.
    6. Let j be 1.
    7. Repeat, while j < n
      1. Increment k by 1.
      2. If the code unit at index k within string is not the code unit 0x0025 (PERCENT SIGN), throw a URIError exception.
      3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
      4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
      5. If the two most significant bits in B are not 10, throw a URIError exception.
      6. Increment k by 2.
      7. Set Octets[j] to B.
      8. Increment j by 1.
    8. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a URIError exception.
    9. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from a List of octets into a 21-bit value.
    10. Let S be the String value whose elements are, in order, the elements in UTF16Encoding(V).
5. Set R to the string-concatenation of the previous value of R and S.
6. Increase k by 1.

Note

This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 44.

Table 44 (Informative): UTF-8 Encodings

Code Unit Value	Representation	1^st Octet	2^nd Octet	3^rd Octet	4^th Octet
`0x0000 - 0x007F`	`00000000 0zzzzzzz`	`0zzzzzzz`
`0x0080 - 0x07FF`	`00000yyy yyzzzzzz`	`110yyyyy`	`10zzzzzz`
`0x0800 - 0xD7FF`	`xxxxyyyy yyzzzzzz`	`1110xxxx`	`10yyyyyy`	`10zzzzzz`
`0xD800 - 0xDBFF` followed by `0xDC00 - 0xDFFF`	`110110vv vvwwwwxx` followed by `110111yy yyzzzzzz`	`11110uuu`	`10uuwwww`	`10xxyyyy`	`10zzzzzz`
`0xD800 - 0xDBFF` not followed by `0xDC00 - 0xDFFF`	causes `URIError`
`0xDC00 - 0xDFFF`	causes `URIError`
`0xE000 - 0xFFFF`	`xxxxyyyy yyzzzzzz`	`1110xxxx`	`10yyyyyy`	`10zzzzzz`

Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).

The above transformation combines each 代理对 (for which code unit values in the inclusive range 0xD800 to 0xDFFF are reserved) into a UTF-32 representation and encodes the resulting 21-bit value into UTF-8. Decoding reconstructs the surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.

18.2.6.2 decodeURI ( `encodedURI` )

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:

Let uriString be ? ToString(encodedURI).
Let reservedURISet be a String containing one instance of each code unit valid in uriReserved plus "#".
Return ? Decode(uriString, reservedURISet).

Note

The code point "#" is not decoded from escape sequences even though it is not a reserved URI code point.

18.2.6.3 decodeURIComponent ( `encodedURIComponent` )

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.

The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

Let componentString be ? ToString(encodedURIComponent).
Let reservedURIComponentSet be the empty String.
Return ? Decode(componentString, reservedURIComponentSet).

18.2.6.4 encodeURI ( `uri` )

The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.

The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:

Let uriString be ? ToString(uri).
Let unescapedURISet be a String containing one instance of each code unit valid in uriReserved and uriUnescaped plus "#".
Return ? Encode(uriString, unescapedURISet).

Note

The code unit "#" is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.

18.2.6.5 encodeURIComponent ( `uriComponent` )

The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.

The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

Let componentString be ? ToString(uriComponent).
Let unescapedURIComponentSet be a String containing one instance of each code unit valid in uriUnescaped.
Return ? Encode(componentString, unescapedURIComponentSet).

18.3 Constructor Properties of the Global Object

18.3.1 Array ( . . . )

See 22.1.1.

18.3.2 ArrayBuffer ( . . . )

See 24.1.2.

18.3.3 Boolean ( . . . )

See 19.3.1.

18.3.4 DataView ( . . . )

See 24.3.2.

18.3.5 Date ( . . . )

See 20.3.2.

18.3.6 Error ( . . . )

See 19.5.1.

18.3.7 EvalError ( . . . )

See 19.5.5.1.

18.3.8 Float32Array ( . . . )

See 22.2.4.

18.3.9 Float64Array ( . . . )

See 22.2.4.

18.3.10 Function ( . . . )

See 19.2.1.

18.3.11 Int8Array ( . . . )

See 22.2.4.

18.3.12 Int16Array ( . . . )

See 22.2.4.

18.3.13 Int32Array ( . . . )

See 22.2.4.

18.3.14 Map ( . . . )

See 23.1.1.

18.3.15 Number ( . . . )

See 20.1.1.

18.3.16 Object ( . . . )

See 19.1.1.

18.3.17 Promise ( . . . )

See 25.6.3.

18.3.18 Proxy ( . . . )

See 26.2.1.

18.3.19 RangeError ( . . . )

See 19.5.5.2.

18.3.20 ReferenceError ( . . . )

See 19.5.5.3.

18.3.21 RegExp ( . . . )

See 21.2.3.

18.3.22 Set ( . . . )

See 23.2.1.

18.3.23 SharedArrayBuffer ( . . . )

See 24.2.2.

18.3.24 String ( . . . )

See 21.1.1.

18.3.25 Symbol ( . . . )

See 19.4.1.

18.3.26 SyntaxError ( . . . )

See 19.5.5.4.

18.3.27 TypeError ( . . . )

See 19.5.5.5.

18.3.28 Uint8Array ( . . . )

See 22.2.4.

18.3.29 Uint8ClampedArray ( . . . )

See 22.2.4.

18.3.30 Uint16Array ( . . . )

See 22.2.4.

18.3.31 Uint32Array ( . . . )

See 22.2.4.

18.3.32 URIError ( . . . )

See 19.5.5.6.

18.3.33 WeakMap ( . . . )

See 23.3.1.

18.3.34 WeakSet ( . . . )

See 23.4.

18.4 Other Properties of the Global Object

18.4.1 Atomics

See 24.4.

18.4.2 JSON

See 24.5.

18.4.3 Math

See 20.2.

18.4.4 Reflect

See 26.1.

19 Fundamental Objects

19.1 Object Objects

19.1.1 The Object Constructor

The Object constructor:

is the intrinsic object %Object%.
is the initial value of the Object property of the global object.
creates a new ordinary object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition.

19.1.1.1 Object ( [ `value` ] )

When Object function is called with optional argument value, the following steps are taken:

If NewTarget is neither undefined nor the active function, then
1. Return ? OrdinaryCreateFromConstructor(NewTarget, "%ObjectPrototype%").
If value is null, undefined or not supplied, return ObjectCreate(%ObjectPrototype%).
Return ! ToObject(value).

The length property of the Object constructor function is 1.

19.1.2 Properties of the Object Constructor

The Object constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has a length property.
has the following additional properties:

19.1.2.1 Object.assign ( `target`, ...`sources` )

The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:

Let to be ? ToObject(target).
If only one argument was passed, return to.
Let sources be the List of argument values starting with the second argument.
For each element nextSource of sources, in ascending index order, do
1. If nextSource is undefined or null, let keys be a new empty List.
2. Else,
  1. Let from be ! ToObject(nextSource).
  2. Let keys be ? from.[[OwnPropertyKeys]]().
3. For each element nextKey of keys in List order, do
  1. Let desc be ? from.[[GetOwnProperty]](nextKey).
  2. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. Let propValue be ? Get(from, nextKey).
    2. Perform ? Set(to, nextKey, propValue, true).
Return to.

The length property of the assign function is 2.

19.1.2.2 Object.create ( `O`, `Properties` )

The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:

If Type(O) is neither Object nor Null, throw a TypeError exception.
Let obj be ObjectCreate(O).
If Properties is not undefined, then
1. Return ? ObjectDefineProperties(obj, Properties).
Return obj.

19.1.2.3 Object.defineProperties ( `O`, `Properties` )

The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:

Return ? ObjectDefineProperties(O, Properties).

19.1.2.3.1 Runtime Semantics: ObjectDefineProperties ( `O`, `Properties` )

The abstract operation ObjectDefineProperties with arguments O and Properties performs the following steps:

If Type(O) is not Object, throw a TypeError exception.
Let props be ? ToObject(Properties).
Let keys be ? props.[[OwnPropertyKeys]]().
Let descriptors be a new empty List.
For each element nextKey of keys in List order, do
1. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
2. If propDesc is not undefined and propDesc.[[Enumerable]] is true, then
  1. Let descObj be ? Get(props, nextKey).
  2. Let desc be ? ToPropertyDescriptor(descObj).
  3. Append the pair (a two element List) consisting of nextKey and desc to the end of descriptors.
For each pair from descriptors in list order, do
1. Let P be the first element of pair.
2. Let desc be the second element of pair.
3. Perform ? DefinePropertyOrThrow(O, P, desc).
Return O.

19.1.2.4 Object.defineProperty ( `O`, `P`, `Attributes` )

The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:

If Type(O) is not Object, throw a TypeError exception.
Let key be ? ToPropertyKey(P).
Let desc be ? ToPropertyDescriptor(Attributes).
Perform ? DefinePropertyOrThrow(O, key, desc).
Return O.

19.1.2.5 Object.entries ( `O` )

When the entries function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, "key+value").
Return CreateArrayFromList(nameList).

19.1.2.6 Object.freeze ( `O` )

When the freeze function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? SetIntegrityLevel(O, "frozen").
If status is false, throw a TypeError exception.
Return O.

19.1.2.7 Object.getOwnPropertyDescriptor ( `O`, `P` )

When the getOwnPropertyDescriptor function is called, the following steps are taken:

Let obj be ? ToObject(O).
Let key be ? ToPropertyKey(P).
Let desc be ? obj.[[GetOwnProperty]](key).
Return FromPropertyDescriptor(desc).

19.1.2.8 Object.getOwnPropertyDescriptors ( `O` )

When the getOwnPropertyDescriptors function is called, the following steps are taken:

Let obj be ? ToObject(O).
Let ownKeys be ? obj.[[OwnPropertyKeys]]().
Let descriptors be ! ObjectCreate(%ObjectPrototype%).
For each element key of ownKeys in List order, do
1. Let desc be ? obj.[[GetOwnProperty]](key).
2. Let descriptor be ! FromPropertyDescriptor(desc).
3. If descriptor is not undefined, perform ! CreateDataProperty(descriptors, key, descriptor).
Return descriptors.

19.1.2.9 Object.getOwnPropertyNames ( `O` )

When the getOwnPropertyNames function is called, the following steps are taken:

Return ? GetOwnPropertyKeys(O, String).

19.1.2.10 Object.getOwnPropertySymbols ( `O` )

When the getOwnPropertySymbols function is called with argument O, the following steps are taken:

Return ? GetOwnPropertyKeys(O, Symbol).

19.1.2.10.1 Runtime Semantics: GetOwnPropertyKeys ( `O`, `Type` )

The abstract operation GetOwnPropertyKeys is called with arguments O and Type where O is an Object and Type is one of the ECMAScript specification types String or Symbol. The following steps are taken:

Let obj be ? ToObject(O).
Let keys be ? obj.[[OwnPropertyKeys]]().
Let nameList be a new empty List.
For each element nextKey of keys in List order, do
1. If Type(nextKey) is Type, then
  1. Append nextKey as the last element of nameList.
Return CreateArrayFromList(nameList).

19.1.2.11 Object.getPrototypeOf ( `O` )

When the getPrototypeOf function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Return ? obj.[[GetPrototypeOf]]().

19.1.2.12 Object.is ( `value1`, `value2` )

When the is function is called with arguments value1 and value2, the following steps are taken:

Return SameValue(value1, value2).

19.1.2.13 Object.isExtensible ( `O` )

When the isExtensible function is called with argument O, the following steps are taken:

If Type(O) is not Object, return false.
Return ? IsExtensible(O).

19.1.2.14 Object.isFrozen ( `O` )

When the isFrozen function is called with argument O, the following steps are taken:

If Type(O) is not Object, return true.
Return ? TestIntegrityLevel(O, "frozen").

19.1.2.15 Object.isSealed ( `O` )

When the isSealed function is called with argument O, the following steps are taken:

If Type(O) is not Object, return true.
Return ? TestIntegrityLevel(O, "sealed").

19.1.2.16 Object.keys ( `O` )

When the keys function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, "key").
Return CreateArrayFromList(nameList).

19.1.2.17 Object.preventExtensions ( `O` )

When the preventExtensions function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? O.[[PreventExtensions]]().
If status is false, throw a TypeError exception.
Return O.

19.1.2.18 Object.prototype

The initial value of Object.prototype is the intrinsic object %ObjectPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.1.2.19 Object.seal ( `O` )

When the seal function is called, the following steps are taken:

If Type(O) is not Object, return O.
Let status be ? SetIntegrityLevel(O, "sealed").
If status is false, throw a TypeError exception.
Return O.

19.1.2.20 Object.setPrototypeOf ( `O`, `proto` )

When the setPrototypeOf function is called with arguments O and proto, the following steps are taken:

Let O be ? RequireObjectCoercible(O).
If Type(proto) is neither Object nor Null, throw a TypeError exception.
If Type(O) is not Object, return O.
Let status be ? O.[[SetPrototypeOf]](proto).
If status is false, throw a TypeError exception.
Return O.

19.1.2.21 Object.values ( `O` )

When the values function is called with argument O, the following steps are taken:

Let obj be ? ToObject(O).
Let nameList be ? EnumerableOwnPropertyNames(obj, "value").
Return CreateArrayFromList(nameList).

19.1.3 Properties of the Object Prototype Object

The Object prototype object:

is the intrinsic object %ObjectPrototype%.
is an immutable prototype exotic object.
has a [[Prototype]] internal slot whose value is null.

19.1.3.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is the intrinsic object %Object%.

19.1.3.2 Object.prototype.hasOwnProperty ( `V` )

When the hasOwnProperty method is called with argument V, the following steps are taken:

Let P be ? ToPropertyKey(V).
Let O be ? ToObject(this value).
Return ? HasOwnProperty(O, P).

Note

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

19.1.3.3 Object.prototype.isPrototypeOf ( `V` )

When the isPrototypeOf method is called with argument V, the following steps are taken:

If Type(V) is not Object, return false.
Let O be ? ToObject(this value).
Repeat,
1. Let V be ? V.[[GetPrototypeOf]]().
2. If V is null, return false.
3. If SameValue(O, V) is true, return true.

Note

The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

19.1.3.4 Object.prototype.propertyIsEnumerable ( `V` )

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

Let P be ? ToPropertyKey(V).
Let O be ? ToObject(this value).
Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return false.
Return desc.[[Enumerable]].

Note 1

This method does not consider objects in the prototype chain.

Note 2

19.1.3.5 Object.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

When the toLocaleString method is called, the following steps are taken:

Let O be the this value.
Return ? Invoke(O, "toString").

The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.

Note 1

This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and Typed Arrays provide their own locale-sensitive toLocaleString methods.

Note 2

ECMA-402 intentionally does not provide an alternative to this default implementation.

19.1.3.6 Object.prototype.toString ( )

When the toString method is called, the following steps are taken:

If the this value is undefined, return "[object Undefined]".
If the this value is null, return "[object Null]".
Let O be ! ToObject(this value).
Let isArray be ? IsArray(O).
If isArray is true, let builtinTag be "Array".
Else if O is a String exotic object, let builtinTag be "String".
Else if O has a [[ParameterMap]] internal slot, let builtinTag be "Arguments".
Else if O has a [[Call]] internal method, let builtinTag be "Function".
Else if O has an [[ErrorData]] internal slot, let builtinTag be "Error".
Else if O has a [[BooleanData]] internal slot, let builtinTag be "Boolean".
Else if O has a [[NumberData]] internal slot, let builtinTag be "Number".
Else if O has a [[DateValue]] internal slot, let builtinTag be "Date".
Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be "RegExp".
Else, let builtinTag be "Object".
Let tag be ? Get(O, @@toStringTag).
If Type(tag) is not String, let tag be builtinTag.
Return the string-concatenation of "[object ", tag, and "]".

This function is the %ObjProto_toString% intrinsic object.

Note

Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.

19.1.3.7 Object.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

Return ? ToObject(this value).

This function is the %ObjProto_valueOf% intrinsic object.

19.1.4 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

19.2 Function Objects

19.2.1 The Function Constructor

The Function constructor:

is the intrinsic object %Function%.
is the initial value of the Function property of the global object.
creates and initializes a new function object when called as a function rather than as a constructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.

19.2.1.1 Function ( `p1`, `p2`, … , `pn`, `body` )

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “ p ” arguments, and where body might also not be provided), the following steps are taken:

Let C be the active function object.
Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
Return ? CreateDynamicFunction(C, NewTarget, "normal", args).

Note

It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

19.2.1.1.1 Runtime Semantics: CreateDynamicFunction ( `constructor`, `newTarget`, `kind`, `args` )

The abstract operation CreateDynamicFunction is called with arguments constructor, newTarget, kind, and args. constructor is the constructor function that is performing this action, newTarget is the constructor that new was initially applied to, kind is either "normal", "generator", "async", or "async generator", and args is a List containing the actual argument values that were passed to constructor. The following steps are taken:

Assert: The execution context stack has at least two elements.
Let callerContext be the second to top element of the execution context stack.
Let callerRealm be callerContext's Realm.
Let calleeRealm be the current Realm Record.
Perform ? HostEnsureCanCompileStrings(callerRealm, calleeRealm).
If newTarget is undefined, set newTarget to constructor.
If kind is "normal", then
1. Let goal be the grammar symbol FunctionBody[~Yield, ~Await].
2. Let parameterGoal be the grammar symbol FormalParameters[~Yield, ~Await].
3. Let fallbackProto be "%FunctionPrototype%".
Else if kind is "generator", then
1. Let goal be the grammar symbol GeneratorBody.
2. Let parameterGoal be the grammar symbol FormalParameters[+Yield, ~Await].
3. Let fallbackProto be "%Generator%".
Else if kind is "async", then
1. Assert: kind is "async".
2. Let goal be the grammar symbol AsyncFunctionBody.
3. Let parameterGoal be the grammar symbol FormalParameters[~Yield, +Await].
4. Let fallbackProto be "%AsyncFunctionPrototype%".
Else,
1. Assert: kind is "async generator".
2. Let goal be the grammar symbol AsyncGeneratorBody.
3. Let parameterGoal be the grammar symbol FormalParameters[+Yield, +Await].
4. Let fallbackProto be "%AsyncGenerator%".
Let argCount be the number of elements in args.
Let P be the empty String.
If argCount = 0, let bodyText be the empty String.
Else if argCount = 1, let bodyText be args[0].
Else argCount > 1,
1. Let firstArg be args[0].
2. Set P to ? ToString(firstArg).
3. Let k be 1.
4. Repeat, while k < argCount-1
  1. Let nextArg be args[k].
  2. Let nextArgString be ? ToString(nextArg).
  3. Set P to the string-concatenation of the previous value of P, "," (a comma), and nextArgString.
  4. Increase k by 1.
5. Let bodyText be args[k].
Set bodyText to ? ToString(bodyText).
Let parameters be the result of parsing P, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using parameterGoal as the goal symbol. Throw a SyntaxError exception if the parse fails.
Let body be the result of parsing bodyText, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using goal as the goal symbol. Throw a SyntaxError exception if the parse fails.
Let strict be ContainsUseStrict of body.
If any static semantics errors are detected for parameters or body, throw a SyntaxError or a ReferenceError exception, depending on the type of the error. If strict is true, the Early Error rules for UniqueFormalParameters:FormalParameters are applied. Parsing and early error detection may be interweaved in an implementation-dependent manner.
If strict is true and IsSimpleParameterList of parameters is false, throw a SyntaxError exception.
If any element of the BoundNames of parameters also occurs in the LexicallyDeclaredNames of body, throw a SyntaxError exception.
If body Contains SuperCall is true, throw a SyntaxError exception.
If parameters Contains SuperCall is true, throw a SyntaxError exception.
If body Contains SuperProperty is true, throw a SyntaxError exception.
If parameters Contains SuperProperty is true, throw a SyntaxError exception.
If kind is "generator" or "async generator", then
1. If parameters Contains YieldExpression is true, throw a SyntaxError exception.
If kind is "async" or "async generator", then
1. If parameters Contains AwaitExpression is true, throw a SyntaxError exception.
If strict is true, then
1. If BoundNames of parameters contains any duplicate elements, throw a SyntaxError exception.
Let proto be ? GetPrototypeFromConstructor(newTarget, fallbackProto).
Let F be FunctionAllocate(proto, strict, kind).
Let realmF be F.[[Realm]].
Let scope be realmF.[[GlobalEnv]].
Perform FunctionInitialize(F, Normal, parameters, body, scope).
If kind is "generator", then
1. Let prototype be ObjectCreate(%GeneratorPrototype%).
2. Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Else if kind is "async generator", then
1. Let prototype be ObjectCreate(%AsyncGeneratorPrototype%).
2. Perform DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Else if kind is "normal", perform MakeConstructor(F).
NOTE: Async functions are not constructable and do not have a [[Construct]] internal method or a "prototype" property.
Perform SetFunctionName(F, "anonymous").
Return F.

Note

A prototype property is created for every non-async function created using CreateDynamicFunction to provide for the possibility that the function will be used as a constructor.

19.2.2 Properties of the Function Constructor

The Function constructor:

is itself a built-in function object.
has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

19.2.2.1 Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.2.2.2 Function.prototype

The value of Function.prototype is %FunctionPrototype%, the intrinsic Function prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.2.3 Properties of the Function Prototype Object

The Function prototype object:

is the intrinsic object %FunctionPrototype%.
is itself a built-in function object.
accepts any arguments and returns undefined when invoked.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.
does not have a prototype property.
has a length property whose value is 0.
has a name property whose value is the empty String.

Note

The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

19.2.3.1 Function.prototype.apply ( `thisArg`, `argArray` )

When the apply method is called on an object func with arguments thisArg and argArray, the following steps are taken:

If IsCallable(func) is false, throw a TypeError exception.
If argArray is undefined or null, then
1. Perform PrepareForTailCall().
2. Return ? Call(func, thisArg).
Let argList be ? CreateListFromArrayLike(argArray).
Perform PrepareForTailCall().
Return ? Call(func, thisArg, argList).

Note 1

The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

Note 2

If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.

19.2.3.2 Function.prototype.bind ( `thisArg`, ...`args` )

When the bind method is called with argument thisArg and zero or more args, it performs the following steps:

Let Target be the this value.
If IsCallable(Target) is false, throw a TypeError exception.
Let args be a new (possibly empty) List consisting of all of the argument values provided after thisArg in order.
Let F be ? BoundFunctionCreate(Target, thisArg, args).
Let targetHasLength be ? HasOwnProperty(Target, "length").
If targetHasLength is true, then
1. Let targetLen be ? Get(Target, "length").
2. If Type(targetLen) is not Number, let L be 0.
3. Else,
  1. Let targetLen be ToInteger(targetLen).
  2. Let L be the larger of 0 and the result of targetLen minus the number of elements of args.
Else, let L be 0.
Perform ! SetFunctionLength(F, L).
Let targetName be ? Get(Target, "name").
If Type(targetName) is not String, let targetName be the empty string.
Perform SetFunctionName(F, targetName, "bound").
Return F.

Note 1

Function objects created using Function.prototype.bind are exotic objects. They also do not have a prototype property.

Note 2

If Target is an arrow function or a bound function then the thisArg passed to this method will not be used by subsequent calls to F.

19.2.3.3 Function.prototype.call ( `thisArg`, ...`args` )

When the call method is called on an object func with argument, thisArg and zero or more args, the following steps are taken:

If IsCallable(func) is false, throw a TypeError exception.
Let argList be a new empty List.
If this method was called with more than one argument, then in left to right order, starting with the second argument, append each argument as the last element of argList.
Perform PrepareForTailCall().
Return ? Call(func, thisArg, argList).

Note 1

Note 2

If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.

19.2.3.4 Function.prototype.constructor

The initial value of Function.prototype.constructor is the intrinsic object %Function%.

19.2.3.5 Function.prototype.toString ( )

When the toString method is called on an object func, the following steps are taken:

If func is a Bound Function exotic object, then
1. Return an implementation-dependent String source code representation of func. The representation must conform to the rules below. It is implementation-dependent whether the representation includes bound function information or information about the target function.
If Type(func) is Object and is either a built-in function object or has an [[ECMAScriptCode]] internal slot, then
1. Return an implementation-dependent String source code representation of func. The representation must conform to the rules below.
Throw a TypeError exception.

toString Representation Requirements:

The string representation must have the syntax of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, ClassDeclaration, ClassExpression, ArrowFunction, AsyncArrowFunction, or MethodDefinition depending upon the actual characteristics of the object.
The use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.
If the object was defined using ECMAScript code and the returned string representation is not in the form of a MethodDefinition or GeneratorMethod then the representation must be such that if the string is evaluated, using eval in a lexical context that is equivalent to the lexical context used to create the original object, it will result in a new functionally equivalent object. In that case the returned source code must not mention freely any variables that were not mentioned freely by the original function's source code, even if these “extra” names were originally in scope.
If the implementation cannot produce a source code string that meets these criteria then it must return a string for which eval will throw a SyntaxError exception.

19.2.3.6 Function.prototype [ @@hasInstance ] ( `V` )

When the @@hasInstance method of an object F is called with value V, the following steps are taken:

Let F be the this value.
Return ? OrdinaryHasInstance(F, V).

The value of the name property of this function is "[Symbol.hasInstance]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as

v instanceof F

evaluates as

F[@@hasInstance](v)

A constructor function can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.

19.2.4 Function Instances

Every Function instance is an ECMAScript function object and has the internal slots listed in Table 27. Function objects created using the Function.prototype.bind method (19.2.3.2) have the internal slots listed in Table 28.

Function instances have the following properties:

19.2.4.1 length

The value of the length property is an integer that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.2.4.2 name

The value of the name property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Anonymous functions objects that do not have a contextual name associated with them by this specification do not have a name own property but inherit the name property of %FunctionPrototype%.

19.2.4.3 prototype

Function instances that can be used as a constructor have a prototype property. Whenever such a Function instance is created another ordinary object is also created and is the initial value of the function's prototype property. Unless otherwise specified, the value of the prototype property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Function objects created using Function.prototype.bind, or by evaluating a MethodDefinition (that is not a GeneratorMethod or AsyncGeneratorMethod) or an ArrowFunction do not have a prototype property.

19.3 Boolean Objects

19.3.1 The Boolean Constructor

The Boolean constructor:

is the intrinsic object %Boolean%.
is the initial value of the Boolean property of the global object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.

19.3.1.1 Boolean ( `value` )

When Boolean is called with argument value, the following steps are taken:

Let b be ToBoolean(value).
If NewTarget is undefined, return b.
Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%BooleanPrototype%", « [[BooleanData]] »).
Set O.[[BooleanData]] to b.
Return O.

19.3.2 Properties of the Boolean Constructor

The Boolean constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

19.3.2.1 Boolean.prototype

The initial value of Boolean.prototype is the intrinsic object %BooleanPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.3.3 Properties of the Boolean Prototype Object

The Boolean prototype object:

is the intrinsic object %BooleanPrototype%.
is an ordinary object.
is itself a Boolean object; it has a [[BooleanData]] internal slot with the value false.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

The abstract operation thisBooleanValue(value) performs the following steps:

If Type(value) is Boolean, return value.
If Type(value) is Object and value has a [[BooleanData]] internal slot, then
1. Assert: value.[[BooleanData]] is a Boolean value.
2. Return value.[[BooleanData]].
Throw a TypeError exception.

19.3.3.1 Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is the intrinsic object %Boolean%.

19.3.3.2 Boolean.prototype.toString ( )

The following steps are taken:

Let b be ? thisBooleanValue(this value).
If b is true, return "true"; else return "false".

19.3.3.3 Boolean.prototype.valueOf ( )

The following steps are taken:

Return ? thisBooleanValue(this value).

19.3.4 Properties of Boolean Instances

Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.

19.4 Symbol Objects

19.4.1 The Symbol Constructor

The Symbol constructor:

is the intrinsic object %Symbol%.
is the initial value of the Symbol property of the global object.
returns a new Symbol value when called as a function.
is not intended to be used with the new operator.
is not intended to be subclassed.
may be used as the value of an extends clause of a class definition but a super call to it will cause an exception.

19.4.1.1 Symbol ( [ `description` ] )

When Symbol is called with optional argument description, the following steps are taken:

If NewTarget is not undefined, throw a TypeError exception.
If description is undefined, let descString be undefined.
Else, let descString be ? ToString(description).
Return a new unique Symbol value whose [[Description]] value is descString.

19.4.2 Properties of the Symbol Constructor

The Symbol constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

19.4.2.1 Symbol.asyncIterator

The initial value of Symbol.asyncIterator is the well known symbol @@asyncIterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.2 Symbol.for ( `key` )

When Symbol.for is called with argument key it performs the following steps:

Let stringKey be ? ToString(key).
For each element e of the GlobalSymbolRegistry List, do
1. If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
Return newSymbol.

The GlobalSymbolRegistry is a List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 45.

Table 45: GlobalSymbolRegistry Record Fields

Field Name	Value	Usage
[[Key]]	A String	A string key used to globally identify a Symbol.
[[Symbol]]	A Symbol	A symbol that can be retrieved from any realm.

19.4.2.3 Symbol.hasInstance

The initial value of Symbol.hasInstance is the well-known symbol @@hasInstance (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.4 Symbol.isConcatSpreadable

The initial value of Symbol.isConcatSpreadable is the well-known symbol @@isConcatSpreadable (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.5 Symbol.iterator

The initial value of Symbol.iterator is the well-known symbol @@iterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.6 Symbol.keyFor ( `sym` )

When Symbol.keyFor is called with argument sym it performs the following steps:

If Type(sym) is not Symbol, throw a TypeError exception.
For each element e of the GlobalSymbolRegistry List (see 19.4.2.2), do
1. If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
Return undefined.

19.4.2.7 Symbol.match

The initial value of Symbol.match is the well-known symbol @@match (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.8 Symbol.prototype

The initial value of Symbol.prototype is the intrinsic object %SymbolPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.9 Symbol.replace

The initial value of Symbol.replace is the well-known symbol @@replace (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.10 Symbol.search

The initial value of Symbol.search is the well-known symbol @@search (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.11 Symbol.species

The initial value of Symbol.species is the well-known symbol @@species (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.12 Symbol.split

The initial value of Symbol.split is the well-known symbol @@split (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.13 Symbol.toPrimitive

The initial value of Symbol.toPrimitive is the well-known symbol @@toPrimitive (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.14 Symbol.toStringTag

The initial value of Symbol.toStringTag is the well-known symbol @@toStringTag (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.2.15 Symbol.unscopables

The initial value of Symbol.unscopables is the well-known symbol @@unscopables (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.4.3 Properties of the Symbol Prototype Object

The Symbol prototype object:

is the intrinsic object %SymbolPrototype%.
is an ordinary object.
is not a Symbol instance and does not have a [[SymbolData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

The abstract operation thisSymbolValue(value) performs the following steps:

If Type(value) is Symbol, return value.
If Type(value) is Object and value has a [[SymbolData]] internal slot, then
1. Assert: value.[[SymbolData]] is a Symbol value.
2. Return value.[[SymbolData]].
Throw a TypeError exception.

19.4.3.1 Symbol.prototype.constructor

The initial value of Symbol.prototype.constructor is the intrinsic object %Symbol%.

19.4.3.2 Symbol.prototype.toString ( )

The following steps are taken:

Let sym be ? thisSymbolValue(this value).
Return SymbolDescriptiveString(sym).

19.4.3.2.1 Runtime Semantics: SymbolDescriptiveString ( `sym` )

When the abstract operation SymbolDescriptiveString is called with argument sym, the following steps are taken:

Assert: Type(sym) is Symbol.
Let desc be sym's [[Description]] value.
If desc is undefined, let desc be the empty string.
Assert: Type(desc) is String.
Return the string-concatenation of "Symbol(", desc, and ")".

19.4.3.3 Symbol.prototype.valueOf ( )

The following steps are taken:

Return ? thisSymbolValue(this value).

19.4.3.4 Symbol.prototype [ @@toPrimitive ] ( `hint` )

This function is called by ECMAScript language operators to convert a Symbol object to a primitive value. The allowed values for hint are "default", "number", and "string".

When the @@toPrimitive method is called with argument hint, the following steps are taken:

Return ? thisSymbolValue(this value).

The value of the name property of this function is "[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.4.3.5 Symbol.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Symbol".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

19.4.4 Properties of Symbol Instances

Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.

19.5 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

19.5.1 The Error Constructor

The Error constructor:

is the intrinsic object %Error%.
is the initial value of the Error property of the global object.
creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

19.5.1.1 Error ( `message` )

When the Error function is called with argument message, the following steps are taken:

If NewTarget is undefined, let newTarget be the active function object, else let newTarget be NewTarget.
Let O be ? OrdinaryCreateFromConstructor(newTarget, "%ErrorPrototype%", « [[ErrorData]] »).
If message is not undefined, then
1. Let msg be ? ToString(message).
2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
3. Perform ! DefinePropertyOrThrow(O, "message", msgDesc).
Return O.

19.5.2 Properties of the Error Constructor

The Error constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

19.5.2.1 Error.prototype

The initial value of Error.prototype is the intrinsic object %ErrorPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.5.3 Properties of the Error Prototype Object

The Error prototype object:

is the intrinsic object %ErrorPrototype%.
is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

19.5.3.1 Error.prototype.constructor

The initial value of Error.prototype.constructor is the intrinsic object %Error%.

19.5.3.2 Error.prototype.message

The initial value of Error.prototype.message is the empty String.

19.5.3.3 Error.prototype.name

The initial value of Error.prototype.name is "Error".

19.5.3.4 Error.prototype.toString ( )

The following steps are taken:

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
Let name be ? Get(O, "name").
If name is undefined, let name be "Error"; otherwise let name be ? ToString(name).
Let msg be ? Get(O, "message").
If msg is undefined, let msg be the empty String; otherwise let msg be ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the string-concatenation of name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.

19.5.4 Properties of Error Instances

Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error and NativeError instances as Error objects within Object.prototype.toString.

19.5.5 Native Error Types Used in This Standard

A new instance of one of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 19.5.6.

19.5.5.1 EvalError

This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.

19.5.5.2 RangeError

Indicates a value that is not in the set or range of allowable values.

19.5.5.3 ReferenceError

Indicate that an invalid reference value has been detected.

19.5.5.4 SyntaxError

Indicates that a parsing error has occurred.

19.5.5.5 TypeError

TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

19.5.5.6 URIError

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

19.5.6 `NativeError` Object Structure

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 19.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the name property of the prototype object, and in the implementation-defined message property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 19.5.5.

19.5.6.1 The `NativeError` Constructors

Each NativeError constructor:

creates and initializes a new NativeError object when called as a function rather than as a constructor. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeError constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

19.5.6.1.1 NativeError ( `message` )

When a NativeError function is called with argument message, the following steps are taken:

If NewTarget is undefined, let newTarget be the active function object, else let newTarget be NewTarget.
Let O be ? OrdinaryCreateFromConstructor(newTarget, "%NativeErrorPrototype%", « [[ErrorData]] »).
If message is not undefined, then
1. Let msg be ? ToString(message).
2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
3. Perform ! DefinePropertyOrThrow(O, "message", msgDesc).
Return O.

The actual value of the string passed in step 2 is either "%EvalErrorPrototype%", "%RangeErrorPrototype%", "%ReferenceErrorPrototype%", "%SyntaxErrorPrototype%", "%TypeErrorPrototype%", or "%URIErrorPrototype%" corresponding to which NativeError constructor is being defined.

19.5.6.2 Properties of the `NativeError` Constructors

Each NativeError constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %Error%.
has a name property whose value is the String value `"NativeError"`.
has the following properties:

19.5.6.2.1 NativeError.prototype

The initial value of NativeError.prototype is a NativeError prototype object (19.5.6.3). Each NativeError constructor has a distinct prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.5.6.3 Properties of the `NativeError` Prototype Objects

Each NativeError prototype object:

is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object %ErrorPrototype%.

19.5.6.3.1 `NativeError`.prototype.constructor

The initial value of the constructor property of the prototype for a given NativeError constructor is the corresponding intrinsic object %NativeError% (19.5.6.1).

19.5.6.3.2 `NativeError`.prototype.message

The initial value of the message property of the prototype for a given NativeError constructor is the empty String.

19.5.6.3.3 `NativeError`.prototype.name

The initial value of the name property of the prototype for a given NativeError constructor is the String value consisting of the name of the constructor (the name used instead of NativeError).

19.5.6.4 Properties of `NativeError` Instances

NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (19.1.3.6) to identify Error or NativeError instances.

20 Numbers and Dates

20.1 Number Objects

20.1.1 The Number Constructor

The Number constructor:

is the intrinsic object %Number%.
is the initial value of the Number property of the global object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.

20.1.1.1 Number ( `value` )

When Number is called with argument value, the following steps are taken:

If no arguments were passed to this function invocation, let n be +0.
Else, let n be ? ToNumber(value).
If NewTarget is undefined, return n.
Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%NumberPrototype%", « [[NumberData]] »).
Set O.[[NumberData]] to n.
Return O.

20.1.2 Properties of the Number Constructor

The Number constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

20.1.2.1 Number.EPSILON

The value of Number.EPSILON is the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 x 10^-16.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.2 Number.isFinite ( `number` )

When Number.isFinite is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, +∞, or -∞, return false.
Otherwise, return true.

20.1.2.3 Number.isInteger ( `number` )

When Number.isInteger is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, +∞, or -∞, return false.
Let integer be ToInteger(number).
If integer is not equal to number, return false.
Otherwise, return true.

20.1.2.4 Number.isNaN ( `number` )

When Number.isNaN is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, return true.
Otherwise, return false.

Note

This function differs from the global isNaN function (18.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.

20.1.2.5 Number.isSafeInteger ( `number` )

When Number.isSafeInteger is called with one argument number, the following steps are taken:

If Type(number) is not Number, return false.
If number is NaN, +∞, or -∞, return false.
Let integer be ToInteger(number).
If integer is not equal to number, return false.
If abs(integer) ≤ 2⁵³-1, return true.
Otherwise, return false.

20.1.2.6 Number.MAX_SAFE_INTEGER

Note

The value of Number.MAX_SAFE_INTEGER is the largest integer n such that n and n + 1 are both exactly representable as a Number value.

The value of Number.MAX_SAFE_INTEGER is 9007199254740991 (2⁵³-1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.7 Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10³⁰⁸.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.8 Number.MIN_SAFE_INTEGER

Note

The value of Number.MIN_SAFE_INTEGER is the smallest integer n such that n and n - 1 are both exactly representable as a Number value.

The value of Number.MIN_SAFE_INTEGER is -9007199254740991 (-(2⁵³-1)).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.9 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10^-324.

In the IEEE 754-2008 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.10 Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.11 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is -∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.12 Number.parseFloat ( `string` )

The value of the Number.parseFloat data property is the same built-in function object that is the value of the parseFloat property of the global object defined in 18.2.4.

20.1.2.13 Number.parseInt ( `string`, `radix` )

The value of the Number.parseInt data property is the same built-in function object that is the value of the parseInt property of the global object defined in 18.2.5.

20.1.2.14 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.15 Number.prototype

The initial value of Number.prototype is the intrinsic object %NumberPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.3 Properties of the Number Prototype Object

The Number prototype object:

is the intrinsic object %NumberPrototype%.
is an ordinary object.
is itself a Number object; it has a [[NumberData]] internal slot with the value +0.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.

The abstract operation thisNumberValue(value) performs the following steps:

If Type(value) is Number, return value.
If Type(value) is Object and value has a [[NumberData]] internal slot, then
1. Assert: value.[[NumberData]] is a Number value.
2. Return value.[[NumberData]].
Throw a TypeError exception.

The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation thisNumberValue with the this value of the method invocation passed as the argument.

20.1.3.1 Number.prototype.constructor

The initial value of Number.prototype.constructor is the intrinsic object %Number%.

20.1.3.2 Number.prototype.toExponential ( `fractionDigits` )

Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits).
Assert: f is 0, when fractionDigits is undefined.
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Let s be "-".
2. Let x be -x.
If x = +∞, then
1. Return the string-concatenation of s and "Infinity".
If f < 0 or f > 100, throw a RangeError exception.
If x = 0, then
1. Let m be the String value consisting of f+1 occurrences of the code unit 0x0030 (DIGIT ZERO).
2. Let e be 0.
Else x ≠ 0,
1. If fractionDigits is not undefined, then
  1. Let e and n be integers such that 10^f ≤ n < 10^f+1 and for which the exact mathematical value of n × 10^e-f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10^e-f is larger.
2. Else fractionDigits is undefined,
  1. Let e, n, and f be integers such that f ≥ 0, 10^f ≤ n < 10^f+1, the Number value for n × 10^e-f is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
3. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
1. Let a be the first element of m, and let b be the remaining f elements of m.
2. Let m be the string-concatenation of a, ".", and b.
If e = 0, then
1. Let c be "+".
2. Let d be "0".
Else,
1. If e > 0, let c be "+".
2. Else e ≤ 0,
  1. Let c be "-".
  2. Let e be -e.
3. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Let m be the string-concatenation of m, "e", c, and d.
Return the string-concatenation of s and m.

Note

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:

Let e, n, and f be integers such that f ≥ 0, 10^f ≤ n < 10^f+1, the Number value for n × 10^e-f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10^e-f is closest in value to x. If there are two such possible values of n, choose the one that is even.

20.1.3.3 Number.prototype.toFixed ( `fractionDigits` )

Note 1

toFixed returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.

The following steps are performed:

Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0.)
If f < 0 or f > 100, throw a RangeError exception.
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Let s be "-".
2. Let x be -x.
If x ≥ 10²¹, then
1. Let m be ! ToString(x).
Else x < 10²¹,
1. Let n be an integer for which the exact mathematical value of n ÷ 10^f - x is as close to zero as possible. If there are two such n, pick the larger n.
2. If n = 0, let m be the String "0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
3. If f ≠ 0, then
  1. Let k be the length of m.
  2. If k ≤ f, then
    1. Let z be the String value consisting of f+1-k occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Let m be the string-concatenation of z and m.
    3. Let k be f + 1.
  3. Let a be the first k-f elements of m, and let b be the remaining f elements of m.
  4. Let m be the string-concatenation of a, ".", and b.
Return the string-concatenation of s and m.

Note 2

The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,

(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".

20.1.3.4 Number.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

20.1.3.5 Number.prototype.toPrecision ( `precision` )

Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision-1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString instead. Specifically, perform the following steps:

Let x be ? thisNumberValue(this value).
If precision is undefined, return ! ToString(x).
Let p be ? ToInteger(precision).
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, then
1. Let s be the code unit 0x002D (HYPHEN-MINUS).
2. Let x be -x.
If x = +∞, then
1. Return the string-concatenation of s and "Infinity".
If p < 1 or p > 100, throw a RangeError exception.
If x = 0, then
1. Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
2. Let e be 0.
Else x ≠ 0,
1. Let e and n be integers such that 10^p-1 ≤ n < 10^p and for which the exact mathematical value of n × 10^e-p+1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10^e-p+1 is larger.
2. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
3. If e < -6 or e ≥ p, then
  1. Assert: e ≠ 0.
  2. If p ≠ 1, then
    1. Let a be the first element of m, and let b be the remaining p-1 elements of m.
    2. Let m be the string-concatenation of a, ".", and b.
  3. If e > 0, then
    1. Let c be the code unit 0x002B (PLUS SIGN).
  4. Else e < 0,
    1. Let c be the code unit 0x002D (HYPHEN-MINUS).
    2. Let e be -e.
  5. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
  6. Return the string-concatenation of s, m, the code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
If e = p-1, return the string-concatenation of s and m.
If e ≥ 0, then
1. Let m be the string-concatenation of the first e+1 elements of m, the code unit 0x002E (FULL STOP), and the remaining p- (e+1) elements of m.
Else e < 0,
1. Let m be the string-concatenation of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e+1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
Return the string-concatenation of s and m.

20.1.3.6 Number.prototype.toString ( [ `radix` ] )

Note

The optional radix should be an integer value in the inclusive range 2 to 36. If radix is not present or is undefined the Number 10 is used as the value of radix.

The following steps are performed:

Let x be ? thisNumberValue(this value).
If radix is not present, let radixNumber be 10.
Else if radix is undefined, let radixNumber be 10.
Else, let radixNumber be ? ToInteger(radix).
If radixNumber < 2 or radixNumber > 36, throw a RangeError exception.
If radixNumber = 10, return ! ToString(x).
Return the String representation of this Number value using the radix specified by radixNumber. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however the algorithm should be a generalization of that specified in 7.1.12.1.

The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

The length property of the toString method is 1.

20.1.3.7 Number.prototype.valueOf ( )

Return ? thisNumberValue(this value).

20.1.4 Properties of Number Instances

Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.

20.2 The Math Object

The Math object:

is the intrinsic object %Math%.
is the initial value of the Math property of the global object.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.
is not a function object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.

Note

In this specification, the phrase “the Number value for x” has a technical meaning defined in 6.1.6.

20.2.1 Value Properties of the Math Object

20.2.1.1 Math.E

The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.1.2 Math.LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.1.3 Math.LN2

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.1.4 Math.LOG10E

The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

20.2.1.5 Math.LOG2E

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

20.2.1.6 Math.PI

The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.1.7 Math.SQRT1_2

The Number value for the square root of ½, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

20.2.1.8 Math.SQRT2

The Number value for the square root of 2, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.1.9 Math [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Math".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.2.2 Function Properties of the Math Object

Each of the following Math object functions applies the ToNumber abstract operation to each of its arguments (in left-to-right order if there is more than one). If ToNumber returns an abrupt completion, that Completion Record is immediately returned. Otherwise, the function performs a computation on the resulting Number value(s). The value returned by each function is a Number.

In the function descriptions below, the symbols NaN, -0, +0, -∞ and +∞ refer to the Number values described in 6.1.6.

Note

The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log,log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2008 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

20.2.2.1 Math.abs ( `x` )

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

If x is NaN, the result is NaN.
If x is -0, the result is +0.
If x is -∞, the result is +∞.

20.2.2.2 Math.acos ( `x` )

Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than -1, the result is NaN.
If x is exactly 1, the result is +0.

20.2.2.3 Math.acosh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic cosine of x.

If x is NaN, the result is NaN.
If x is less than 1, the result is NaN.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.2.2.4 Math.asin ( `x` )

Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians and ranges from -π/2 to +π/2.

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than -1, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.

20.2.2.5 Math.asinh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic sine of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

20.2.2.6 Math.atan ( `x` )

Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed in radians and ranges from -π/2 to +π/2.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is an implementation-dependent approximation to +π/2.
If x is -∞, the result is an implementation-dependent approximation to -π/2.

20.2.2.7 Math.atanh ( `x` )

Returns an implementation-dependent approximation to the inverse hyperbolic tangent of x.

If x is NaN, the result is NaN.
If x is less than -1, the result is NaN.
If x is greater than 1, the result is NaN.
If x is -1, the result is -∞.
If x is +1, the result is +∞.
If x is +0, the result is +0.
If x is -0, the result is -0.

20.2.2.8 Math.atan2 ( `y`, `x` )

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π.

If either x or y is NaN, the result is NaN.
If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
If y>0 and x is -0, the result is an implementation-dependent approximation to +π/2.
If y is +0 and x>0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is -0, the result is an implementation-dependent approximation to +π.
If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
If y is -0 and x>0, the result is -0.
If y is -0 and x is +0, the result is -0.
If y is -0 and x is -0, the result is an implementation-dependent approximation to -π.
If y is -0 and x<0, the result is an implementation-dependent approximation to -π.
If y<0 and x is +0, the result is an implementation-dependent approximation to -π/2.
If y<0 and x is -0, the result is an implementation-dependent approximation to -π/2.
If y>0 and y is finite and x is +∞, the result is +0.
If y>0 and y is finite and x is -∞, the result is an implementation-dependent approximation to +π.
If y<0 and y is finite and x is +∞, the result is -0.
If y<0 and y is finite and x is -∞, the result is an implementation-dependent approximation to -π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
If y is -∞ and x is finite, the result is an implementation-dependent approximation to -π/2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
If y is +∞ and x is -∞, the result is an implementation-dependent approximation to +3π/4.
If y is -∞ and x is +∞, the result is an implementation-dependent approximation to -π/4.
If y is -∞ and x is -∞, the result is an implementation-dependent approximation to -3π/4.

20.2.2.9 Math.cbrt ( `x` )

Returns an implementation-dependent approximation to the cube root of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

20.2.2.10 Math.ceil ( `x` )

Returns the smallest (closest to -∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is less than 0 but greater than -1, the result is -0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

20.2.2.11 Math.clz32 ( `x` )

When Math.clz32 is called with one argument x, the following steps are taken:

Let n be ToUint32(x).
Let p be the number of leading zero bits in the 32-bit binary representation of n.
Return p.

Note

If n is 0, p will be 32. If the most significant bit of the 32-bit binary encoding of n is 1, p will be 0.

20.2.2.12 Math.cos ( `x` )

Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is NaN.
If x is -∞, the result is NaN.

20.2.2.13 Math.cosh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic cosine of x.

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is +∞.
If x is -∞, the result is +∞.

Note

The value of cosh(x) is the same as (exp(x) + exp(-x))/2.

20.2.2.14 Math.exp ( `x` )

Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is -0, the result is 1.
If x is +∞, the result is +∞.
If x is -∞, the result is +0.

20.2.2.15 Math.expm1 ( `x` )

Returns an implementation-dependent approximation to subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close 0.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -1.

20.2.2.16 Math.floor ( `x` )

Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.

Note

The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

20.2.2.17 Math.fround ( `x` )

When Math.fround is called with argument x, the following steps are taken:

If x is NaN, return NaN.
If x is one of +0, -0, +∞, -∞, return x.
Let x32 be the result of converting x to a value in IEEE 754-2008 binary32 format using roundTiesToEven.
Let x64 be the result of converting x32 to a value in IEEE 754-2008 binary64 format.
Return the ECMAScript Number value corresponding to x64.

20.2.2.18 Math.hypot ( `value1`, `value2`, ...`values` )

Math.hypot returns an implementation-dependent approximation of the square root of the sum of squares of its arguments.

If no arguments are passed, the result is +0.
If any argument is +∞, the result is +∞.
If any argument is -∞, the result is +∞.
If no argument is +∞ or -∞, and any argument is NaN, the result is NaN.
If all arguments are either +0 or -0, the result is +0.

Note

Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.

20.2.2.19 Math.imul ( `x`, `y` )

When Math.imul is called with arguments x and y, the following steps are taken:

Let a be ToUint32(x).
Let b be ToUint32(y).
Let product be (a × b) modulo 2³².
If product ≥ 2³¹, return product - 2³²; otherwise return product.

20.2.2.20 Math.log ( `x` )

Returns an implementation-dependent approximation to the natural logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0 or -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.2.2.21 Math.log1p ( `x` )

Returns an implementation-dependent approximation to the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.

If x is NaN, the result is NaN.
If x is less than -1, the result is NaN.
If x is -1, the result is -∞.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.

20.2.2.22 Math.log10 ( `x` )

Returns an implementation-dependent approximation to the base 10 logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is -∞.
If x is -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.2.2.23 Math.log2 ( `x` )

Returns an implementation-dependent approximation to the base 2 logarithm of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is -∞.
If x is -0, the result is -∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

20.2.2.24 Math.max ( `value1`, `value2`, ...`values` )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

If no arguments are given, the result is -∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the largest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.

20.2.2.25 Math.min ( `value1`, `value2`, ...`values` )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.

If no arguments are given, the result is +∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the smallest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.

20.2.2.26 Math.pow ( `base`, `exponent` )

Return the result of Applying the ** operator with base and exponent as specified in 12.6.4.

20.2.2.27 Math.random ( )

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.

20.2.2.28 Math.round ( `x` )

Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is -0.

Note 1

Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

Note 2

The value of Math.round(x) is not always the same as the value of Math.floor(x+0.5). When x is -0 or is less than 0 but greater than or equal to -0.5, Math.round(x) returns -0, but Math.floor(x+0.5) returns +0. Math.round(x) may also differ from the value of Math.floor(x+0.5)because of internal rounding when computing x+0.5.

20.2.2.29 Math.sign ( `x` )

Returns the sign of x, indicating whether x is positive, negative, or zero.

If x is NaN, the result is NaN.
If x is -0, the result is -0.
If x is +0, the result is +0.
If x is negative and not -0, the result is -1.
If x is positive and not +0, the result is +1.

20.2.2.30 Math.sin ( `x` )

Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞ or -∞, the result is NaN.

20.2.2.31 Math.sinh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic sine of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.

Note

The value of sinh(x) is the same as (exp(x) - exp(-x))/2.

20.2.2.32 Math.sqrt ( `x` )

Returns an implementation-dependent approximation to the square root of x.

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.

20.2.2.33 Math.tan ( `x` )

Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞ or -∞, the result is NaN.

20.2.2.34 Math.tanh ( `x` )

Returns an implementation-dependent approximation to the hyperbolic tangent of x.

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +1.
If x is -∞, the result is -1.

Note

The value of tanh(x) is the same as (exp(x) - exp(-x))/(exp(x) + exp(-x)).

20.2.2.35 Math.trunc ( `x` )

Returns the integral part of the number x, removing any fractional digits. If x is already an integer, the result is x.

If x is NaN, the result is NaN.
If x is -0, the result is -0.
If x is +0, the result is +0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.
If x is less than 0 but greater than -1, the result is -0.

20.3 Date Objects

20.3.1 Overview of Date Objects and Definitions of Abstract Operations

The following functions are abstract operations that operate on time values (defined in 20.3.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

20.3.1.1 Time Values and Time Range

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called a time value. A time value may also be NaN, indicating that the Date object does not represent a specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly -100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.

20.3.1.2 Day Number and Time within Day

A given time value t belongs to day number

Day(t) = floor(t / msPerDay)

where the number of milliseconds per day is

msPerDay = 86400000

The remainder is called the time within the day:

TimeWithinDay(t) = t

modulo

msPerDay

20.3.1.3 Year Number

ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y)

= 365 if (y modulo 4) ≠ 0

= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0

= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0

= 366 if (y modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) = 365 × (y-1970) + floor((y-1969)/4) - floor((y-1901)/100) + floor((y-1601)/400)

The time value of the start of a year is:

TimeFromYear(y) = msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t)

= 0 if DaysInYear(YearFromTime(t)) = 365

= 1 if DaysInYear(YearFromTime(t)) = 366

20.3.1.4 Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:

MonthFromTime(t)

= 0 if 0 ≤ DayWithinYear(t) < 31

= 1 if 31 ≤ DayWithinYear(t) < 59+InLeapYear(t)

= 2 if 59+InLeapYear(t) ≤ DayWithinYear(t) < 90+InLeapYear(t)

= 3 if 90+InLeapYear(t) ≤ DayWithinYear(t) < 120+InLeapYear(t)

= 4 if 120+InLeapYear(t) ≤ DayWithinYear(t) < 151+InLeapYear(t)

= 5 if 151+InLeapYear(t) ≤ DayWithinYear(t) < 181+InLeapYear(t)

= 6 if 181+InLeapYear(t) ≤ DayWithinYear(t) < 212+InLeapYear(t)

= 7 if 212+InLeapYear(t) ≤ DayWithinYear(t) < 243+InLeapYear(t)

= 8 if 243+InLeapYear(t) ≤ DayWithinYear(t) < 273+InLeapYear(t)

= 9 if 273+InLeapYear(t) ≤ DayWithinYear(t) < 304+InLeapYear(t)

= 10 if 304+InLeapYear(t) ≤ DayWithinYear(t) < 334+InLeapYear(t)

= 11 if 334+InLeapYear(t) ≤ DayWithinYear(t) < 365+InLeapYear(t)

where

DayWithinYear(t) = Day(t)-DayFromYear(YearFromTime(t))

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

20.3.1.5 Date Number

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time value t to a date number is defined by:

DateFromTime(t)

= DayWithinYear(t)+1 if MonthFromTime(t)=0

= DayWithinYear(t)-30 if MonthFromTime(t)=1

= DayWithinYear(t)-58-InLeapYear(t) if MonthFromTime(t)=2

= DayWithinYear(t)-89-InLeapYear(t) if MonthFromTime(t)=3

= DayWithinYear(t)-119-InLeapYear(t) if MonthFromTime(t)=4

= DayWithinYear(t)-150-InLeapYear(t) if MonthFromTime(t)=5

= DayWithinYear(t)-180-InLeapYear(t) if MonthFromTime(t)=6

= DayWithinYear(t)-211-InLeapYear(t) if MonthFromTime(t)=7

= DayWithinYear(t)-242-InLeapYear(t) if MonthFromTime(t)=8

= DayWithinYear(t)-272-InLeapYear(t) if MonthFromTime(t)=9

= DayWithinYear(t)-303-InLeapYear(t) if MonthFromTime(t)=10

= DayWithinYear(t)-333-InLeapYear(t) if MonthFromTime(t)=11

20.3.1.6 Week Day

The weekday for a particular time value t is defined as

WeekDay(t) = (Day(t) + 4) modulo 7

A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

20.3.1.7 LocalTZA ( `t`, `isUTC` )

LocalTZA( t, isUTC ) is an implementation-defined algorithm that must return a number representing milliseconds suitable for adding to a Time Value. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in the following three paragraphs.

When isUTC is true, LocalTZA( t, true ) should return the offset of the local time zone from UTC measured in milliseconds at time represented by time value t (UTC). When the result is added to t (UTC), it should yield the local time.

When isUTC is false, LocalTZA( t, false ) should return the offset of the local time zone from UTC measured in milliseconds at local time represented by time value t_local = t. When the result is subtracted from the local time t_local, it should yield the corresponding UTC.

When t_local represents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone adjustment is decreased due to a time zone rule change) or skipped local time at a positive time zone transitions (e.g. when the daylight saving time starts or the time zone adjustment is increased due to a time zone rule change), t_local must be interpreted with the time zone adjustment before the transition.

If an implementation does not support a conversion described above or if political rules for time t are not available within the implementation, the result must be 0.

Note

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

1:30 AM on November 5, 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. LocalTZA(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0))), false) is -4 × msPerHour.

2:30 AM on March 12, 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). LocalTZA(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0))), false) is -5 × msPerHour.

20.3.1.8 LocalTime ( `t` )

The abstract operation LocalTime with argument t converts t from UTC to local time by performing the following steps:

Return t + LocalTZA(t, true).

Note

Two different time values (t (UTC)) are converted to the same local time t_local at a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).

20.3.1.9 UTC ( `t` )

The abstract operation UTC with argument t converts t from local time to UTC. It performs the following steps:

Return t - LocalTZA(t, false).

Note

UTC(LocalTime(t)) is not necessarily always equal to t. LocalTime(UTC(t_local)) is not necessarily always equal to t_local, either.

20.3.1.10 Hours, Minutes, Second, and Milliseconds

The following abstract operations are useful in decomposing time values:

HourFromTime(t) = floor(t / msPerHour) modulo

HoursPerDay

MinFromTime(t) = floor(t / msPerMinute) modulo

MinutesPerHour

SecFromTime(t) = floor(t / msPerSecond) modulo

SecondsPerMinute

msFromTime(t) = t

modulo

msPerSecond

where

HoursPerDay = 24

MinutesPerHour = 60

SecondsPerMinute = 60

msPerSecond = 1000

msPerMinute = 60000 = msPerSecond × SecondsPerMinute

msPerHour = 3600000 = msPerMinute × MinutesPerHour

20.3.1.11 MakeTime ( `hour`, `min`, `sec`, `ms` )

The abstract operation MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:

If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
Let h be ! ToInteger(hour).
Let m be ! ToInteger(min).
Let s be ! ToInteger(sec).
Let milli be ! ToInteger(ms).
Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according to IEEE 754-2008 rules (that is, as if using the ECMAScript operators * and +).
Return t.

20.3.1.12 MakeDay ( `year`, `month`, `date` )

The abstract operation MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:

If year is not finite or month is not finite or date is not finite, return NaN.
Let y be ! ToInteger(year).
Let m be ! ToInteger(month).
Let dt be ! ToInteger(date).
Let ym be y + floor(m / 12).
Let mn be m modulo 12.
Find a value t such that YearFromTime(t) is ym and MonthFromTime(t) is mn and DateFromTime(t) is 1; but if this is not possible (because some argument is out of range), return NaN.
Return Day(t) + dt - 1.

20.3.1.13 MakeDate ( `day`, `time` )

The abstract operation MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:

If day is not finite or time is not finite, return NaN.
Return day × msPerDay + time.

20.3.1.14 TimeClip ( `time` )

The abstract operation TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:

If time is not finite, return NaN.
If abs(time) > 8.64 × 10¹⁵, return NaN.
Let clippedTime be ! ToInteger(time).
If clippedTime is -0, set clippedTime to +0.
Return clippedTime.

Note

The point of step 4 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish -0 and +0.

20.3.1.15 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

`YYYY`	is the decimal digits of the year 0000 to 9999 in the proleptic Gregorian calendar.
`-`	`"-"` (hyphen) appears literally twice in the string.
`MM`	is the month of the year from 01 (January) to 12 (December).
`DD`	is the day of the month from 01 to 31.
`T`	`"T"` appears literally in the string, to indicate the beginning of the time element.
`HH`	is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
`:`	`":"` (colon) appears literally twice in the string.
`mm`	is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
`ss`	is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
`.`	`"."` (dot) appears literally in the string.
`sss`	is the number of complete milliseconds since the start of the second as three decimal digits.
`Z`	is the time zone offset specified as `"Z"` (for UTC) or either `"+"` or `"-"` followed by a time expression `HH:mm`

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

All numbers must be base 10. If the MM or DD fields are absent "01" is used as the value. If the HH, mm, or ss fields are absent "00" is used as the value and the value of an absent sss field is "000". When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.

Note 1

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00

Note 2

There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.

20.3.1.15.1 Extended Years

ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign.

Note

Examples of extended years:

-283457-03-21T15:00:59.008Z	283458 B.C.
-000001-01-01T00:00:00Z	2 B.C.
+000000-01-01T00:00:00Z	1 B.C.
+000001-01-01T00:00:00Z	1 A.D.
+001970-01-01T00:00:00Z	1970 A.D.
+002009-12-15T00:00:00Z	2009 A.D.
+287396-10-12T08:59:00.992Z	287396 A.D.

20.3.2 The Date Constructor

The Date constructor:

is the intrinsic object %Date%.
is the initial value of the Date property of the global object.
creates and initializes a new Date object when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than as a constructor.
is a single function whose behaviour is overloaded based upon the number and types of its arguments.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
has a length property whose value is 7.

20.3.2.1 Date ( `year`, `month` [ , `date` [ , `hours` [ , `minutes` [ , `seconds` [ , `ms` ] ] ] ] ] )

This description applies only if the Date constructor is called with at least two arguments.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs ≥ 2.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. Let y be ? ToNumber(year).
2. Let m be ? ToNumber(month).
3. If date is present, let dt be ? ToNumber(date); else let dt be 1.
4. If hours is present, let h be ? ToNumber(hours); else let h be 0.
5. If minutes is present, let min be ? ToNumber(minutes); else let min be 0.
6. If seconds is present, let s be ? ToNumber(seconds); else let s be 0.
7. If ms is present, let milli be ? ToNumber(ms); else let milli be 0.
8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, let yr be 1900+ToInteger(y); otherwise, let yr be y.
9. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
10. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%DatePrototype%", « [[DateValue]] »).
11. Set O.[[DateValue]] to TimeClip(UTC(finalDate)).
12. Return O.

20.3.2.2 Date ( `value` )

This description applies only if the Date constructor is called with exactly one argument.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 1.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. If Type(value) is Object and value has a [[DateValue]] internal slot, then
  1. Let tv be thisTimeValue(value).
2. Else,
  1. Let v be ? ToPrimitive(value).
  2. If Type(v) is String, then
    1. Assert: The next step never returns an abrupt completion because v is a String value.
    2. Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (20.3.3.2).
  3. Else,
    1. Let tv be ? ToNumber(v).
3. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%DatePrototype%", « [[DateValue]] »).
4. Set O.[[DateValue]] to TimeClip(tv).
5. Return O.

20.3.2.3 Date ( )

This description applies only if the Date constructor is called with no arguments.

When the Date function is called, the following steps are taken:

Let numberOfArgs be the number of arguments passed to this function call.
Assert: numberOfArgs = 0.
If NewTarget is undefined, then
1. Let now be the Number that is the time value (UTC) identifying the current time.
2. Return ToDateString(now).
Else,
1. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%DatePrototype%", « [[DateValue]] »).
2. Set O.[[DateValue]] to the time value (UTC) identifying the current time.
3. Return O.

20.3.3 Properties of the Date Constructor

The Date constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

20.3.3.1 Date.now ( )

The now function returns a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.

20.3.3.2 Date.parse ( `string` )

The parse function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules (including extended years) called out in Date Time String Format (20.3.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognizable Strings or dates containing illegal element values in the format String shall cause Date.parse to return NaN.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (20.3.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

20.3.3.3 Date.prototype

The initial value of Date.prototype is the intrinsic object %DatePrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.3.4 Date.UTC ( `year` [ , `month` [ , `date` [ , `hours` [ , `minutes` [ , `seconds` [ , `ms` ] ] ] ] ] ] )

When the UTC function is called, the following steps are taken:

Let y be ? ToNumber(year).
If month is present, let m be ? ToNumber(month); else let m be 0.
If date is present, let dt be ? ToNumber(date); else let dt be 1.
If hours is present, let h be ? ToNumber(hours); else let h be 0.
If minutes is present, let min be ? ToNumber(minutes); else let min be 0.
If seconds is present, let s be ? ToNumber(seconds); else let s be 0.
If ms is present, let milli be ? ToNumber(ms); else let milli be 0.
If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, let yr be 1900+ToInteger(y); otherwise, let yr be y.
Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The length property of the UTC function is 7.

Note

The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

20.3.4 Properties of the Date Prototype Object

The Date prototype object:

is the intrinsic object %DatePrototype%.
is itself an ordinary object.
is not a Date instance and does not have a [[DateValue]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.

The abstract operation thisTimeValue(value) performs the following steps:

If Type(value) is Object and value has a [[DateValue]] internal slot, then
1. Return value.[[DateValue]].
Throw a TypeError exception.

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. If the Type of the this value is not Object, a TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the this value of the method invocation passed as the argument.

20.3.4.1 Date.prototype.constructor

The initial value of Date.prototype.constructor is the intrinsic object %Date%.

20.3.4.2 Date.prototype.getDate ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return DateFromTime(LocalTime(t)).

20.3.4.3 Date.prototype.getDay ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return WeekDay(LocalTime(t)).

20.3.4.4 Date.prototype.getFullYear ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)).

20.3.4.5 Date.prototype.getHours ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return HourFromTime(LocalTime(t)).

20.3.4.6 Date.prototype.getMilliseconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return msFromTime(LocalTime(t)).

20.3.4.7 Date.prototype.getMinutes ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MinFromTime(LocalTime(t)).

20.3.4.8 Date.prototype.getMonth ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MonthFromTime(LocalTime(t)).

20.3.4.9 Date.prototype.getSeconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return SecFromTime(LocalTime(t)).

20.3.4.10 Date.prototype.getTime ( )

The following steps are performed:

Return ? thisTimeValue(this value).

20.3.4.11 Date.prototype.getTimezoneOffset ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return (t - LocalTime(t)) / msPerMinute.

20.3.4.12 Date.prototype.getUTCDate ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return DateFromTime(t).

20.3.4.13 Date.prototype.getUTCDay ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return WeekDay(t).

20.3.4.14 Date.prototype.getUTCFullYear ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return YearFromTime(t).

20.3.4.15 Date.prototype.getUTCHours ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return HourFromTime(t).

20.3.4.16 Date.prototype.getUTCMilliseconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return msFromTime(t).

20.3.4.17 Date.prototype.getUTCMinutes ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MinFromTime(t).

20.3.4.18 Date.prototype.getUTCMonth ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return MonthFromTime(t).

20.3.4.19 Date.prototype.getUTCSeconds ( )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, return NaN.
Return SecFromTime(t).

20.3.4.20 Date.prototype.setDate ( `date` )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

20.3.4.21 Date.prototype.setFullYear ( `year` [ , `month` [ , `date` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, let t be +0; otherwise, let t be LocalTime(t).
Let y be ? ToNumber(year).
If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The length property of the setFullYear method is 3.

Note

If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().

20.3.4.22 Date.prototype.setHours ( `hour` [ , `min` [ , `sec` [ , `ms` ] ] ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let h be ? ToNumber(hour).
If min is not present, let m be MinFromTime(t); otherwise, let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The length property of the setHours method is 4.

Note

If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().

20.3.4.23 Date.prototype.setMilliseconds ( `ms` )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let ms be ? ToNumber(ms).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ms).
Let u be TimeClip(UTC(MakeDate(Day(t), time))).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

20.3.4.24 Date.prototype.setMinutes ( `min` [ , `sec` [ , `ms` ] ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The length property of the setMinutes method is 3.

Note

If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().

20.3.4.25 Date.prototype.setMonth ( `month` [ , `date` ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The length property of the setMonth method is 2.

Note

If date is not present, this method behaves as if date was present with the value getDate().

20.3.4.26 Date.prototype.setSeconds ( `sec` [ , `ms` ] )

The following steps are performed:

Let t be LocalTime(? thisTimeValue(this value)).
Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[DateValue]] internal slot of this Date object to u.
Return u.

The length property of the setSeconds method is 2.

Note

If ms is not present, this method behaves as if ms was present with the value getMilliseconds().

20.3.4.27 Date.prototype.setTime ( `time` )

The following steps are performed:

Perform ? thisTimeValue(this value).
Let t be ? ToNumber(time).
Let v be TimeClip(t).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.3.4.28 Date.prototype.setUTCDate ( `date` )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.3.4.29 Date.prototype.setUTCFullYear ( `year` [ , `month` [ , `date` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
If t is NaN, let t be +0.
Let y be ? ToNumber(year).
If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The length property of the setUTCFullYear method is 3.

Note

If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().

20.3.4.30 Date.prototype.setUTCHours ( `hour` [ , `min` [ , `sec` [ , `ms` ] ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let h be ? ToNumber(hour).
If min is not present, let m be MinFromTime(t); otherwise, let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t); otherwise, let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t); otherwise, let milli be ? ToNumber(ms).
Let newDate be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The length property of the setUTCHours method is 4.

Note

If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().

20.3.4.31 Date.prototype.setUTCMilliseconds ( `ms` )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let milli be ? ToNumber(ms).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), milli).
Let v be TimeClip(MakeDate(Day(t), time)).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

20.3.4.32 Date.prototype.setUTCMinutes ( `min` [ , `sec` [ , `ms` ] ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let m be ? ToNumber(min).
If sec is not present, let s be SecFromTime(t).
Else,
1. Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t).
Else,
1. Let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let v be TimeClip(date).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The length property of the setUTCMinutes method is 3.

Note

If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it function behaves as if ms was present with the value return by getUTCMilliseconds().

20.3.4.33 Date.prototype.setUTCMonth ( `month` [ , `date` ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let m be ? ToNumber(month).
If date is not present, let dt be DateFromTime(t).
Else,
1. Let dt be ? ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The length property of the setUTCMonth method is 2.

Note

If date is not present, this method behaves as if date was present with the value getUTCDate().

20.3.4.34 Date.prototype.setUTCSeconds ( `sec` [ , `ms` ] )

The following steps are performed:

Let t be ? thisTimeValue(this value).
Let s be ? ToNumber(sec).
If ms is not present, let milli be msFromTime(t).
Else,
1. Let milli be ? ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let v be TimeClip(date).
Set the [[DateValue]] internal slot of this Date object to v.
Return v.

The length property of the setUTCSeconds method is 2.

Note

If ms is not present, this method behaves as if ms was present with the value getUTCMilliseconds().

20.3.4.35 Date.prototype.toDateString ( )

The following steps are performed:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return DateString(t).

20.3.4.36 Date.prototype.toISOString ( )

This function returns a String value representing the instance in time corresponding to this time value. The format of the String is the Date Time string format defined in 20.3.1.15. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If this time value is not a finite Number or if the year is not a value that can be represented in that format (if necessary using extended year format), a RangeError exception is thrown.

20.3.4.37 Date.prototype.toJSON ( `key` )

This function provides a String representation of a Date object for use by JSON.stringify (24.5.2).

When the toJSON method is called with argument key, the following steps are taken:

Let O be ? ToObject(this value).
Let tv be ? ToPrimitive(O, hint Number).
If Type(tv) is Number and tv is not finite, return null.
Return ? Invoke(O, "toISOString").

Note 1

The argument is ignored.

Note 2

The toJSON function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.

20.3.4.38 Date.prototype.toLocaleDateString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

20.3.4.39 Date.prototype.toLocaleString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

20.3.4.40 Date.prototype.toLocaleTimeString ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

20.3.4.41 Date.prototype.toString ( )

The following steps are performed:

Let tv be ? thisTimeValue(this value).
Return ToDateString(tv).

Note 1

For any Date object d whose milliseconds amount is zero, the result of Date.parse(d.toString()) is equal to d.valueOf(). See 20.3.3.2.

Note 2

The toString function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method.

20.3.4.41.1 Runtime Semantics: TimeString ( `tv` )

The following steps are performed:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let hour be the String representation of HourFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let minute be the String representation of MinFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let second be the String representation of SecFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Return the string-concatenation of hour, ":", minute, ":", second, the code unit 0x0020 (SPACE), and "GMT".

20.3.4.41.2 Runtime Semantics: DateString ( `tv` )

The following steps are performed:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let weekday be the Name of the entry in Table 46 with the Number WeekDay(tv).
Let month be the Name of the entry in Table 47 with the Number MonthFromTime(tv).
Let day be the String representation of DateFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let year be the String representation of YearFromTime(tv), formatted as a decimal number of at least four digits, padded to the left with zeroes if necessary.
Return the string-concatenation of weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), and year.

Table 46: Names of days of the week

Number	Name
0	`"Sun"`
1	`"Mon"`
2	`"Tue"`
3	`"Wed"`
4	`"Thu"`
5	`"Fri"`
6	`"Sat"`

Table 47: Names of months of the year

Number	Name
0	`"Jan"`
1	`"Feb"`
2	`"Mar"`
3	`"Apr"`
4	`"May"`
5	`"Jun"`
6	`"Jul"`
7	`"Aug"`
8	`"Sep"`
9	`"Oct"`
10	`"Nov"`
11	`"Dec"`

20.3.4.41.3 Runtime Semantics: TimeZoneString ( `tv` )

The following steps are performed:

Assert: Type(tv) is Number.
Assert: tv is not NaN.
Let offset be LocalTZA(tv, true).
If offset ≥ 0, let offsetSign be "+"; otherwise, let offsetSign be "-".
Let offsetMin be the String representation of MinFromTime(abs(offset)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let offsetHour be the String representation of HourFromTime(abs(offset)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let tzName be an implementation-defined string that is either the empty string or the string-concatenation of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-dependent timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
Return the string-concatenation of offsetSign, offsetHour, offsetMin, and tzName.

20.3.4.41.4 Runtime Semantics: ToDateString ( `tv` )

The following steps are performed:

Assert: Type(tv) is Number.
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return the string-concatenation of DateString(t), the code unit 0x0020 (SPACE), TimeString(t), and TimeZoneString(tv).

20.3.4.42 Date.prototype.toTimeString ( )

The following steps are performed:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let t be LocalTime(tv).
Return the string-concatenation of TimeString(t) and TimeZoneString(tv).

20.3.4.43 Date.prototype.toUTCString ( )

The following steps are performed:

Let O be this Date object.
Let tv be ? thisTimeValue(O).
If tv is NaN, return "Invalid Date".
Let weekday be the Name of the entry in Table 46 with the Number WeekDay(tv).
Let month be the Name of the entry in Table 47 with the Number MonthFromTime(tv).
Let day be the String representation of DateFromTime(tv), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let year be the String representation of YearFromTime(tv), formatted as a decimal number of at least four digits, padded to the left with zeroes if necessary.
Return the string-concatenation of weekday, ",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), year, the code unit 0x0020 (SPACE), and TimeString(tv).

20.3.4.44 Date.prototype.valueOf ( )

The following steps are performed:

Return ? thisTimeValue(this value).

20.3.4.45 Date.prototype [ @@toPrimitive ] ( `hint` )

This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are "default", "number", and "string". Date objects, are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".

When the @@toPrimitive method is called with argument hint, the following steps are taken:

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If hint is the String value "string" or the String value "default", then
1. Let tryFirst be "string".
Else if hint is the String value "number", then
1. Let tryFirst be "number".
Else, throw a TypeError exception.
Return ? OrdinaryToPrimitive(O, tryFirst).

The value of the name property of this function is "[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.3.5 Properties of Date Instances

Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date object.

21 Text Processing

21.1 String Objects

21.1.1 The String Constructor

The String constructor:

is the intrinsic object %String%.
is the initial value of the String property of the global object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.

21.1.1.1 String ( `value` )

When String is called with argument value, the following steps are taken:

If no arguments were passed to this function invocation, let s be "".
Else,
1. If NewTarget is undefined and Type(value) is Symbol, return SymbolDescriptiveString(value).
2. Let s be ? ToString(value).
If NewTarget is undefined, return s.
Return ? StringCreate(s, ? GetPrototypeFromConstructor(NewTarget, "%StringPrototype%")).

21.1.2 Properties of the String Constructor

The String constructor:

has a [[Prototype]] internal slot whose value is the intrinsic object %FunctionPrototype%.
has the following properties:

21.1.2.1 String.fromCharCode ( ...`codeUnits` )

The String.fromCharCode function may be called with any number of arguments which form the rest parameter codeUnits. The following steps are taken:

Let codeUnits be a List containing the arguments passed to this function.
Let length be the number of elements in codeUnits.
Let elements be a new empty List.
Let nextIndex be 0.
Repeat, while nextIndex < length
1. Let next be codeUnits[nextIndex].
2. Let nextCU be ? ToUint16(next).
3. Append nextCU to the end of elements.
4. Let nextIndex be nextIndex + 1.
Return the String value whose elements are, in order, the elements in the List elements. If length is 0, the empty string is returned.

The length property of the fromCharCode function is 1.

21.1.2.2 String.fromCodePoint ( ...`codePoints` )

The String.fromCodePoint function may be called with any number of arguments which form the rest parameter codePoints. The following steps are taken:

Let codePoints be a List containing the arguments passed to this function.
Let length be the number of elements in codePoints.
Let elements be a new empty List.
Let nextIndex be 0.
Repeat, while nextIndex < length
1. Let next be codePoints[nextIndex].
2. Let nextCP be ? ToNumber(next).
3. If SameValue(nextCP, ToInteger(nextCP)) is false, throw a RangeError exception.
4. If nextCP < 0 or nextCP > 0x10FFFF, throw a RangeError exception.
5. Append the elements of the UTF16Encoding of nextCP to the end of elements.
6. Let nextIndex be nextIndex + 1.
Return the String value whose elements are, in order, the elements in the List elements. If length is 0, the empty string is returned.

The length property of the fromCodePoint function is 1.

21.1.2.3 String.prototype

The initial value of String.prototype is the intrinsic object %StringPrototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.4 String.raw ( `template`, ...`substitutions` )

The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the List substitutions. The following steps are taken:

Let substitutions be a List consisting of all of the arguments passed to this function, starting with the second argument. If fewer than two arguments were passed, the List is empty.
Let numberOfSubstitutions be the number of elements in substitutions.
Let cooked be ? ToObject(template).
Let raw be ? ToObject(? Get(cooked, "raw")).
Let literalSegments be ? ToLength(? Get(raw, "length")).
If literalSegments ≤ 0, return the empty string.
Let stringElements be a new empty List.
Let nextIndex be 0.
Repeat,
1. Let nextKey be ! ToString(nextIndex).
2. Let nextSeg be ? ToString(? Get(raw, nextKey)).
3. Append in order the code unit elements of nextSeg to the end of stringElements.
4. If nextIndex + 1 = literalSegments, then
  1. Return the String value whose code units are, in order, the elements in the List stringElements. If stringElements has no elements, the empty string is returned.
5. If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
6. Else, let next be the empty String.
7. Let nextSub be ? ToString(next).
8. Append in order the code unit elements of nextSub to the end of stringElements.
9. Let nextIndex be nextIndex + 1.

Note

String.raw is intended for use as a tag function of a Tagged Template (12.3.7). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.

21.1.3 Properties of the String Prototype Object

The String prototype object:

is the intrinsic object %StringPrototype%.
is a String exotic object and has the internal methods specified for such objects.
has a [[StringData]] internal slot whose value is the empty String.
has a length property whose initial value is 0 and whose attributes are { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
has a [[Prototype]] internal slot whose value is the intrinsic object %ObjectPrototype%.

Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.

The abstract operation thisStringValue(value) performs the following steps:

If Type(value) is String, return value.
If Type(value) is Object and value has a [[StringData]] internal slot, then
1. Assert: value.[[StringData]] is a String value.
2. Return value.[[StringData]].
Throw a TypeError exception.

21.1.3.1 String.prototype.charAt ( `pos` )

Note 1

Returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return the empty String.
Return the String value of length 1, containing one code unit from S, namely the code unit at index position.

Note 2

The charAt function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.2 String.prototype.charCodeAt ( `pos` )

Note 1

Returns a Number (a nonnegative integer less than 2¹⁶) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return NaN.
Return a value of Number type, whose value is the numeric value of the code unit at index position within the String S.

Note 2

The charCodeAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.3 String.prototype.codePointAt ( `pos` )

Note 1

Returns a nonnegative integer Number less than 0x110000 that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 代理对 does not begin at pos, the result is the code unit at pos.

When the codePointAt method is called with one argument pos, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let position be ? ToInteger(pos).
Let size be the length of S.
If position < 0 or position ≥ size, return undefined.
Let first be the numeric value of the code unit at index position within the String S.
If first < 0xD800 or first > 0xDBFF or position+1 = size, return first.
Let second be the numeric value of the code unit at index position+1 within the String S.
If second < 0xDC00 or second > 0xDFFF, return first.
Return UTF16Decode(first, second).

Note 2

The codePointAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.4 String.prototype.concat ( ...`args` )

Note 1

When the concat method is called it returns the String value consisting of the code units of the this object (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.

When the concat method is called with zero or more arguments, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let args be a List whose elements are the arguments passed to this function.
Let R be S.
Repeat, while args is not empty
1. Remove the first element from args and let next be the value of that element.
2. Let nextString be ? ToString(next).
3. Set R to the string-concatenation of the previous value of R and nextString.
Return R.

The length property of the concat method is 1.

Note 2

The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.5 String.prototype.constructor

The initial value of String.prototype.constructor is the intrinsic object %String%.

21.1.3.6 String.prototype.endsWith ( `searchString` [ , `endPosition` ] )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let len be the length of S.
If endPosition is undefined, let pos be len, else let pos be ? ToInteger(endPosition).
Let end be min(max(pos, 0), len).
Let searchLength be the length of searchStr.
Let start be end - searchLength.
If start is less than 0, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.

Note 1

Returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at endPosition - length(this). Otherwise returns false.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The endsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.7 String.prototype.includes ( `searchString` [ , `position` ] )

The includes method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position). (If position is undefined, this step produces the value 0.)
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
If there exists any integer k not smaller than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j within S is the same as the code unit at index j within searchStr, return true; but if there is no such integer k, return false.

Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, return true; otherwise, returns false. If position is undefined, 0 is assumed, so as to search all of the String.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The includes function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.8 String.prototype.indexOf ( `searchString` [ , `position` ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position). (If position is undefined, this step produces the value 0.)
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
Return the smallest possible integer k not smaller than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j within S is the same as the code unit at index j within searchStr; but if there is no such integer k, return the value -1.

Note 2

The indexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.9 String.prototype.lastIndexOf ( `searchString` [ , `position` ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let searchStr be ? ToString(searchString).
Let numPos be ? ToNumber(position). (If position is undefined, this step produces the value NaN.)
If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLen be the length of searchStr.
Return the largest possible nonnegative integer k not larger than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j within S is the same as the code unit at index j within searchStr; but if there is no such integer k, return the value -1.

Note 2

The lastIndexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.10 String.prototype.localeCompare ( `that` [ , `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.

When the localeCompare method is called with argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by a host default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before performing the comparisons, the following steps are performed to prepare the Strings:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let That be ? ToString(that).

The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.

The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 22.1.3.25) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.

Note 1

The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

Note 2

This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. However, regardless of the host provided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (https://unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (https://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (https://unicode.org/reports/tr10/).

Note 3

The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.11 String.prototype.match ( `regexp` )

When the match method is called with argument regexp, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If regexp is neither undefined nor null, then
1. Let matcher be ? GetMethod(regexp, @@match).
2. If matcher is not undefined, then
  1. Return ? Call(matcher, regexp, « O »).
Let S be ? ToString(O).
Let rx be ? RegExpCreate(regexp, undefined).
Return ? Invoke(rx, @@match, « S »).

Note

The match function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.12 String.prototype.normalize ( [ `form` ] )

When the normalize method is called with one argument form, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
If form is not present or form is undefined, let form be "NFC".
Let f be ? ToString(form).
If f is not one of "NFC", "NFD", "NFKC", or "NFKD", throw a RangeError exception.
Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
Return ns.

Note

The normalize function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.13 String.prototype.padEnd ( `maxLength` [ , `fillString` ] )

When the padEnd method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let intMaxLength be ? ToLength(maxLength).
Let stringLength be the length of S.
If intMaxLength is not greater than stringLength, return S.
If fillString is undefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
Else, let filler be ? ToString(fillString).
If filler is the empty String, return S.
Let fillLen be intMaxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
Return the string-concatenation of S and truncatedStringFiller.

Note 1

The first argument maxLength will be clamped such that it can be no smaller than the length of the this value.

Note 2

The optional second argument fillString defaults to " " (the String value consisting of the code unit 0x0020 SPACE).

21.1.3.14 String.prototype.padStart ( `maxLength` [ , `fillString` ] )

When the padStart method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let intMaxLength be ? ToLength(maxLength).
Let stringLength be the length of S.
If intMaxLength is not greater than stringLength, return S.
If fillString is undefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
Else, let filler be ? ToString(fillString).
If filler is the empty String, return S.
Let fillLen be intMaxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
Return the string-concatenation of truncatedStringFiller and S.

Note 1

The first argument maxLength will be clamped such that it can be no smaller than the length of the this value.

Note 2

The optional second argument fillString defaults to " " (the String value consisting of the code unit 0x0020 SPACE).

21.1.3.15 String.prototype.repeat ( `count` )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let n be ? ToInteger(count).
If n < 0, throw a RangeError exception.
If n is +∞, throw a RangeError exception.
Let T be the String value that is made from n copies of S appended together. If n is 0, T is the empty String.
Return T.

Note 1

This method creates the String value consisting of the code units of the this object (converted to String) repeated count times.

Note 2

The repeat function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.16 String.prototype.replace ( `searchValue`, `replaceValue` )

When the replace method is called with arguments searchValue and replaceValue, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If searchValue is neither undefined nor null, then
1. Let replacer be ? GetMethod(searchValue, @@replace).
2. If replacer is not undefined, then
  1. Return ? Call(replacer, searchValue, « O, replaceValue »).
Let string be ? ToString(O).
Let searchString be ? ToString(searchValue).
Let functionalReplace be IsCallable(replaceValue).
If functionalReplace is false, then
1. Let replaceValue be ? ToString(replaceValue).
Search string for the first occurrence of searchString and let pos be the index within string of the first code unit of the matched substring and let matched be searchString. If no occurrences of searchString were found, return string.
If functionalReplace is true, then
1. Let replValue be ? Call(replaceValue, undefined, « matched, pos, string »).
2. Let replStr be ? ToString(replValue).
Else,
1. Let captures be a new empty List.
2. Let replStr be GetSubstitution(matched, string, pos, captures, undefined, replaceValue).
Let tailPos be pos + the number of code units in matched.
Let newString be the string-concatenation of the first pos code units of string, replStr, and the trailing substring of string starting at index tailPos. If pos is 0, the first element of the concatenation will be the empty String.
Return newString.

Note

The replace function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.16.1 Runtime Semantics: GetSubstitution ( `matched`, `str`, `position`, `captures`, `namedCaptures`, `replacement` )

The abstract operation GetSubstitution performs the following steps:

Assert: Type(matched) is String.
Let matchLength be the number of code units in matched.
Assert: Type(str) is String.
Let stringLength be the number of code units in str.
Assert: position is a nonnegative integer.
Assert: position ≤ stringLength.
Assert: captures is a possibly empty List of Strings.
Assert: Type(replacement) is String.
Let tailPos be position + matchLength.
Let m be the number of elements in captures.
If namedCaptures is not undefined, then
1. Set namedCaptures to ? ToObject(namedCaptures).
Let result be the String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified in Table 48. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
Return result.

Table 48: Replacement Text Symbol Substitutions

Code units	Unicode Characters	Replacement text
0x0024, 0x0024	`$$`	`$`
0x0024, 0x0026	`$&`	`matched`
0x0024, 0x0060	$`	If `position` is 0, the replacement is the empty String. Otherwise the replacement is the substring of `str` that starts at index 0 and whose last code unit is at index `position` - 1.
0x0024, 0x0027	`$'`	If `tailPos` ≥ `stringLength`, the replacement is the empty String. Otherwise the replacement is the substring of `str` that starts at index `tailPos` and continues to the end of `str`.
0x0024, N Where 0x0031 ≤ N ≤ 0x0039	`$n` where `n` is one of `1 2 3 4 5 6 7 8 9` and `$n` is not followed by a decimal digit	The `n`^th element of `captures`, where `n` is a single digit in the range 1 to 9. If `n`≤`m` and the `n`^th element of `captures` is undefined, use the empty String instead. If `n`>`m`, no replacement is done.
0x0024, N, N Where 0x0030 ≤ N ≤ 0x0039	`$nn` where `n` is one of `0 1 2 3 4 5 6 7 8 9`	The `nn`^th element of `captures`, where `nn` is a two-digit decimal number in the range 01 to 99. If `nn`≤`m` and the `nn`^th element of `captures` is undefined, use the empty String instead. If `nn` is 00 or `nn`>`m`, no replacement is done.
0x0024, 0x003C	`$<`	If `namedCaptures` is undefined, the replacement text is the String `"$<"`. Else, Scan until the next `>` U+003E (GREATER-THAN SIGN). If none is found, the replacement text is the String `"$<"`. Else, Let `groupName` be the enclosed substring. Let `capture` be ? Get(`namedCaptures`, `groupName`). If `capture` is undefined, replace the text through `>` with the empty string. Otherwise, replace the text through `>` with ? ToString(`capture`).
0x0024	`$` in any context that does not match any of the above.	`$`

21.1.3.17 String.prototype.search ( `regexp` )

When the search method is called with argument regexp, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If regexp is neither undefined nor null, then
1. Let searcher be ? GetMethod(regexp, @@search).
2. If searcher is not undefined, then
  1. Return ? Call(searcher, regexp, « O »).
Let string be ? ToString(O).
Let rx be ? RegExpCreate(regexp, undefined).
Return ? Invoke(rx, @@search, « string »).

Note

The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.18 String.prototype.slice ( `start`, `end` )

The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let len be the length of S.
Let intStart be ? ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ? ToInteger(end).
If intStart < 0, let from be max(len + intStart, 0); otherwise let from be min(intStart, len).
If intEnd < 0, let to be max(len + intEnd, 0); otherwise let to be min(intEnd, len).
Let span be max(to - from, 0).
Return the String value containing span consecutive elements from S beginning with the element at index from.

Note

The slice function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

21.1.3.19 String.prototype.split ( `separator`, `limit` )

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a @@split method.

When the split method is called, the following steps are taken:

Let O be ? RequireObjectCoercible(this value).
If separator is neither undefined nor null, then
1. Let splitter be ? GetMethod(separator, @@split).
2. If splitter is not undefined, then
  1. Return ? Call(splitter, separator, « O, limit »).
Let S be ? ToString(O).
Let A be ! ArrayCreate(0).
Let lengthA be 0.
If limit is undefined, let lim be 2³²-1; else let lim be ? ToUint32(limit).
Let s be the length of S.
Let p be 0.
Let R be ? ToString(separator).
If lim = 0, return A.
If separator is undefined, then
1. Perform ! CreateDataProperty(A, "0", S).
2. Return A.
If s = 0, then
1. Let z be SplitMatch(S, 0, R).
2. If z is not false, return A.
3. Perform ! CreateDataProperty(A, "0", S).
4. Return A.
Let q be p.
Repeat, while q ≠ s
1. Let e be SplitMatch(S, q, R).
2. If e is false, let q be q+1.
3. Else e is an integer index ≤ s,
  1. If e = p, let q be q+1.
  2. Else e ≠ p,
    1. Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through q (exclusive).
    2. Perform ! CreateDataProperty(A, ! ToString(lengthA), T).
    3. Increment lengthA by 1.
    4. If lengthA = lim, return A.
    5. Let p be e.
    6. Let q be p.
Let T be the String value equal to the substring of S consisting of the code units at indices p (inclusive) through s (exclusive).
Perform ! CreateDataProperty(A, ! ToString(lengthA), T).
Return A.

Note 1

The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.

If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

Note 2

The split function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.19.1 Runtime Semantics: SplitMatch ( `S`, `q`, `R` )

The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String R, and performs the following steps in order to return either false or the end index of a match:

Assert: Type(R) is String.
Let r be the number of code units in R.
Let s be the number of code units in S.
If q+r > s, return false.
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the code unit at index q+i within S is different from the code unit at index i within R, return false.
Return q+r.

21.1.3.20 String.prototype.startsWith ( `searchString` [ , `position` ] )

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let isRegExp be ? IsRegExp(searchString).
If isRegExp is true, throw a TypeError exception.
Let searchStr be ? ToString(searchString).
Let pos be ? ToInteger(position). (If position is undefined, this step produces the value 0.)
Let len be the length of S.
Let start be min(max(pos, 0), len).
Let searchLength be the length of searchStr.
If searchLength+start is greater than len, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.

Note 1

This method returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at index position. Otherwise returns false.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The startsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.21 String.prototype.substring ( `start`, `end` )

The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let len be the length of S.
Let intStart be ? ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ? ToInteger(end).
Let finalStart be min(max(intStart, 0), len).
Let finalEnd be min(max(intEnd, 0), len).
Let from be min(finalStart, finalEnd).
Let to be max(finalStart, finalEnd).
Return the String value whose length is to - from, containing code units from S, namely the code units with indices from through to - 1, in ascending order.

Note

The substring function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.22 String.prototype.toLocaleLowerCase ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

Note

The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.23 String.prototype.toLocaleUpperCase ( [ `reserved1` [ , `reserved2` ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

Note

The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.24 String.prototype.toLowerCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let cpList be a List containing in order the code points as defined in 6.1.4 of S, starting at the first element of S.
Let cuList be a List where the elements are the result of toLowercase(cplist), according to the Unicode Default Case Conversion algorithm.
Let L be the String value whose elements are the UTF16Encoding of the code points of cuList.
Return L.

The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).

Note 1

The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

Note 2

The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.25 String.prototype.toString ( )

When the toString method is called, the following steps are taken:

Return ? thisStringValue(this value).

Note

For a String object, the toString method happens to return the same thing as the valueOf method.

21.1.3.26 String.prototype.toUpperCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.

Note

The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.27 String.prototype.trim ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Let T be the String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.
Return T.

Note

The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

21.1.3.28 String.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

Return ? thisStringValue(this value).

21.1.3.29 String.prototype [ @@iterator ] ( )

When the @@iterator method is called it returns an Iterator object (25.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:

Let O be ? RequireObjectCoercible(this value).
Let S be ? ToString(O).
Return CreateStringIterator(S).

The value of the name property of this function is "[Symbol.iterator]".

21.1.4 Properties of String Instances

String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.

String instances have a length property, and a set of enumerable properties with integer-indexed names.

21.1.4.1 length

The number of elements in the String value represented by this String object.

Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.5 String Iterator Objects

A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.

21.1.5.1 CreateStringIterator ( `string` )

Several methods of String objects return Iterator objects. The abstract operation CreateStringIterator with argument string is used to create such iterator objects. It performs the following steps:

Assert: Type(string) is String.
Let iterator be ObjectCreate(%StringIteratorPrototype%, « [[IteratedString]], [[StringIteratorNextIndex]] »).
Set iterator.[[IteratedString]] to string.
Set iterator.[[StringIteratorNextIndex]] to 0.
Return iterator.

21.1.5.2 The %StringIteratorPrototype% Object

The %StringIteratorPrototype% object:

has properties that are inherited by all String Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object %IteratorPrototype%.
has the following properties:

21.1.5.2.1 %StringIteratorPrototype%.next ( )

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have all of the internal slots of a String Iterator Instance (21.1.5.3), throw a TypeError exception.
Let s be O.[[IteratedString]].
If s is undefined, return CreateIterResultObject(undefined, true).
Let position be O.[[StringIteratorNextIndex]].
Let len be the length of s.
If position ≥ len, then
1. Set O.[[IteratedString]] to undefined.
2. Return CreateIterResultObject(undefined, true).
Let first be the numeric value of the code unit at index position within s.
If first < 0xD800 or first > 0xDBFF or position+1 = len, let resultString be the String value consisting of the single code unit first.
Else,
1. Let second be the numeric value of the code unit at index position+1 within the String S.
2. If second < 0xDC00 or second > 0xDFFF, let resultString be the String value consisting of the single code unit first.
3. Else, let resultString be the string-concatenation of the code unit first and the code unit second.
Let resultSize be the number of code units in resultString.
Set O.[[StringIteratorNextIndex]] to position + resultSize.
Return CreateIterResultObject(resultString, false).

21.1.5.2.2 %StringIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "String Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

21.1.5.3 Properties of String Iterator Instances

String Iterator instances are ordinary objects that inherit properties from the %StringIteratorPrototype% intrinsic object. String Iterator instances are initially created with the internal slots listed in Table 49.

Table 49: Internal Slots of String Iterator Instances

Internal Slot	Description
[[IteratedString]]	The String value whose elements are being iterated.
[[StringIteratorNextIndex]]	The integer index of the next string index to be examined by this iteration.

21.2 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

Note

The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

21.2.1 Patterns

The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.

Syntax

[U, N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

[empty]

[?U, ?N]

[?U, ?N]

[U, N]

[?U, ?N]

[?U, ?N]

[?U, ?N]

[U, N]

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

(

[?U, ?N]

)

Quantifier

QuantifierPrefix

{

}

{

}

{

}

[U, N]

[?U, ?N]

[?U]

(

GroupSpecifier

[?U]

[?U, ?N]

)

(

[?U, ?N]

)

SyntaxCharacter

one of

(

)

[

]

{

}

PatternCharacter

but not SyntaxCharacter

[U, N]

[?U]

[?U]

[+N]

[?U]

[U]

[lookahead ∉ DecimalDigit]

[?U]

[?U]

one of

one of

[U]

[empty]

[?U]

[U]

[?U]

[U]

[?U]

RegExpIdentifierName

[?U]

[?U]

[U]

UnicodeIDStart

[?U]

[U]

UnicodeIDContinue

[?U]

<ZWNJ>

<ZWJ>

[U]

[+U]

[+U]

[+U]

[+U]

[~U]

[+U]

}

Each \u TrailSurrogate for which the choice of associated u LeadSurrogate is ambiguous shall be associated with the nearest possible u LeadSurrogate that would otherwise have no corresponding \u TrailSurrogate.

LeadSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xD800 to 0xDBFF

TrailSurrogate

but only if the SV of Hex4Digits is in the inclusive range 0xDC00 to 0xDFFF

NonSurrogate

but only if the SV of Hex4Digits is not in the inclusive range 0xD800 to 0xDFFF

IdentityEscape

[U]

[+U]

SyntaxCharacter

[+U]

[~U]

but not UnicodeIDContinue

opt

[lookahead ∉ DecimalDigit]

[U]

[+U]

}

[+U]

}

LoneUnicodePropertyNameOrValue

UnicodePropertyName

UnicodePropertyValue

UnicodePropertyName

UnicodePropertyNameCharacter

opt

UnicodePropertyValue

LoneUnicodePropertyNameOrValue

UnicodePropertyValueCharacter

UnicodePropertyValueCharacter

opt

UnicodePropertyNameCharacter

ControlLetter

CharacterClass

[U]

[

[lookahead ∉ { ^ }]

[?U]

]

[

[?U]

]

[U]

[empty]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[?U]

[?U]

[?U]

[?U]

[?U]

[U]

[?U]

[U]

but not one of \ or ] or -

[?U]

[U]

[+U]

[?U]

[?U]

21.2.1.1 Static Semantics: Early Errors

Pattern

It is a Syntax Error if NcapturingParens ≥ 2³²-1.
It is a Syntax Error if Pattern contains multiple GroupSpecifiers whose enclosed RegExpIdentifierNames have the same StringValue.

QuantifierPrefix

{

}

It is a Syntax Error if the MV of the first DecimalDigits is larger than the MV of the second DecimalDigits.

AtomEscape

GroupName

It is a Syntax Error if the enclosing Pattern does not contain a GroupSpecifier with an enclosed RegExpIdentifierName whose StringValue equals the StringValue of the RegExpIdentifierName of this production's GroupName.

AtomEscape

It is a Syntax Error if the CapturingGroupNumber of DecimalEscape is larger than NcapturingParens (21.2.2.1).

NonemptyClassRanges

It is a Syntax Error if IsCharacterClass of the first ClassAtom is true or IsCharacterClass of the second ClassAtom is true.
It is a Syntax Error if IsCharacterClass of the first ClassAtom is false and IsCharacterClass of the second ClassAtom is false and the CharacterValue of the first ClassAtom is larger than the CharacterValue of the second ClassAtom.

It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is true or IsCharacterClass of ClassAtom is true.
It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is false and IsCharacterClass of ClassAtom is false and the CharacterValue of ClassAtomNoDash is larger than the CharacterValue of ClassAtom.

[U]

[?U]

It is a Syntax Error if SV(RegExpUnicodeEscapeSequence) is none of "$", or "_", or the UTF16Encoding of a code point matched by the UnicodeIDStart lexical grammar production.

[U]

[?U]

It is a Syntax Error if SV(RegExpUnicodeEscapeSequence) is none of "$", or "_", or the UTF16Encoding of either <ZWNJ> or <ZWJ>, or the UTF16Encoding of a Unicode code point that would be matched by the UnicodeIDContinue lexical grammar production.

UnicodePropertyName

UnicodePropertyValue

It is a Syntax Error if the List of Unicode code points that is SourceText of UnicodePropertyName is not identical to a List of Unicode code points that is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 51.
It is a Syntax Error if the List of Unicode code points that is SourceText of UnicodePropertyValue is not identical to a List of Unicode code points that is a value or value alias for the Unicode property or property alias given by SourceText of UnicodePropertyName listed in the “Property value and aliases” column of the corresponding tables Table 53 or Table 54.

LoneUnicodePropertyNameOrValue

It is a Syntax Error if the List of Unicode code points that is SourceText of LoneUnicodePropertyNameOrValue is not identical to a List of Unicode code points that is a Unicode general category or general category alias listed in the “Property value and aliases” column of Table 53, nor a binary property or binary property alias listed in the “Property name and aliases” column of Table 52.

21.2.1.2 Static Semantics: CapturingGroupNumber

Return the MV of NonZeroDigit.

Let n be the number of code points in DecimalDigits.
Return (the MV of NonZeroDigit × 10ⁿ) plus the MV of DecimalDigits.

The definitions of “the MV of NonZeroDigit” and “the MV of DecimalDigits” are in 11.8.3.

21.2.1.3 Static Semantics: IsCharacterClass

but not one of \ or ] or -

Return false.

Return true.

21.2.1.4 Static Semantics: CharacterValue

Return the code point value of U+002D (HYPHEN-MINUS).

but not one of \ or ] or -

Let ch be the code point matched by SourceCharacter.
Return the code point value of ch.

Return the code point value of U+0008 (BACKSPACE).

Return the code point value of U+002D (HYPHEN-MINUS).

ControlEscape

Return the code point value according to Table 50.

Table 50: ControlEscape Code Point Values

ControlEscape	Code Point Value	Code Point	Unicode Name	Symbol
`t`	9	`U+0009`	CHARACTER TABULATION	<HT>
`n`	10	`U+000A`	LINE FEED (LF)	<LF>
`v`	11	`U+000B`	LINE TABULATION	<VT>
`f`	12	`U+000C`	FORM FEED (FF)	<FF>
`r`	13	`U+000D`	CARRIAGE RETURN (CR)	<CR>

ControlLetter

Let ch be the code point matched by ControlLetter.
Let i be ch's code point value.
Return the remainder of dividing i by 32.

[lookahead ∉ DecimalDigit]

Return the code point value of U+0000 (NULL).

Note

\0 represents the <NUL> character and cannot be followed by a decimal digit.

Return the numeric value of the code unit that is the SV of HexEscapeSequence.

LeadSurrogate

TrailSurrogate

Let lead be the CharacterValue of LeadSurrogate.
Let trail be the CharacterValue of TrailSurrogate.
Let cp be UTF16Decode(lead, trail).
Return the code point value of cp.

LeadSurrogate

Return the CharacterValue of LeadSurrogate.

TrailSurrogate

Return the CharacterValue of TrailSurrogate.

NonSurrogate

Return the CharacterValue of NonSurrogate.

Return the MV of Hex4Digits.

CodePoint

}

Return the MV of CodePoint.

Return the MV of HexDigits.

IdentityEscape

Let ch be the code point matched by IdentityEscape.
Return the code point value of ch.

21.2.1.5 Static Semantics: SourceText

UnicodePropertyNameCharacter

opt

UnicodePropertyValueCharacter

opt

Return the List, in source text order, of Unicode code points in the source text matched by this production.

21.2.1.6 Static Semantics: StringValue

RegExpIdentifierName

[U]

[?U]

RegExpIdentifierName

[?U]

[?U]

Return the String value consisting of the sequence of code units corresponding to RegExpIdentifierName. In determining the sequence any occurrences of \ RegExpUnicodeEscapeSequence are first replaced with the code point represented by the RegExpUnicodeEscapeSequence and then the code points of the entire RegExpIdentifierName are converted to code units by UTF16Encoding each code point.

21.2.2 Pattern Semantics

A regular expression pattern is converted into an internal procedure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The internal procedure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.

A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a "u". A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.

The syntax and semantics of Pattern is defined as if the source code for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.

Note

For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.

Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16Decode must be used in producing a List consisting of a single pattern character, the code point U+1D11E.

An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.

21.2.2.1 Notation

The descriptions below use the following variables:

Input is a List consisting of all of the characters, in order, of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the n^th character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in Input.
NcapturingParens is the total number of left-capturing parentheses (i.e. the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes) in the pattern. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom::(GroupSpecifierDisjunction) production.
DotAll is true if the RegExp object's [[OriginalFlags]] internal slot contains "s" and otherwise is false.
IgnoreCase is true if the RegExp object's [[OriginalFlags]] internal slot contains "i" and otherwise is false.
Multiline is true if the RegExp object's [[OriginalFlags]] internal slot contains "m" and otherwise is false.
Unicode is true if the RegExp object's [[OriginalFlags]] internal slot contains "u" and otherwise is false.

Furthermore, the descriptions below use the following internal data structures:

A CharSet is a mathematical set of characters, either code units or code points depending up the state of the Unicode flag. “All characters” means either all code unit values or all code point values also depending upon the state of Unicode.
A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is a List of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The n^th element of captures is either a List that represents the value obtained by the n^th set of capturing parentheses or undefined if the n^th set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special token failure that indicates that the match failed.
A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to values) that takes one State argument and returns a MatchResult result. If an internal closure references variables which are bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. The Continuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returns failure.
A Matcher procedure is an internal closure that takes two arguments — a State and a Continuation — and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
An AssertionTester procedure is an internal closure that takes a State argument and returns a Boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in Input and returns true if the condition matched or false if not.

21.2.2.2 Pattern

The production Pattern::Disjunction evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return an internal closure that takes two arguments, a String str and an integer index, and performs the following steps:
1. Assert: index ≤ the length of str.
2. If Unicode is true, let Input be a List consisting of the sequence of code points of str interpreted as a UTF-16 encoded (6.1.4) Unicode string. Otherwise, let Input be a List consisting of the sequence of code units that are the elements of str. Input will be used throughout the algorithms in 21.2.2. Each element of Input is considered to be a character.
3. Let InputLength be the number of characters contained in Input. This variable will be used throughout the algorithms in 21.2.2.
4. Let listIndex be the index into Input of the character that was obtained from element index of str.
5. Let c be a Continuation that always returns its State argument as a successful MatchResult.
6. Let cap be a List of NcapturingParens undefined values, indexed 1 through NcapturingParens.
7. Let x be the State (listIndex, cap).
8. Call m(x, c) and return its result.

Note

A Pattern evaluates (“compiles”) to an internal procedure value. RegExpBuiltinExec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 21.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting internal procedure to find a match in a String cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).

21.2.2.3 Disjunction

With parameter direction.

The production Disjunction::Alternative evaluates as follows:

Evaluate Alternative to obtain a Matcher m.
Return m.

The production Disjunction::Alternative|Disjunction evaluates as follows:

Evaluate Alternative with argument direction to obtain a Matcher m1.
Evaluate Disjunction with argument direction to obtain a Matcher m2.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
1. Call m1(x, c) and let r be its result.
2. If r is not failure, return r.
3. Call m2(x, c) and return its result.

Note

The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,

/a|ab/.exec("abc")

returns the result "a" and not "ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

The order in which the two alternatives are tried is independent of the value of direction.

21.2.2.4 Alternative

With parameter direction.

The production Alternative::[empty] evaluates as follows:

Return a Matcher that takes two arguments, a State x and a Continuation c, and returns the result of calling c(x).

The production Alternative::AlternativeTerm evaluates as follows:

Evaluate Alternative with argument direction to obtain a Matcher m1.
Evaluate Term with argument direction to obtain a Matcher m2.
If direction is equal to +1, then
1. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
  1. Let d be a Continuation that takes a State argument y and returns the result of calling m2(y, c).
  2. Call m1(x, d) and return its result.
Else,
1. Assert: direction is equal to -1.
2. Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
  1. Let d be a Continuation that takes a State argument y and returns the result of calling m1(y, c).
  2. Call m2(x, d) and return its result.

Note

Consecutive Terms try to simultaneously match consecutive portions of Input. When direction is equal to +1, if the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative. When direction is equal to -1, the evaluation order of Alternative and Term are reversed.

21.2.2.5 Term

With parameter direction.

The production Term::Assertion evaluates as follows:

Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
1. Evaluate Assertion to obtain an AssertionTester t.
2. Call t(x) and let r be the resulting Boolean value.
3. If r is false, return failure.
4. Call c(x) and return its result.

Note

The AssertionTester is independent of direction.

The production Term::Atom evaluates as follows:

Return the Matcher that is the result of evaluating Atom with argument direction.

The production Term::AtomQuantifier evaluates as follows:

Evaluate Atom with argument direction to obtain a Matcher m.
Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and Boolean greedy.
Assert: If max is finite, then max is not less than min.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this Term. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing this Term.
Let parenCount be the number of left-capturing parentheses in Atom. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes enclosed by Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
1. Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.

21.2.2.5.1 Runtime Semantics: RepeatMatcher ( `m`, `min`, `max`, `greedy`, `x`, `c`, `parenIndex`, `parenCount` )

The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞) max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer parenCount, and performs the following steps:

If max is zero, return c(x).
Let d be an internal Continuation closure that takes one State argument y and performs the following steps when evaluated:
1. If min is zero and y's endIndex is equal to x's endIndex, return failure.
2. If min is zero, let min2 be zero; otherwise let min2 be min-1.
3. If max is ∞, let max2 be ∞; otherwise let max2 be max-1.
4. Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its result.
Let cap be a copy of x's captures List.
For each integer k that satisfies parenIndex < k and k ≤ parenIndex+parenCount, set cap[k] to undefined.
Let e be x's endIndex.
Let xr be the State (e, cap).
If min is not zero, return m(xr, d).
If greedy is false, then
1. Call c(x) and let z be its result.
2. If z is not failure, return z.
3. Call m(xr, d) and return its result.
Call m(xr, d) and let z be its result.
If z is not failure, return z.
Call c(x) and return its result.

Note 1

An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.

Note 2

If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (n^th) repetition of Atom are tried before moving on to the next choice in the next-to-last (n-1)^st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1)^st repetition of Atom and so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns "abcde" with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns "abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"]
["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/, "$1")

which returns the gcd in unary notation "aaaaa".

Note 3

Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression

/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

Note 4

Step 1 of the RepeatMatcher's d closure states that, once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1+/.exec("baaaac")

which returns the array

["b", ""]

21.2.2.6 Assertion

The production Assertion::^ evaluates as follows:

Return an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
1. Let e be x's endIndex.
2. If e is zero, return true.
3. If Multiline is false, return false.
4. If the character Input[e-1] is one of LineTerminator, return true.
5. Return false.

Note

Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline is true) at the beginning of a line.

The production Assertion::$ evaluates as follows:

Return an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
1. Let e be x's endIndex.
2. If e is equal to InputLength, return true.
3. If Multiline is false, return false.
4. If the character Input[e] is one of LineTerminator, return true.
5. Return false.

The production Assertion::\b evaluates as follows:

Return an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
1. Let e be x's endIndex.
2. Call IsWordChar(e-1) and let a be the Boolean result.
3. Call IsWordChar(e) and let b be the Boolean result.
4. If a is true and b is false, return true.
5. If a is false and b is true, return true.
6. Return false.

The production Assertion::\B evaluates as follows:

Return an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
1. Let e be x's endIndex.
2. Call IsWordChar(e-1) and let a be the Boolean result.
3. Call IsWordChar(e) and let b be the Boolean result.
4. If a is true and b is false, return false.
5. If a is false and b is true, return false.
6. Return true.

The production Assertion::(?=Disjunction) evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r is failure, return failure.
4. Let y be r's State.
5. Let cap be y's captures List.
6. Let xe be x's endIndex.
7. Let z be the State (xe, cap).
8. Call c(z) and return its result.

The production Assertion::(?!Disjunction) evaluates as follows:

Evaluate Disjunction with +1 as its direction argument to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r is not failure, return failure.
4. Call c(x) and return its result.

The production Assertion::(?<=Disjunction) evaluates as follows:

Evaluate Disjunction with -1 as its direction argument to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r is failure, return failure.
4. Let y be r's State.
5. Let cap be y's captures List.
6. Let xe be x's endIndex.
7. Let z be the State (xe, cap).
8. Call c(z) and return its result.

The production Assertion::(?<!Disjunction) evaluates as follows:

Evaluate Disjunction with -1 as its direction argument to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r is not failure, return failure.
4. Call c(x) and return its result.

21.2.2.6.1 Runtime Semantics: WordCharacters ( )

The abstract operation WordCharacters performs the following steps:

Let A be a set of characters containing the sixty-three characters:

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

0 1 2 3 4 5 6 7 8 9 _

Let U be an empty set.
For each character c not in set A where Canonicalize(c) is in A, add c to U.
Assert: Unless Unicode and IgnoreCase are both true, U is empty.
Add the characters in set U to set A.
Return A.

21.2.2.6.2 Runtime Semantics: IsWordChar ( `e` )

The abstract operation IsWordChar takes an integer parameter e and performs the following steps:

If e is -1 or e is InputLength, return false.
Let c be the character Input[e].
Let wordChars be the result of ! WordCharacters().
If c is in wordChars, return true.
Return false.

21.2.2.7 Quantifier

The production Quantifier::QuantifierPrefix evaluates as follows:

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min, max, and true.

The production Quantifier::QuantifierPrefix? evaluates as follows:

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min, max, and false.

The production QuantifierPrefix::* evaluates as follows:

Return the two results 0 and ∞.

The production QuantifierPrefix::+ evaluates as follows:

Return the two results 1 and ∞.

The production QuantifierPrefix::? evaluates as follows:

Return the two results 0 and 1.

The production QuantifierPrefix::{DecimalDigits} evaluates as follows:

Let i be the MV of DecimalDigits (see 11.8.3).
Return the two results i and i.

The production QuantifierPrefix::{DecimalDigits,} evaluates as follows:

Let i be the MV of DecimalDigits.
Return the two results i and ∞.

The production QuantifierPrefix::{DecimalDigits,DecimalDigits} evaluates as follows:

Let i be the MV of the first DecimalDigits.
Let j be the MV of the second DecimalDigits.
Return the two results i and j.

21.2.2.8 Atom

With parameter direction.

The production Atom::PatternCharacter evaluates as follows:

Let ch be the character matched by PatternCharacter.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production Atom::. evaluates as follows:

If DotAll is true, then
1. Let A be the set of all characters.
Otherwise, let A be the set of all characters except LineTerminator.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production Atom::\AtomEscape evaluates as follows:

Return the Matcher that is the result of evaluating AtomEscape with argument direction.

The production Atom::CharacterClass evaluates as follows:

Evaluate CharacterClass to obtain a CharSet A and a Boolean invert.
Call CharacterSetMatcher(A, invert, direction) and return its Matcher result.

The production Atom::(GroupSpecifierDisjunction) evaluates as follows:

Evaluate Disjunction with argument direction to obtain a Matcher m.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this Atom. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing this Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let d be an internal Continuation closure that takes one State argument y and performs the following steps:
  1. Let cap be a copy of y's captures List.
  2. Let xe be x's endIndex.
  3. Let ye be y's endIndex.
  4. If direction is equal to +1, then
    1. Assert: xe ≤ ye.
    2. Let s be a new List whose elements are the characters of Input at indices xe (inclusive) through ye (exclusive).
  5. Else,
    1. Assert: direction is equal to -1.
    2. Assert: ye ≤ xe.
    3. Let s be a new List whose elements are the characters of Input at indices ye (inclusive) through xe (exclusive).
  6. Set cap[parenIndex+1] to s.
  7. Let z be the State (ye, cap).
  8. Call c(z) and return its result.
2. Call m(x, d) and return its result.

The production Atom::(?:Disjunction) evaluates as follows:

Return the Matcher that is the result of evaluating Disjunction with argument direction.

21.2.2.8.1 Runtime Semantics: CharacterSetMatcher ( `A`, `invert`, `direction` )

The abstract operation CharacterSetMatcher takes three arguments, a CharSet A, a Boolean flag invert, and an integer direction, and performs the following steps:

Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
1. Let e be x's endIndex.
2. Let f be e + direction.
3. If f < 0 or f > InputLength, return failure.
4. Let index be min(e, f).
5. Let ch be the character Input[index].
6. Let cc be Canonicalize(ch).
7. If invert is false, then
  1. If there does not exist a member a of set A such that Canonicalize(a) is cc, return failure.
8. Else,
  1. Assert: invert is true.
  2. If there exists a member a of set A such that Canonicalize(a) is cc, return failure.
9. Let cap be x's captures List.
10. Let y be the State (f, cap).
11. Call c(y) and return its result.

21.2.2.8.2 Runtime Semantics: Canonicalize ( `ch` )

The abstract operation Canonicalize takes a character parameter ch and performs the following steps:

If IgnoreCase is false, return ch.
If Unicode is true, then
1. If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
2. Return ch.
Else,
1. Assert: ch is a UTF-16 code unit.
2. Let s be the String value consisting of the single code unit ch.
3. Let u be the same result produced as if by performing the algorithm for String.prototype.toUpperCase using s as the this value.
4. Assert: u is a String value.
5. If u does not consist of a single code unit, return ch.
6. Let cu be u's single code unit element.
7. If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
8. Return cu.

Note 1

Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching internal procedure. To inhibit the capturing behaviour of parentheses, use the form (?: Disjunction ) instead.

Note 2

The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a+))/.exec("baaabac")

matches the empty String immediately after the first b and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a+))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]

Note 3

The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]

Note 4

In case-insignificant matches when Unicode is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, "ß" (U+00DF) to "SS". It may however map a code point outside the Basic Latin range to a character within, for example, "ſ" (U+017F) to "s". Such characters are not mapped if Unicode is false. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.

21.2.2.8.3 Runtime Semantics: UnicodeMatchProperty ( `p` )

The abstract operation UnicodeMatchProperty takes a parameter p that is a List of Unicode code points and performs the following steps:

Assert: p is a List of Unicode code points that is identical to a List of Unicode code points that is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 51 or Table 52.
Let c be the canonical property name of p as given in the “Canonical property name” column of the corresponding row.
Return the List of Unicode code points of c.

Implementations must support the Unicode property names and aliases listed in Table 51 and Table 52. To ensure interoperability, implementations must not support any other property names or aliases.

Note 1

For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.

Note 2

The listed properties form a superset of what UTS18 RL1.2 requires.

Table 51: Non-binary Unicode property aliases and their canonical property names

Property name and aliases	Canonical property name
`General_Category` `gc`	`General_Category`
`Script` `sc`	`Script`
`Script_Extensions` `scx`	`Script_Extensions`

Table 52: Binary Unicode property aliases and their canonical property names

Property name and aliases	Canonical property name
`ASCII`	`ASCII`
`ASCII_Hex_Digit` `AHex`	`ASCII_Hex_Digit`
`Alphabetic` `Alpha`	`Alphabetic`
`Any`	`Any`
`Assigned`	`Assigned`
`Bidi_Control` `Bidi_C`	`Bidi_Control`
`Bidi_Mirrored` `Bidi_M`	`Bidi_Mirrored`
`Case_Ignorable` `CI`	`Case_Ignorable`
`Cased`	`Cased`
`Changes_When_Casefolded` `CWCF`	`Changes_When_Casefolded`
`Changes_When_Casemapped` `CWCM`	`Changes_When_Casemapped`
`Changes_When_Lowercased` `CWL`	`Changes_When_Lowercased`
`Changes_When_NFKC_Casefolded` `CWKCF`	`Changes_When_NFKC_Casefolded`
`Changes_When_Titlecased` `CWT`	`Changes_When_Titlecased`
`Changes_When_Uppercased` `CWU`	`Changes_When_Uppercased`
`Dash`	`Dash`
`Default_Ignorable_Code_Point` `DI`	`Default_Ignorable_Code_Point`
`Deprecated` `Dep`	`Deprecated`
`Diacritic` `Dia`	`Diacritic`
`Emoji`	`Emoji`
`Emoji_Component`	`Emoji_Component`
`Emoji_Modifier`	`Emoji_Modifier`
`Emoji_Modifier_Base`	`Emoji_Modifier_Base`
`Emoji_Presentation`	`Emoji_Presentation`
`Extender` `Ext`	`Extender`
`Grapheme_Base` `Gr_Base`	`Grapheme_Base`
`Grapheme_Extend` `Gr_Ext`	`Grapheme_Extend`
`Hex_Digit` `Hex`	`Hex_Digit`
`IDS_Binary_Operator` `IDSB`	`IDS_Binary_Operator`
`IDS_Trinary_Operator` `IDST`	`IDS_Trinary_Operator`
`ID_Continue` `IDC`	`ID_Continue`
`ID_Start` `IDS`	`ID_Start`
`Ideographic` `Ideo`	`Ideographic`
`Join_Control` `Join_C`	`Join_Control`
`Logical_Order_Exception` `LOE`	`Logical_Order_Exception`
`Lowercase` `Lower`	`Lowercase`
`Math`	`Math`
`Noncharacter_Code_Point` `NChar`	`Noncharacter_Code_Point`
`Pattern_Syntax` `Pat_Syn`	`Pattern_Syntax`
`Pattern_White_Space` `Pat_WS`	`Pattern_White_Space`
`Quotation_Mark` `QMark`	`Quotation_Mark`
`Radical`	`Radical`
`Regional_Indicator` `RI`	`Regional_Indicator`
`Sentence_Terminal` `STerm`	`Sentence_Terminal`
`Soft_Dotted` `SD`	`Soft_Dotted`
`Terminal_Punctuation` `Term`	`Terminal_Punctuation`
`Unified_Ideograph` `UIdeo`	`Unified_Ideograph`
`Uppercase` `Upper`	`Uppercase`
`Variation_Selector` `VS`	`Variation_Selector`
`White_Space` `space`	`White_Space`
`XID_Continue` `XIDC`	`XID_Continue`
`XID_Start` `XIDS`	`XID_Start`

21.2.2.8.4 Runtime Semantics: UnicodeMatchPropertyValue ( `p`, `v` )

The abstract operation UnicodeMatchPropertyValue takes two parameters p and v, each of which is a List of Unicode code points, and performs the following steps:

Assert: p is a List of Unicode code points that is identical to a List of Unicode code points that is a canonical, unaliased Unicode property name listed in the “Canonical property name” column of Table 51.
Assert: v is a List of Unicode code points that is identical to a List of Unicode code points that is a property value or property value alias for Unicode property p listed in the “Property value and aliases” column of Table 53 or Table 54.
Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
Return the List of Unicode code points of value.

Implementations must support the Unicode property value names and aliases listed in Table 53 and Table 54. To ensure interoperability, implementations must not support any other property value names or aliases.

Note 1

For example, Xpeo and Old_Persian are valid Script_Extension values, but xpeo and Old Persian aren't.

Note 2

This algorithm differs from the matching rules for symbolic values listed in UAX44: case, white space, U+002D (HYPHEN-MINUS), and U+005F (LOW LINE) are not ignored, and the Is prefix is not supported.

Table 53: Value aliases and canonical values for the Unicode property General_Category

Property value and aliases	Canonical property value
`Cased_Letter` `LC`	`Cased_Letter`
`Close_Punctuation` `Pe`	`Close_Punctuation`
`Connector_Punctuation` `Pc`	`Connector_Punctuation`
`Control` `Cc` `cntrl`	`Control`
`Currency_Symbol` `Sc`	`Currency_Symbol`
`Dash_Punctuation` `Pd`	`Dash_Punctuation`
`Decimal_Number` `Nd` `digit`	`Decimal_Number`
`Enclosing_Mark` `Me`	`Enclosing_Mark`
`Final_Punctuation` `Pf`	`Final_Punctuation`
`Format` `Cf`	`Format`
`Initial_Punctuation` `Pi`	`Initial_Punctuation`
`Letter` `L`	`Letter`
`Letter_Number` `Nl`	`Letter_Number`
`Line_Separator` `Zl`	`Line_Separator`
`Lowercase_Letter` `Ll`	`Lowercase_Letter`
`Mark` `M` `Combining_Mark`	`Mark`
`Math_Symbol` `Sm`	`Math_Symbol`
`Modifier_Letter` `Lm`	`Modifier_Letter`
`Modifier_Symbol` `Sk`	`Modifier_Symbol`
`Nonspacing_Mark` `Mn`	`Nonspacing_Mark`
`Number` `N`	`Number`
`Open_Punctuation` `Ps`	`Open_Punctuation`
`Other` `C`	`Other`
`Other_Letter` `Lo`	`Other_Letter`
`Other_Number` `No`	`Other_Number`
`Other_Punctuation` `Po`	`Other_Punctuation`
`Other_Symbol` `So`	`Other_Symbol`
`Paragraph_Separator` `Zp`	`Paragraph_Separator`
`Private_Use` `Co`	`Private_Use`
`Punctuation` `P` `punct`	`Punctuation`
`Separator` `Z`	`Separator`
`Space_Separator` `Zs`	`Space_Separator`
`Spacing_Mark` `Mc`	`Spacing_Mark`
`Surrogate` `Cs`	`Surrogate`
`Symbol` `S`	`Symbol`
`Titlecase_Letter` `Lt`	`Titlecase_Letter`
`Unassigned` `Cn`	`Unassigned`
`Uppercase_Letter` `Lu`	`Uppercase_Letter`

Table 54: Value aliases and canonical values for the Unicode properties Script and Script_Extensions

Property value and aliases	Canonical property value
`Adlam` `Adlm`	`Adlam`
`Ahom` `Ahom`	`Ahom`
`Anatolian_Hieroglyphs` `Hluw`	`Anatolian_Hieroglyphs`
`Arabic` `Arab`	`Arabic`
`Armenian` `Armn`	`Armenian`
`Avestan` `Avst`	`Avestan`
`Balinese` `Bali`	`Balinese`
`Bamum` `Bamu`	`Bamum`
`Bassa_Vah` `Bass`	`Bassa_Vah`
`Batak` `Batk`	`Batak`
`Bengali` `Beng`	`Bengali`
`Bhaiksuki` `Bhks`	`Bhaiksuki`
`Bopomofo` `Bopo`	`Bopomofo`
`Brahmi` `Brah`	`Brahmi`
`Braille` `Brai`	`Braille`
`Buginese` `Bugi`	`Buginese`
`Buhid` `Buhd`	`Buhid`
`Canadian_Aboriginal` `Cans`	`Canadian_Aboriginal`
`Carian` `Cari`	`Carian`
`Caucasian_Albanian` `Aghb`	`Caucasian_Albanian`
`Chakma` `Cakm`	`Chakma`
`Cham` `Cham`	`Cham`
`Cherokee` `Cher`	`Cherokee`
`Common` `Zyyy`	`Common`
`Coptic` `Copt` `Qaac`	`Coptic`
`Cuneiform` `Xsux`	`Cuneiform`
`Cypriot` `Cprt`	`Cypriot`
`Cyrillic` `Cyrl`	`Cyrillic`
`Deseret` `Dsrt`	`Deseret`
`Devanagari` `Deva`	`Devanagari`
`Duployan` `Dupl`	`Duployan`
`Egyptian_Hieroglyphs` `Egyp`	`Egyptian_Hieroglyphs`
`Elbasan` `Elba`	`Elbasan`
`Ethiopic` `Ethi`	`Ethiopic`
`Georgian` `Geor`	`Georgian`
`Glagolitic` `Glag`	`Glagolitic`
`Gothic` `Goth`	`Gothic`
`Grantha` `Gran`	`Grantha`
`Greek` `Grek`	`Greek`
`Gujarati` `Gujr`	`Gujarati`
`Gurmukhi` `Guru`	`Gurmukhi`
`Han` `Hani`	`Han`
`Hangul` `Hang`	`Hangul`
`Hanunoo` `Hano`	`Hanunoo`
`Hatran` `Hatr`	`Hatran`
`Hebrew` `Hebr`	`Hebrew`
`Hiragana` `Hira`	`Hiragana`
`Imperial_Aramaic` `Armi`	`Imperial_Aramaic`
`Inherited` `Zinh` `Qaai`	`Inherited`
`Inscriptional_Pahlavi` `Phli`	`Inscriptional_Pahlavi`
`Inscriptional_Parthian` `Prti`	`Inscriptional_Parthian`
`Javanese` `Java`	`Javanese`
`Kaithi` `Kthi`	`Kaithi`
`Kannada` `Knda`	`Kannada`
`Katakana` `Kana`	`Katakana`
`Kayah_Li` `Kali`	`Kayah_Li`
`Kharoshthi` `Khar`	`Kharoshthi`
`Khmer` `Khmr`	`Khmer`
`Khojki` `Khoj`	`Khojki`
`Khudawadi` `Sind`	`Khudawadi`
`Lao` `Laoo`	`Lao`
`Latin` `Latn`	`Latin`
`Lepcha` `Lepc`	`Lepcha`
`Limbu` `Limb`	`Limbu`
`Linear_A` `Lina`	`Linear_A`
`Linear_B` `Linb`	`Linear_B`
`Lisu` `Lisu`	`Lisu`
`Lycian` `Lyci`	`Lycian`
`Lydian` `Lydi`	`Lydian`
`Mahajani` `Mahj`	`Mahajani`
`Malayalam` `Mlym`	`Malayalam`
`Mandaic` `Mand`	`Mandaic`
`Manichaean` `Mani`	`Manichaean`
`Marchen` `Marc`	`Marchen`
`Masaram_Gondi` `Gonm`	`Masaram_Gondi`
`Meetei_Mayek` `Mtei`	`Meetei_Mayek`
`Mende_Kikakui` `Mend`	`Mende_Kikakui`
`Meroitic_Cursive` `Merc`	`Meroitic_Cursive`
`Meroitic_Hieroglyphs` `Mero`	`Meroitic_Hieroglyphs`
`Miao` `Plrd`	`Miao`
`Modi` `Modi`	`Modi`
`Mongolian` `Mong`	`Mongolian`
`Mro` `Mroo`	`Mro`
`Multani` `Mult`	`Multani`
`Myanmar` `Mymr`	`Myanmar`
`Nabataean` `Nbat`	`Nabataean`
`New_Tai_Lue` `Talu`	`New_Tai_Lue`
`Newa` `Newa`	`Newa`
`Nko` `Nkoo`	`Nko`
`Nushu` `Nshu`	`Nushu`
`Ogham` `Ogam`	`Ogham`
`Ol_Chiki` `Olck`	`Ol_Chiki`
`Old_Hungarian` `Hung`	`Old_Hungarian`
`Old_Italic` `Ital`	`Old_Italic`
`Old_North_Arabian` `Narb`	`Old_North_Arabian`
`Old_Permic` `Perm`	`Old_Permic`
`Old_Persian` `Xpeo`	`Old_Persian`
`Old_South_Arabian` `Sarb`	`Old_South_Arabian`
`Old_Turkic` `Orkh`	`Old_Turkic`
`Oriya` `Orya`	`Oriya`
`Osage` `Osge`	`Osage`
`Osmanya` `Osma`	`Osmanya`
`Pahawh_Hmong` `Hmng`	`Pahawh_Hmong`
`Palmyrene` `Palm`	`Palmyrene`
`Pau_Cin_Hau` `Pauc`	`Pau_Cin_Hau`
`Phags_Pa` `Phag`	`Phags_Pa`
`Phoenician` `Phnx`	`Phoenician`
`Psalter_Pahlavi` `Phlp`	`Psalter_Pahlavi`
`Rejang` `Rjng`	`Rejang`
`Runic` `Runr`	`Runic`
`Samaritan` `Samr`	`Samaritan`
`Saurashtra` `Saur`	`Saurashtra`
`Sharada` `Shrd`	`Sharada`
`Shavian` `Shaw`	`Shavian`
`Siddham` `Sidd`	`Siddham`
`SignWriting` `Sgnw`	`SignWriting`
`Sinhala` `Sinh`	`Sinhala`
`Sora_Sompeng` `Sora`	`Sora_Sompeng`
`Soyombo` `Soyo`	`Soyombo`
`Sundanese` `Sund`	`Sundanese`
`Syloti_Nagri` `Sylo`	`Syloti_Nagri`
`Syriac` `Syrc`	`Syriac`
`Tagalog` `Tglg`	`Tagalog`
`Tagbanwa` `Tagb`	`Tagbanwa`
`Tai_Le` `Tale`	`Tai_Le`
`Tai_Tham` `Lana`	`Tai_Tham`
`Tai_Viet` `Tavt`	`Tai_Viet`
`Takri` `Takr`	`Takri`
`Tamil` `Taml`	`Tamil`
`Tangut` `Tang`	`Tangut`
`Telugu` `Telu`	`Telugu`
`Thaana` `Thaa`	`Thaana`
`Thai` `Thai`	`Thai`
`Tibetan` `Tibt`	`Tibetan`
`Tifinagh` `Tfng`	`Tifinagh`
`Tirhuta` `Tirh`	`Tirhuta`
`Ugaritic` `Ugar`	`Ugaritic`
`Vai` `Vaii`	`Vai`
`Warang_Citi` `Wara`	`Warang_Citi`
`Yi` `Yiii`	`Yi`
`Zanabazar_Square` `Zanb`	`Zanabazar_Square`

21.2.2.9 AtomEscape

With parameter direction.

The production AtomEscape::DecimalEscape evaluates as follows:

Evaluate DecimalEscape to obtain an integer n.
Assert: n ≤ NcapturingParens.
Call BackreferenceMatcher(n, direction) and return its Matcher result.

The production AtomEscape::CharacterEscape evaluates as follows:

Evaluate CharacterEscape to obtain a character ch.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

The production AtomEscape::CharacterClassEscape evaluates as follows:

Evaluate CharacterClassEscape to obtain a CharSet A.
Call CharacterSetMatcher(A, false, direction) and return its Matcher result.

Note

An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the nth set of capturing parentheses (21.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured anything, then the backreference always succeeds.

The production AtomEscape::kGroupName evaluates as follows:

Search the enclosing Pattern for an instance of a GroupSpecifier for a RegExpIdentifierName which has a StringValue equal to the StringValue of the RegExpIdentifierName contained in GroupName.
Assert: A unique such GroupSpecifier is found.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of the located GroupSpecifier. This is the total number of Atom::(GroupSpecifierDisjunction) Parse Nodes prior to or enclosing the located GroupSpecifier.
Call BackreferenceMatcher(parenIndex, direction) and return its Matcher result.

21.2.2.9.1 Runtime Semantics: BackreferenceMatcher ( `n`, `direction` )

The abstract operation BackreferenceMatcher takes two arguments, an integer n and an integer direction, and performs the following steps:

Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
1. Let cap be x's captures List.
2. Let s be cap[n].
3. If s is undefined, return c(x).
4. Let e be x's endIndex.
5. Let len be the number of elements in s.
6. Let f be e + direction × len.
7. If f < 0 or f > InputLength, return failure.
8. Let g be min(e, f).
9. If there exists an integer i between 0 (inclusive) and len (exclusive) such that Canonicalize(s[i]) is not the same character value as Canonicalize(Input[g + i]), return failure.
10. Let y be the State (f, cap).
11. Call c(y) and return its result.

21.2.2.10 CharacterEscape

The CharacterEscape productions evaluate as follows:

ControlEscape

ControlLetter

[lookahead ∉ DecimalDigit]

IdentityEscape

Let cv be the CharacterValue of this CharacterEscape.
Return the character whose character value is cv.

21.2.2.11 DecimalEscape

The DecimalEscape productions evaluate as follows:

opt

Return the CapturingGroupNumber of this DecimalEscape.

Note

If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left-capturing parentheses in the entire regular expression.

21.2.2.12 CharacterClassEscape

The production CharacterClassEscape::d evaluates as follows:

Return the ten-element set of characters containing the characters 0 through 9 inclusive.

The production CharacterClassEscape::D evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::d .

The production CharacterClassEscape::s evaluates as follows:

Return the set of characters containing the characters that are on the right-hand side of the WhiteSpace or LineTerminator productions.

The production CharacterClassEscape::S evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::s .

The production CharacterClassEscape::w evaluates as follows:

Return the set of all characters returned by WordCharacters().

The production CharacterClassEscape::W evaluates as follows:

Return the set of all characters not included in the set returned by CharacterClassEscape::w .

The production CharacterClassEscape::p{UnicodePropertyValueExpression} evaluates by returning the CharSet containing all Unicode code points included in the CharSet returned by UnicodePropertyValueExpression.

The production CharacterClassEscape::P{UnicodePropertyValueExpression} evaluates by returning the CharSet containing all Unicode code points not included in the CharSet returned by UnicodePropertyValueExpression.

The production UnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValue evaluates as follows:

Let ps be SourceText of UnicodePropertyName.
Let p be ! UnicodeMatchProperty(ps).
Assert: p is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 51.
Let vs be SourceText of UnicodePropertyValue.
Let v be ! UnicodeMatchPropertyValue(p, vs).
Return the CharSet containing all Unicode code points whose character database definition includes the property p with value v.

The production UnicodePropertyValueExpression::LoneUnicodePropertyNameOrValue evaluates as follows:

Let s be SourceText of LoneUnicodePropertyNameOrValue.
If ! UnicodeMatchPropertyValue("General_Category", s) is identical to a List of Unicode code points that is the name of a Unicode general category or general category alias listed in the “Property value and aliases” column of Table 53, then
1. Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
Let p be ! UnicodeMatchProperty(s).
Assert: p is a binary Unicode property or binary property alias listed in the “Property name and aliases” column of Table 52.
Return the CharSet containing all Unicode code points whose character database definition includes the property p with value “True”.

21.2.2.13 CharacterClass

The production CharacterClass::[ClassRanges] evaluates as follows:

Evaluate ClassRanges to obtain a CharSet A.
Return the two results A and false.

The production CharacterClass::[^ClassRanges] evaluates as follows:

Evaluate ClassRanges to obtain a CharSet A.
Return the two results A and true.

21.2.2.14 ClassRanges

The production ClassRanges::[empty] evaluates as follows:

Return the empty CharSet.

The production ClassRanges::NonemptyClassRanges evaluates as follows:

Return the CharSet that is the result of evaluating NonemptyClassRanges.

21.2.2.15 NonemptyClassRanges

The production NonemptyClassRanges::ClassAtom evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtom.

The production NonemptyClassRanges::ClassAtomNonemptyClassRangesNoDash evaluates as follows:

Evaluate ClassAtom to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRanges::ClassAtom-ClassAtomClassRanges evaluates as follows:

Evaluate the first ClassAtom to obtain a CharSet A.
Evaluate the second ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

21.2.2.15.1 Runtime Semantics: CharacterRange ( `A`, `B` )

The abstract operation CharacterRange takes two CharSet parameters A and B and performs the following steps:

Assert: A and B each contain exactly one character.
Let a be the one character in CharSet A.
Let b be the one character in CharSet B.
Let i be the character value of character a.
Let j be the character value of character b.
Assert: i ≤ j.
Return the set containing all characters numbered i through j, inclusive.

21.2.2.16 NonemptyClassRangesNoDash

The production NonemptyClassRangesNoDash::ClassAtom evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtom.

The production NonemptyClassRangesNoDash::ClassAtomNoDashNonemptyClassRangesNoDash evaluates as follows:

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRanges evaluates as follows:

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

Note 1

ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is greater than the second ClassAtom's character value.

Note 2

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.

Note 3

A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

21.2.2.17 ClassAtom

The production ClassAtom::- evaluates as follows:

Return the CharSet containing the single character - U+002D (HYPHEN-MINUS).

The production ClassAtom::ClassAtomNoDash evaluates as follows:

Return the CharSet that is the result of evaluating ClassAtomNoDash.

21.2.2.18 ClassAtomNoDash

The production ClassAtomNoDash::SourceCharacterbut not one of \ or ] or - evaluates as follows:

Return the CharSet containing the character matched by SourceCharacter.

The production ClassAtomNoDash::\ClassEscape evaluates as follows:

Return the CharSet that is the result of evaluating ClassEscape.

21.2.2.19 ClassEscape

The ClassEscape productions evaluate as follows:

Let cv be the CharacterValue of this ClassEscape.
Let c be the character whose character value is cv.
Return the CharSet containing the single character c.