5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 7. This grammar has as its terminal symbols characters (Unicode code units) that conform to the rules for SourceCharacter defined in Clause 6. It defines a set of productions, starting from the goal symbol InputElementDiv or InputElementRegExp, that describe how sequences of such characters are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (7.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form “/*…*/” regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 15.10. This grammar also has as its terminal symbols the characters as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of characters are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This grammar appears in 9.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from the goal symbol Script, that describe how sequences of tokens can form syntactically correct independent components of an ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript script, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The script is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal Script, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which token sequences are accepted as correct ECMAScript scripts. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a terminator character appears in certain “awkward” places.

In certain cases in order to avoid ambiguities the syntactic grammar uses generalize productions that permit token sequences that are not valid ECMAScript scripts. For example, this technique is used in with object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. In certain contexts, when explicitly specific, the input elements corresponding to such a production are parsed again using a goal symbol of a supplemental grammar. The script is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the supplemental goal symbol, with no tokens left over.

5.1.5 The JSON Grammar

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects. The JSON grammar is given in 15.12.1.

The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON lexical grammar is used to translate character sequences into tokens and is similar to parts of the ECMAScript lexical grammar. The JSON syntactic grammar describes how sequences of tokens from the JSON lexical grammar can form syntactically correct JSON object descriptions.

Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The JSON syntactic grammar is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON syntactic grammar are distinguished by using one colon “:” as separating punctuation.

5.1.6 Grammar Notation

Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal symbols of the other grammars, are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script either exactly as written or using equalvant Unicode escape sequences (see clause 6). All terminal symbol characters specified in this way are to be understood as the appropriate Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement :

while ( Expression ) Statement

states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

ArgumentList :

AssignmentExpression

ArgumentList , AssignmentExpression

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix “_opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration :

Identifier Initialiser_opt

is a convenient abbreviation for:

VariableDeclaration :

Identifier

Identifier Initialiser

and that:

IterationStatement :

for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

is a convenient abbreviation for:

IterationStatement :

for ( ; Expression_opt ; Expression_opt ) Statement

for ( ExpressionNoIn ; Expression_opt ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; Expression_opt ) Statement

for ( ; Expression ; Expression_opt ) Statement

for ( ExpressionNoIn ; ; Expression_opt ) Statement

for ( ExpressionNoIn ; Expression ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; ) Statement

for ( ; ; Expression ) Statement

for ( ; Expression ; ) Statement

for ( ; Expression ; Expression ) Statement

for ( ExpressionNoIn ; ; ) Statement

for ( ExpressionNoIn ; ; Expression ) Statement

for ( ExpressionNoIn ; Expression ; ) Statement

for ( ExpressionNoIn ; Expression ; Expression ) Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit ::

1

2

3

4

5

6

7

8

9

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token is a member of the given set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. For example, given the definitions

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

DecimalDigits ::

DecimalDigit

DecimalDigits DecimalDigit

the definition

LookaheadExample ::

n [lookahead ∉ {1, 3, 5, 7, 9}] DecimalDigits

DecimalDigit [lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

ThrowStatement :

throw [no LineTerminator here] Expression ;

indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

The lexical grammar has multiple goal symbols and the appropriate goal symbol to use depends upon the syntactic grammar context. If a phrase of the form “[Lexical goal LexicalGoalSymbol]” appears on the right-hand-side of a syntactic production then the next token must be lexically recognized using the indicated goal symbol. In the absence of such a phrase the default lexical goal symbol is used.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character token, it represents the sequence of characters that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier ::

IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of characters that could replace IdentifierName provided that the same sequence of characters could not replace ReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

SourceCharacter ::

any Unicode character

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

Algorithms may be explicitly parameterised, in which case the names and usage of the parameters must be provided as part of the algorithm’s definition. In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterised functional form so that they may be referenced by name from within other algorithms.

Algorithms may be associated with productions of one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and non-terminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, non-terminal symbols refer to the actual alternative definition that is matched when parsing the script souce code.

Unless explicitly specified otherwise, all chain productions have an implicit associated definition for every algorithm that is might be applied to that production’s left-hand side nonterminal. The implicit simply reapplies the same algorithm name with the same parameters, if any, to the chain production’s sole right-hand side nonterminal and then result. For example, assume there is a production

Block :

{ StatementList }

but there is no evalution algorithm that is explicitly specified for that production. If in some algorithm there is a statement of the form: “Return the result of evaluating Block” it is implicit that the algorithm has an evalution algorithm of the form:

Runtime Semantics: Evaluation

Block : { StatementList }

Return the result of evaluating StatementList

For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

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

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step may assert an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is +0 or −0 then the corresponding mathematical value is simply 0.

The mathematical function abs(x) yields the absolute value of x, which is −x if x is negative (less than zero) and otherwise is x itself.

The mathematical function sign(x) yields 1 if x is positive and −1 if x is negative. The sign function is not used in this standard for cases when x is zero.

The mathematical function min(x₁, x₂, ..., x_n) yields the mathematically smallest of x₁ through x_n.

The notation “x modulo y” (y must be finite and nonzero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x−k = q × y for some integer q.

The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is not larger than x.

NOTE floor(x) = x−(x modulo 1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result is returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the exception, using terminology such as “If an exception was thrown…”. Once such an algorithm step has been encountered the exception is no longer considered to have occurred.

5.3 Static Semantic Rules

Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements make up a valid ECMAScript script that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.

Unless otherwise specified every grammar production alternative in this specification implicitly has a definition for a static semantic rule named Contains which takes an argument named symbol whose value is a terminal or non-terminal of the grammar that includes the associated production. The default definition of Contains is:

For each terminal and non-terminal grammar symbol, sym, in the definition of this production do
1. If sym is the same grammar symbol as symbol, return true.
2. If sym is a non-terminal, then
  1. Let contained be the result of Contains for sym with argument symbol.
  2. If contained is true, return true.
Return false.

The above definition is explicitly over-ridden for specific productions.

A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 16) that are associate with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A comforming implementation must, prior to the first evaluation of a Script, validate all of the early error rules of the productions used to parse that Script. If any of the early error rules are violated the Script is invalid and can not be evaluated.

7 Lexical Conventions

The source text of an ECMAScript script 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 characters 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 InputElementDiv goal symbol is the default goal symbol and is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted. The InputElementTemplateTail goal is used in syntactic grammar contexts where a TemplateLiteral logically continues after a substitution element.

NOTE 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 7.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment character after a LineTerminator is slash (/) 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

InputElementDiv ::

WhiteSpace

LineTerminator

Comment

Token

DivPunctuator

RightBracePunctuator

InputElementRegExp ::

WhiteSpace

LineTerminator

Comment

Token

RightBracePunctuator

RegularExpressionLiteral

InputElementTemplateTail ::

WhiteSpace

LineTerminator

Comment

Token

DivPunctuator

TemplateSubstitutionTail

7.1 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.

<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text, <ZWNJ> and <ZWJ> may also be used in an identifier after the first character.

<BOM> 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. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarised in Table 1.

Table 1 — Format-Control Character Usage

Code Point	Name	Formal Name	Usage
`U+200C`	Zero width non-joiner	<ZWNJ>	IdentifierPart
`U+200D`	Zero width joiner	<ZWJ>	IdentifierPart
`U+FEFF`	Byte Order Mark	<BOM>	Whitespace

7.2 White Space

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end of input. White space characters may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant characters 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 characters are listed in Table 2.

Table 2 — Whitespace Characters

Code Point	Name	Formal Name
`U+0009`	Tab	<TAB>
`U+000B`	Vertical Tab	<VT>
`U+000C`	Form Feed	<FF>
`U+0020`	Space	<SP>
`U+00A0`	No-break space	<NBSP>
`U+FEFF`	Byte Order Mark	<BOM>
Other category “Zs”	Any other Unicode “space separator”	<USP>

ECMAScript implementations must recognise all of the white space characters defined in Unicode 5.1. Later editions of the Unicode Standard may define other white space characters. ECMAScript implementations may recognise white space characters from later editions of the Unicode Standard.

Syntax

WhiteSpace ::

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<BOM>

<USP>

7.3 Line Terminators

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, 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 (7.9). 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 (7.4) but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space characters that are matched by the \s class in regular expressions.

The ECMAScript line terminator characters are listed in Table 3.

Table 3 — Line Terminator Characters

Code Point	Name	Formal Name
`U+000A`	Line Feed	<LF>
`U+000D`	Carriage Return	<CR>
`U+2028`	Line separator	<LS>
`U+2029`	Paragraph separator	<PS>

Only the Unicode characters in Table 3 are treated as line terminators. Other new line or line breaking Unicode characters are treated as white space but not as line terminators. 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>

7.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode character except a LineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters 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 recognised 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 7.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator character, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment ::

MultiLineComment

SingleLineComment

MultiLineComment ::

/* MultiLineCommentChars_opt */

MultiLineCommentChars ::

MultiLineNotAsteriskChar MultiLineCommentChars_opt

* PostAsteriskCommentChars_opt

PostAsteriskCommentChars ::

MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentChars_opt

* PostAsteriskCommentChars_opt

MultiLineNotAsteriskChar ::

SourceCharacter but not *

MultiLineNotForwardSlashOrAsteriskChar ::

SourceCharacter but not one of / or *

SingleLineComment ::

// SingleLineCommentChars_opt

SingleLineCommentChars ::

SingleLineCommentChar SingleLineCommentChars_opt

SingleLineCommentChar ::

SourceCharacter but not LineTerminator

7.5 Tokens

Syntax

Token ::

IdentifierName

Punctuator

NumericLiteral

StringLiteral

Template

NOTE The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions define tokens, but are not included in the Token production.

7.6 Identifier Names and Identifiers

IdentifierName, Identifier, 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 is an enumerated subset of IdentifierName and Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1). The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode characters in the specified categories in version 5.1.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognise identifier characters defined in later editions of the Unicode Standard.

NOTE 1 This standard specifies specific character additions: The dollar sign (U+0024) and the underscore (U+005f) are permitted anywhere in an IdentifierName, and the characters zero width non-joiner (U+200C) and zero width joiner (U+200D) are permitted anywhere after the first character of an IdentifierName.

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode character to the IdentifierName. The code point of the contributed character is expressed by the HexDigits of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } characters, if they appear, do not contribute characters to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the Unicode character it constributes, the result must still be a valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual characters regardless of whether or not an escape sequence was used to contribute any particular characters.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values).

NOTE 2 If maximal portability is a concern, programmers should only employ the identifier characters that were defined in Unicode 3.0.

Syntax

Identifier ::

IdentifierName but not ReservedWord

IdentifierName ::

IdentifierStart

IdentifierName IdentifierPart

IdentifierStart ::

UnicodeIDStart

$

_

\ UnicodeEscapeSequence

IdentifierPart ::

UnicodeIDContinue

$

_

\ UnicodeEscapeSequence

<ZWNJ>

<ZWJ>

UnicodeIDStart ::

any Unicode character with the Unicode property “ID_Start”.

UnicodeIDContinue ::

any Unicode character with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 7.8.4

Static Semantics: StringValue

Identifier :: IdentifierName but not ReservedWord

Return the StringValue of IdentifierName.

IdentifierName ::

IdentifierStart

IdentifierName IdentifierPart

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 UTF-16 Encoding (clause 6) each code point.

7.6.1 Reserved Words

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax

ReservedWord ::

Keyword

FutureReservedWord

NullLiteral

BooleanLiteral

The ReservedWord definitions are specified as literal sequences of Unicode characters. However, any Unicode character in a ReservedWord can also be expressed by a \ UnicodeEscapeSequence that expresses that same Unicode character’s code point. Use of such escape sequences does not change the meaning of the ReservedWord.

7.6.1.1 Keywords

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.

Syntax

Keyword :: one of

break	delete	import	this
case	do	in	throw
catch	else	instanceof	try
class	export	let	typeof
continue	finally	new	var
const	for	return	void
debugger	function	super	while
default	if	switch	with

7.6.1.2 Future Reserved Words

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.

Syntax

FutureReservedWord :: one of

	enum	extends

The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:

implements		private	public	yield
interface	package	protected	static

7.7 Punctuators

Syntax

Punctuator :: one of

{	(	)	[	]	.
...	;	,	<	>	<=
>=	==	!=	===	!==
+	-	*	%	++	--
<<	>>	>>>	&	\|	^
!	~	&&	\|\|	?	:
=	+=	-=	*=	%=	<<=
>>=	>>>=	&=	\|=	^=	=>

DivPunctuator :: one of

RightBracePunctuator ::

}

7.8 Literals

7.8.1 Null Literals

Syntax

NullLiteral ::

null

7.8.2 Boolean Literals

Syntax

BooleanLiteral ::

true

false

7.8.3 Numeric Literals

Syntax

NumericLiteral ::

DecimalLiteral

BinaryIntegerLiteral

OctalIntegerLiteral

HexIntegerLiteral

DecimalLiteral ::

DecimalIntegerLiteral . DecimalDigits_opt ExponentPart_opt

. DecimalDigits ExponentPart_opt

DecimalIntegerLiteral ExponentPart_opt

DecimalIntegerLiteral ::

0

NonZeroDigit DecimalDigits_opt

DecimalDigits ::

DecimalDigit

DecimalDigits DecimalDigit

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

ExponentPart ::

ExponentIndicator SignedInteger

ExponentIndicator :: one of

e E

SignedInteger ::

DecimalDigits

+ DecimalDigits

- DecimalDigits

BinaryIntegerLiteral ::

0b BinaryDigit

0B BinaryDigit

BinaryIntegerLiteral BinaryDigit

BinaryDigit :: one of

0 1

OctalIntegerLiteral ::

0o OctalDigit

0O OctalDigit

OctalIntegerLiteral OctalDigit

OctalDigit :: one of

0 1 2 3 4 5 6 7

HexIntegerLiteral ::

0x HexDigits

0X HexDigits

HexDigits ::

HexDigit

HexDigits HexDigit

HexDigit :: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

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 (see 10.1.1), must not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

Static Semantics: MV’s

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 times 10^–n), where n is the number of characters in DecimalDigits.
The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral times 10^e, where e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10^–n)) times 10^e, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits times 10^–n, where n is the number of characters in DecimalDigits.
The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10^e–n, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral times 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 :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times 10ⁿ) plus the MV of DecimalDigits, where n is the number of characters in DecimalDigits.
The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.
The MV of ExponentPart :: ExponentIndicator SignedInteger 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 :: 0b BinaryDigit is the MV of BinaryDigit.
The MV of BinaryIntegerLiteral :: 0B BinaryDigit is the MV of BinaryDigit.
The MV of BinaryIntegerLiteral :: BinaryIntegerLiteral BinaryDigit is (the MV of BinaryIntegerLiteral times 2) plus the MV of BinaryDigit.
The MV of OctalIntegerLiteral :: 0o OctalDigit is the MV of OctalDigit.
The MV of OctalIntegerLiteral :: 0O OctalDigit is the MV of OctalDigit.
The MV of OctalIntegerLiteral :: OctalIntegerLiteral OctalDigit is (the MV of OctalIntegerLiteral times 8) plus the MV of OctalDigit.
The MV of HexIntegerLiteral :: 0x HexDigits is the MV of HexDigits.
The MV of HexIntegerLiteral :: 0X HexDigits is the MV of HexDigits.
The MV of HexDigits :: HexDigit is the MV of HexDigit.
The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits times 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 8.5), 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.

∉

7.8.5 Regular Expression Literals

NOTE A regular expression literal is an input element that is converted to a RegExp object (see 15.10) 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 (see 15.10.4) or calling the RegExp constructor as a function (15.10.3).

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 code comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed using the more stringent ECMAScript Regular Expression grammar (15.10.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 15.10.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral ::

/ RegularExpressionBody / RegularExpressionFlags

RegularExpressionBody ::

RegularExpressionFirstChar RegularExpressionChars

RegularExpressionChars ::

[empty]

RegularExpressionChars RegularExpressionChar

RegularExpressionFirstChar ::

RegularExpressionNonTerminator but not one of * or \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionChar ::

RegularExpressionNonTerminator but not one of \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionBackslashSequence ::

\ RegularExpressionNonTerminator

RegularExpressionNonTerminator ::

SourceCharacter but not LineTerminator

RegularExpressionClass ::

[ RegularExpressionClassChars ]

RegularExpressionClassChars ::

[empty]

RegularExpressionClassChars RegularExpressionClassChar

RegularExpressionClassChar ::

RegularExpressionNonTerminator but not one of ] or \

RegularExpressionBackslashSequence

RegularExpressionFlags ::

[empty]

RegularExpressionFlags IdentifierPart

NOTE Regular expression literals may not be empty; instead of representing an empty regular expression literal, the characters // start a single-line comment. To specify an empty regular expression, use: /(?:)/.

Static Semantics: BodyText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags

Return the source code that was recognized as RegularExpressionBody.

Static Semantics: FlagText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags

Return the source code that was recognized as RegularExpressionFlags.

7.8.6 Template Literal Lexical Components

Syntax

Template ::

NoSubstitutionTemplate

TemplateHead

NoSubstitutionTemplate ::

` TemplateCharacters_opt `

TemplateHead ::

` TemplateCharacters_opt ${

TemplateSubstitutionTail ::

TemplateMiddle

TemplateTail

TemplateMiddle ::

} TemplateCharacters_opt ${

TemplateTail ::

} TemplateCharacters_opt `

TemplateCharacters ::

TemplateCharacter TemplateCharacters_opt

TemplateCharacter ::

SourceCharacter but not one of ` or \ or $

$ [lookahead ∉ { ]

\ EscapeSequence

LineContinuation

Static Semantics: TV’s and TRV’s

A template literal component is interpreted as a sequence of Unicode characters. The Template Value (TV) of a literal component is described in terms of code unit values (CV, 7.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode characters within the template component are interpreted as having a mathematical value (MV, 7.8.3). In determining a TV, escape sequences are replaced by the code unit of the Unicode characters 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 :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TV of TemplateCharacter followed by all the code units in the TV of TemplateCharacters in order.
The TV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ is the UTF-16 Encoding (clause 6) of the code point value of SourceCharacter.
The TV of TemplateCharacter :: $ [lookahead ∉ { ] is the code unit value 0x0024.
The TV of TemplateCharacter :: \ EscapeSequence is the CV of EscapeSequence.
The TV of TemplateCharacter :: LineContinuation is the TV of LineContinuation.
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 :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TRV of TemplateCharacter followed by all the code units in the TRV of TemplateCharacters, in order.
The TRV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ is the UTF-16 Encoding (clause 6) of the code point value of SourceCharacter.
The TRV of TemplateCharacter :: $ [lookahead ∉ { ] is the code unit value 0x0024.
The TRV of TemplateCharacter :: \ EscapeSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of EscapeSequence.
The TRV of TemplateCharacter :: LineContinuation is the TRV of LineContinuation.
The TRV of EscapeSequence :: CharacterEscapeSequence is the TRV of the CharacterEscapeSequence.
The TRV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is the code unit value 0x0030.
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 CharacterEscapeSequence :: SingleEscapeCharacter is the TRV of the SingleEscapeCharacter.
The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.
The TRV of SingleEscapeCharacter :: one of ' " \ b f n r t v is the CV of the SourceCharacter that is that single character.
The TRV of HexEscapeSequence :: x HexDigit HexDigit is the sequence consisting of code unit value 0x0078 followed by TRV of the first HexDigit followed by the TRV of the second HexDigit.
The TRV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the sequence consisting of code unit value 0x0075 followed by TRV of the first HexDigit followed by the TRV of the second HexDigit followed by TRV of the third HexDigit followed by the TRV of the fourth HexDigit.
The TRV of UnicodeEscapeSequence :: u{ HexDigits } is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit value 0x007D.
The TRV of HexDigits :: HexDigit is the TRV of HexDigit.
The TRV of HexDigits :: HexDigits HexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.
The TRV of a HexDigit is the CV of the SourceCharacter that is that HexDigit.
The TRV of LineContinuation :: \ LineTerminatorSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of LineTerminatorSequence.
The TRV of LineTerminatorSequence :: <LF> is the code unit value 0x000A.
The TRV of LineTerminatorSequence :: <CR> [lookahead ∉ <LF> ] is the code unit value 0x000D.
The TRV of LineTerminatorSequence :: <LS> is the code unit value 0x2028.
The TRV of LineTerminatorSequence :: <PS> is the code unit value 0x2029.
The TRV of LineTerminatorSequence :: <CR><LF> is the sequence consisting of the code unit value 0x000D followed by the code unit value 0x000A.

NOTE TV excludes the code units of LineContinuation while TRV includes them.

7.8.4 String Literals

NOTE A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be be represented by an escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash, carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape sequence. String literals evaluate to ECAMScript String values. When generating these string values Unicode code points are UTF-16 encoded as defined in clause 6. Code points belonging to 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

StringLiteral ::

" DoubleStringCharacters_opt "

' SingleStringCharacters_opt '

DoubleStringCharacters ::

DoubleStringCharacter DoubleStringCharacters_opt

SingleStringCharacters ::

SingleStringCharacter SingleStringCharacters_opt

DoubleStringCharacter ::

SourceCharacter but not one of " or \ or LineTerminator

\ EscapeSequence

LineContinuation

SingleStringCharacter ::

SourceCharacter but not one of ' or \ or LineTerminator

\ EscapeSequence

LineContinuation

LineContinuation ::

\ LineTerminatorSequence

EscapeSequence ::

CharacterEscapeSequence

0 [lookahead ∉ DecimalDigit]

HexEscapeSequence

UnicodeEscapeSequence

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

CharacterEscapeSequence ::

SingleEscapeCharacter

NonEscapeCharacter

SingleEscapeCharacter :: one of

' " \ b f n r t v

NonEscapeCharacter ::

SourceCharacter but not one of EscapeCharacter or LineTerminator

EscapeCharacter ::

SingleEscapeCharacter

DecimalDigit

x

u

HexEscapeSequence ::

x HexDigit HexDigit

UnicodeEscapeSequence ::

u HexDigit HexDigit HexDigit HexDigit

u{ HexDigits }

The definition of the nonterminal HexDigit is given in 7.8.3. SourceCharacter is defined in clause 6.

NOTE A line terminator character cannot appear in a string literal, except as part of a LineContinuation to produce the empty character sequence. The correct way to cause a line terminator character to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.

Static Semantics

Static Semantics: Early Errors

UnicodeEscapeSequence :: u{ HexDigits }

It is a Syntax Error if the MV of HexDigits > 1114111.

Static Semantics: SV’s and CV’s

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 (CV) contributed by the various parts of the string literal. As part of this process, some Unicode characters within the string literal are interpreted as having a mathematical value (MV), as described below or in 7.8.3.

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 one or two code units that is the CV of DoubleStringCharacter.
The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of one or two code units that is the CV of DoubleStringCharacter followed by all the code units in the SV of DoubleStringCharacters in order.
The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one or two code units that is the CV of SingleStringCharacter.
The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of one or two code units that is the CV of SingleStringCharacter followed by all the code units in the SV of SingleStringCharacters in order.
The SV of LineContinuation :: \ LineTerminatorSequence is the empty code unit sequence.
The CV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator is the UTF-16 Encoding (clause 6) of the code point value of SourceCharacter.
The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.
The CV of DoubleStringCharacter :: LineContinuation is the empty character sequence.
The CV of SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator is the UTF-16 Encoding (clause 6) of the code point value of SourceCharacter .
The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.
The CV of SingleStringCharacter :: LineContinuation is the empty character sequence.
The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.
The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is the code unit value 0.
The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.
The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.
The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the character whose code unit value is determined by the SingleEscapeCharacter according to { REF _Ref327353613 \h \* MERGEFORMAT }:

Table 4 — String Single Character Escape Sequences

Escape Sequence	Code Unit Value	Name	Symbol
`\b`	`0x0008`	backspace	<BS>
`\t`	`0x0009`	horizontal tab	<HT>
`\n`	`0x000A`	line feed (new line)	<LF>
`\v`	`0x000B`	vertical tab	<VT>
`\f`	`0x000C`	form feed	<FF>
`\r`	`0x000D`	carriage return	<CR>
`\"`	`0x0022`	double quote	`"`
`\'`	`0x0027`	single quote	`'`
`\\`	`0x005C`	backslash	`\`

The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.
The CV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the UTF-16 Encoding (clause 6) of the code point value of SourceCharacter .
The CV of HexEscapeSequence :: x HexDigit HexDigit is the code unit value that is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.
The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the code unit value that is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.
The CV of UnicodeEscapeSequence :: u{ HexDigits } the is the UTF-16 Encoding (clause 6) of the MV of HexDigits.

7.9 Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-while statement, continue statement, break statement, return statement, and throw statement) must be terminated with semicolons. 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.

7.9.1 Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

When, as the script 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 }.
When, as the script 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 complete ECMAScript script, then a semicolon is automatically inserted at the end of the input stream.
When, as the script 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 12.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression :

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

ContinueStatement :

continue [no LineTerminator here] Identifier ;

BreakStatement :

break [no LineTerminator here] Identifier ;

ReturnStatement :

return [no LineTerminator here] Expression ;

ThrowStatement :

throw [no LineTerminator here] Expression ;

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, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw 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 should start on the same line as the return or throw token.

An Identifier in a break or continue statement should be on the same line as the break or continue token.

7.9.2 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 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 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 parenthesised 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.

8 Types

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.

8.1 ECMAScript Language Types

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.

8.1.1 The Undefined Type

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.

8.1.2 The Null Type

The Null type has exactly one value, called null.

8.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, called true and false.

8.1.4 The String Type

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values (“elements”). The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

Where ECMAScript operations interpret String values, each element is interpreted as a single UTF-16 code unit. However, ECMAScript does not place any restrictions or requirements on the sequence of code units in a String value, so they may be ill-formed when interpreted as UTF-16 code unit sequences. Operations that do not interpret String contents treat them as sequences of undifferentiated 16-bit unsigned integers. No operations ensure that Strings are in a normalized form. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source code is in Normalised Form C, string literals are guaranteed to also be normalised, as long as they do not contain any Unicode escape sequences.

Some operations interpret String contents as UTF-16 encoded Unicode code points. In that case the interpretation is:

A code unit in the range 0 to 0xD7FF or in the range 0xE000 to 0xFFFF is interpreted as a code point with the same value.
A sequence of two code units, where the first code unit c1 is in the range 0xD800 to 0xDBFF and the second code unit c2 is in the range 0xDC00 to 0xDFFF, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 – 0xDC00) + 0x10000.
A code unit that is in the range 0xD800 to 0xDFFF, but is not part of a surrogate pair, is interpreted as a code point with the same value.

8.1.5 The Number Type

The Number type has exactly 18437736874454810627 (that is, 2⁶⁴−2⁵³+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 2⁵³−2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞ and −∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18437736874454810624 (that is, 2⁶⁴−2⁵³) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0 and −0, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18437736874454810622 (that is, 2⁶⁴−2⁵³−2) finite nonzero values are of two kinds:

18428729675200069632 (that is, 2⁶⁴−2⁵⁴) of them are normalised, having the form

s \times m \times 2 e

where s is +1 or −1, m is a positive integer less than 2⁵³ but not less than 2⁵², and e is an integer ranging from −1074 to 971, inclusive.

The remaining 9007199254740990 (that is, 2⁵³−2) values are denormalised, having the form

s \times m \times 2 e

where s is +1 or −1, m is a positive integer less than 2⁵², and e is −1074.

Note that all the positive and negative integers whose magnitude is no greater than 2⁵³ are representable in the Number type (indeed, the integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact nonzero real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with −0 removed and with two additional values added to it that are not representable in the Number type, namely 2¹⁰²⁴ (which is +1 × 2⁵³ × 2⁹⁷¹) and −2¹⁰²⁴ (which is −1 × 2⁵³ × 2⁹⁷¹). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 2¹⁰²⁴ and −2¹⁰²⁴ are considered to have even significands. Finally, if 2¹⁰²⁴ was chosen, replace it with +∞; if −2¹⁰²⁴ was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range −2³¹ through 2³¹−1, inclusive, or in the range 0 through 2³²−1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 2³² integer values. See the descriptions of the ToInt32 and ToUint32 operators in 9.5 and 9.6, respectively.

8.1.6 The Object Type

An Object is logically a collection of properties. Each property is either a data property, or an accessor property:

A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
A accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.

Properties are identified using key values. A key value is either an ECMAScript String value or an Exotic Symbol object (8.4.3).

Property keys are used to access properties and their values.

There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object.

All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Ordinary objects are the most common form of objects and have the default object semantics. An exotic object is any form of object whose property semantics differ in any way from the default semantics.

8.1.6.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 5.

Table 5 — Attributes of a Data Property

Attribute Name	Value Domain	Description
[[Value]]	Any ECMAScript language type	The value retrieved by a get access of the property.
[[Writable]]	Boolean	If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]	Boolean	If true, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

An accessor property associates a key value with the attributes listed in Table 6.

Table 6 — Attributes of an Accessor Property

Attribute Name	Value Domain	Description
[[Get]]	Object or Undefined	If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]	Object or Undefined	If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]	Boolean	If true, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the initial values of a property’s attributes are not explicitly specified by this specification, the default value defined in Table 7 is used.

Table 7 — Default Attribute Values

Attribute Name	Default Value
[[Value]]	undefined
[[Get]]	undefined
[[Set]]	undefined
[[Writable]]	false
[[Enumerable]]	false
[[Configurable]]	false

8.1.6.2 Object Internal Methods and Internal Data Properties

The actual semantics of ECMAScript objects are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.

Internal methods are identified within this specification using names enclosed in double square brackets [[ ]]. Internal method names are polymorphic. This means that different ECMAScript object values may perform different algorithms when a common internal method name is invoked upon them. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.

Internal data properties correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Depending upon the specific internal data property such state may consist of values of any ECMAScript language type or of specific ECMA specification type values. Unless explicitly specified otherwise, internal data properties are allocated as part of the process of creating an an ECMAScript object and may not be dynamically added to ECMAScript objects. Unless specified otherwise, the initial value of an internal data property is the value undefined.

Table 8{ REF _Ref327354012 \h } summarises the essential internal methods used by this specification that are applicable to all ECMAScript objects. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

The “Signature” column of Table 8 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesised list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in Clause 8 augmented by the following additional names. “any” means the value may be any ECMAScript language type. “primitive” means Undefined, Null, Boolean, String, or Number. An internal method implicitly returns a Completion Record as described in 8.8. In addition to its parameters, an internal method always has access to the object upon which it is invoked as a method.

Table 8 — Essential Internal Methods

Internal Method	Signature	Description
[[GetInheritance]]	()→Object or Null	Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties. an object.
[[SetInheritance]]	(Object or Null)→Boolean	Associate with an object another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[HasIntegrity]]	(String)→Boolean	Determine whether the property structure of an object is fixed to at least the specified level. The argument is one of the values "`nonextensible`", "`sealed`", or "`frozen`".
[[SetIntegrity]]	(String)→Boolean	Restrict the mutability of an object’s properties to that which is allowed for the specified integtry level. The argument is one of the values "`nonextensible`", "`sealed`", or "`frozen`". Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful. The integrity level of an object may be raised but may not be lower.
[[HasOwnProperty]]	(propertyKey) → Boolean	Returns a Boolean value indicating whether the object already has an own property whose key is propertyKey.
[[GetOwnProperty]]	(propertyKey) → Undefined or Property Descriptor	Returns a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[HasProperty]]	(propertyKey) → Boolean	Returns a Boolean value indicating whether the object already has either an own or inherited property whose key is propertyKey.
[[Set]]	(propertyKey,value, Receiver) → Boolean	Try to set the value of an object’s property indentified by propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the this value when evaluating the code. Returns true indicating that the property value was set or false indicating that it could not be set.
[[Delete]]	(propertyKey) → Boolean	Removes the own property indentified by the propertyKey parameter from the object. Return false if the property was not deleted and is still present. Return true if the property was deleted or was not present.
[[DefineOwnProperty]]	(propertyKey, PropertyDescriptor) → Boolean	Creates or alters the named own property to have the state described by a Property Descriptor. Returns true indicating that the property was successfully created/updated or false indicating that the property could not be created or updated.
[[Enumerate]]	()→Object	Returns an iterator object over the string values of the keys of the enumerable properties of the object.

[[OwnPropertyKeys]]	()→Object	Returns an interator object that produces all of the own property keys for the object except those that are private Symbols.

Table 9 summarises additional essential internal methods that must be supported by all objects that may be called as functions..

Table 9 — Additional Essential Internal Methods of Function Objects

Internal Method	Signature	Description
[[Call]]	(any, a List of any) → any or Reference	Executes code associated with the object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values.
[[Construct]]	(a List of any) → Object	Creates an object. Invoked via the `new` operator. The arguments to the internal are the arguments passed to the new operator. Objects that implement this internal method are called constructors.

8.1.6.3 Invariants of the Essential Internal Methods

Current this section is just a bunch of material merged together from the ES5 spec. and from the wiki Proxy pages. It need to be completely reworked.

The intent is that it lists all invariants of the Essential Internal Methods. This includes both invariants that are enforced for Proxy objects and other invariants that may not be enfored.

Definitions:

The target of an internal method is the object the internal method is called upon.
A sealed property is a non-configurable own property of a target.
A frozen property is a non-configurable non-writable own property of a target.
A new property is a property that does not exist on a non-extensible target.
Two property descriptors desc1 and desc2 for a property key value are incompatible if:
1. Descl is produced by calling [[GetOwnPropertyDescriptor]] of target with key, and
2. Calling [[DefineOwnProperty]] of target with arguments key and desc2 would throw a TypeError exception.

Exotic objects may define additional constraints upon their [[Set]] internal method behavior. If possible, exotic objects should not allow [[Set]] operations in situations where this definition of [[CanPut]] returns false.

[[GetInheritance]]

Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal data property must eventually lead to a null value).

getOwnPropertyDescriptor

Non-configurability invariant: cannot return incompatible descriptors for sealed properties

Non-extensibility invariant: must return undefined for new properties

Invariant checks:

if trap returns undefined, check if the property is configurable

if property exists on target, check if the returned descriptor is compatible

if returned descriptor is non-configurable, check if the property exists on the target and is also non-configurable

defineProperty

Non-configurability invariant: cannot succeed (return true) for incompatible changes to sealed properties

Non-extensibility invariant: must reject (return false) for new properties

Invariant checks:

on success, if property exists on target, check if existing descriptor is compatible with argument descriptor

on success, if argument descriptor is non-configurable, check if the property exists on the target and is also non-configurable

getOwnPropertyNames

Non-configurability invariant: must report all sealed properties

Non-extensibility invariant: must not list new property names

Invariant checks:

check whether all sealed target properties are present in the trap result

If the target is non-extensible, check that no new properties are listed in the trap result

deleteProperty

Non-configurability invariant: cannot succeed (return true) for sealed properties

Invariant checks:

on success, check if the target property is configurable

getPrototypeOf

Invariant check: check whether the target’s prototype and the trap result are identical (according to the egal operator)

freeze | seal | preventExtensions

Invariant checks:

on success, check if isFrozen(target), isSealed(target) or !isExtensible(target)

isFrozen | isSealed | isExtensible

Invariant check: check whether the boolean trap result is equal to isFrozen(target), isSealed(target) or isExtensible(target)

hasOwn

Non-configurability invariant: cannot return false for sealed properties

Non-extensibility invariant: must return false for new properties

Invariant checks:

if false is returned, check if the target property is configurable

if false is returned, the property does not exist on target, and the target is non-extensible, throw a TypeError

has

Non-configurability invariant: cannot return false for sealed properties

Invariant checks:

if false is returned, check if the target property is configurable

get

Non-configurability invariant: cannot return inconsistent values for frozen data properties, and must return undefined for sealed accessors with an undefined getter

Invariant checks:

if property exists on target as a data property, check whether the target property’s value and the trap result are identical (according to the egal operator)

if property exists on target as an accessor, and the accessor’s get attribute is undefined, check whether the trap result is also undefined.

set

Non-configurability invariant: cannot succeed (return true) for frozen data properties or sealed accessors with an undefined setter

Invariant checks:

on success, if property exists on target as a data property, check whether the target property’s value and the update value are identical (according to the egal operator)

on success, if property exists on target as an accessor, check whether the accessor’s set attribute is not undefined

keys

Non-configurability invariant: must report all enumerable sealed properties

Non-extensibility invariant: must not list new property names

Invariant checks:

Check whether all enumerable sealed target properties are listed in the trap result

If the target is non-extensible, check that no new properties are listed in the trap result

enumerate

Non-configurability invariant: must report all enumerable sealed properties

Invariant checks:

Check whether all enumerable sealed target properties are listed in the trap result

NOTE This specification defines no ECMAScript language operators or built-in functions that permit a program to modify an object’s [[Prototype]] internal properties or to change the value of [[Extensible]] from false to true. Implementation specific extensions that modify [[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

Unless otherwise specified, the standard ECMAScript objects are ordinary objects and behave as described in 8.3. Some standard objects are exotic objects and have behaviour defined in 8.4..

Exotic objects may implement internal methods in any manner unless specified otherwise; for example, one possibility is that [[Get]] and [[Set]] for a particular exotic object indeed fetch and store property values but [[HasOwnProperty]] always generates false. However, if any specified manipulation of an exotic object's internal properties is not supported by an implementation, that manipulation must throw a TypeError exception when attempted.

The [[GetOwnProperty]] internal method of all objects must conform to the following invariants for each property of the object:

If a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other internal methods.
If a property is described as a data property and its [[Writable]] and [[Configurable]] are both false, then the SameValue (according to 9.12) must be returned for the [[Value]] attribute of the property on all calls to [[GetOwnProperty]].
If the attributes other than [[Writable]] may change over time or if the property might disappear, then the [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
If the result of calling an object’s [[IsExtensible]] internal method has been observed by ECMAScript code to be false, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also describe that property as non-existent.

The [[DefineOwnProperty]] internal method of all objects must not permit the addition of a new property to an object if the [[Extensible]] internal method of that object has been observed by ECMAScript code to be false.

If the result of calling the [[IsExtensible]] internal method of an object has been observed by ECMAScript code to be false then it must not subsequently become true.

8.1.6.4 Well-Known Symbols and Intrinsics

Well-known symbols are built-in exotic symbol objects (8.4.4) that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all Code Realms (10.3) and the value of their [[Private]] internal data property (forward ref) is false.

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 10.

Table 10--Well-known Symbols

Specification Name	Value and Purpose
@@create	A method used to allocate an object. Called from the [[Construct]] internal method.
@@hasInstance	A method that determines if a constructor object recognizes an object as one of the constructor’s instances. Called by the semantics of the `instanceof` operator.
@@isRegExp	A Boolean value that if true indicates that an object may be used as a regular expression.
@@iterator	A method that returns the default iterator for an object. Called by the semantics of the for-of statement.
@@ToPrimitive	A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag	A string value that is used in the creation of the default string description of an object. Called by the built-in method Object.prototype.toString.

Well-known intransics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have Code Realm specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per Code Realm.

Within this specification a reference such as %name% means the intrinsic object, associated with the current Code Realm, corresponding to the name. Determine of the current Code Realmand its intransics is described in 10.3. The well-known intrincs are listed in Table 11.

Table 11 — Well-known Intrinsic Objects

Intrinsic Name	ECMAScript Language Association
%Object%	The initial value of the global object property named `"Object"`.
%ObjectPrototype%	The initial value of the `"prototype"` data property of the intrinsic %Object%.
%ObjProto_toString%	The initial value of the `"toString"` data property of the intrinsic %ObjectPrototype%.
%Function%	The initial value of the global object property named `"Function"`.
%FunctionPrototype%	The initial value of the `"prototype"` data property of the intrinsic %Function%.
%Array%	The initial value of the global object property named `"Array"`.
%ArrayPrototype%	The initial value of the `"prototype"` data property of the intrinsic %Array%.
%ArrayIteratorPrototype%	The prototype object used for interator objects created by the CreateArrayIterator abstract operation.
%Map%	The initial value of the global object property named `"Map"`.
%MapPrototype%	The initial value of the `"prototype"` data property of the intrinsic %Map%.
%MapIteratorPrototype%	The prototype object used for interator objects created by the CreateMapIterator abstract operation
%WeakMap%	The initial value of the global object property named `"WeakMap"`.
%WeakMapPrototype%	The initial value of the `"prototype"` data property of the intrinsic %WeakMap%.
%Set%	The initial value of the global object property named `"Set"`.
%SetPrototype%	The initial value of the `"prototype"` data property of the intrinsic %Set%.
%SetIteratorPrototype%	The prototype object used for interator objects created by the CreateSetterator abstract operation
%StopIteration%
%ErrorPrototype%
%EvalErrorPrototype%
%RangePrototype%
%ReferenceErrorPrototype%
%SyntaxErrorPrototype%
%TypeErrorPrototype%
%URIErrorPrototype%
???

8.2 ECMAScript Specification Types

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types are Reference, List, Completion, Property Descriptor, Lexical Environment, Environment Record, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

8.2.1 Data Blocks

This section is a placeholder for describing the Data Block internal type. The following material is verbatium from the the Binary Data ES wiki proposal. The material has not yet been reviewed or integrated with the rest of this spec.

This spec introduces a new, spec-internal block datatype, intuitively representing a contiguously allocated block of binary data. Blocks are not ECMAScript language values and appear only in the program store (aka heap).

A block is one of:

a number-block
an array-block[t, n]
a struct-block[t1, ..., tn]

A number-block is one of:

an unsigned-integer; i.e., one of uint8, uint16, uint32, or uint64
a signed-integer; i.e., one of int8, int16, int32, or int64
a floating-point; i.e., one of float32 or float64

A uintk is an integer in the range [0, 2k). An intk is an integer in the range [-2k-1, 2k-1). A floatk is a floating-point number representable as a k-bit IEE754 value.

An array-block[t, n] is an ordered sequence of n blocks of homogeneous block type t. Each element of the array is stored at in independently addressable location in the program store, and multiple Data objects may contain references to the element.

A struct-block[t1, ..., tn] is an ordered sequence of n blocks of heterogeneous types t1 to tn, respectively. Each field of the struct is stored at in independently addressable location in the program store, and multiple Data objects may contain references to the field.

The spec also introduces a datatype of Data objects, which are ECMAScript objects that encapsulate references to block data in the program store. Every Data object has the following properties:

[[Class]] = “Data”

[[Value]] : reference[block] – a reference to a block in the program store

[[DataType]] : reference[Type] – a reference to a Type object describing this object’s data block

8.2.2 The List and Record Specification Type

The List type is used to explain the evaluation of argument lists (see 11.2.4) in new expressions, in function calls, and in other algorithms where a simple list of values is needed. Values of the List type are simply ordered sequences of values. These sequences may be of any length.

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[value]]

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, {[[field1]]: 42, [[field2]]: false, [[field3]]: empty} defines a Record value that has three fields each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[field2]] is shorthand for “the field of R named [[field2]]”.

Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true}.

8.2.3 The Completion Record Specification Type

The Completion type is a Record used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Values of the Completion type are Record values whole fields are defined as by Table 12.

Table 12 — Completion Record Fields

Field Name	Value	Meaning
[[type]]	One of normal, break, continue, return, or throw	The type of completion that occurred.
[[value]]	any ECMAScript language value or empty	The value that was produced.
[[target]]	any ECMAScript identifier or empty	The target label for directed control transfers.

The term “abrupt completion” refers to any completion with a [[type]] value other than normal.

8.2.3.1 NormalCompletion

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

Return NormalCompletion(argument).

Is a short hand that is defined as follows:

Return Completion {[[type]]: normal, [[value]]: argument, [[target]]:empty}.

8.2.3.2 Implicit Completion Values

The algorithms of this specification often implicitly return Completion Records whose [[type]] is normal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a Completion Record, such as:

Return "Infinity".

Generally means the same thing as:

Return NormalCompletion("Infinity").

A “return” statement without a value in an algoritm step means the same thing as:

Return NormalCompletion(undefined).

Similarly, any reference to a Completion Record value that is in a context that does not explicitly require a complete Completion Record value is equivalent to an explicit reference to the [[value]] field of the Completion Record value unless the Completion Record is an abrupt completion.

8.2.3.3 Throw an Exception

Algorithms steps that say to throw an exception, such as

Throw a TypeError exception.

Mean the same things as:

Return Completion {[[type]]: throw, [[value]]: a newly created TypeError object, [[target]]:empty}.

8.2.3.4 ReturnIfAbrupt

Algorithms steps that say

ReturnIfAbrupt(argument).

mean the same things as:

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

8.2.4 The Reference Specification Type

NOTE The Reference type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super keyword and other lanauge features. For example, the left-hand operand of an assignment is expected to produce a reference.

A Reference is a resolved name binding. A Reference consists of three components, the base value, the referenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value of undefined indicates that the Reference could not be resolved to a binding. The referenced name is a String.

A Super Reference is a Reference that is used to represents a name binding that was expressed using the super keyword. A Super Reference has an additional thisValue component and its base value will never be an environment record.

The following abstract operations are used in this specification to access the components of references:

GetBase(V). Returns the base value component of the reference V.
GetReferencedName(V). Returns the referenced name component of the reference V.
GetThisValue(V). Returns the thisValue component of the reference V.
IsStrictReference(V). Returns the strict reference flag component of the reference V.
HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.
IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true; otherwise returns false.
IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.
IsSuperReference(V). Returns true if this reference has a thisValue component.

The following abstract operations are used in this specification to operate on references:

8.2.4.1 GetValue (V)

ReturnIfAbrupt(V).
If Type(V) is not Reference, return V.
Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), throw a ReferenceError exception.
If IsPropertyReference(V), then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be null or undefined.
  2. Set base to ToObject(base).
2. Return the result of calling the [[Get]] internal method of base passing GetReferencedName(V) and GetThisValue(V) as the arguments.
Else base must be an environment record,
1. Return the result of calling the GetBindingValue (see 10.2.1) concrete method of base passing GetReferencedName(V) and IsStrictReference(V) as arguments.

NOTE The object that may be created in step 5.a.ii is not accessible outside of the above method. An implementation might choose to avoid the actual creation of the object.

8.2.4.2 PutValue (V, W)

ReturnIfAbrupt(V).
ReturnIfAbrupt(W).
If Type(V) is not Reference, throw a ReferenceError exception.
Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), then
1. If IsStrictReference(V) is true, then
  1. Throw ReferenceError exception.
2. Let globalObj be the result of the abstract operation GetGlobalObject.
3. Return the result of calling Put(globalObj,GetReferencedName(V), W, false).
Else if IsPropertyReference(V), then
1. If HasPrimitiveBase(V) is true, then
  1. Assert: In this case, base will never be null or undefined.
  2. Set base to ToObject(base).
2. Let succeeded be the
3. result of calling the [[Set]] internal method of base passing GetReferencedName(V), W, and GetThisValue(V) as arguments.
4. ReturnIfAbrupt(succeeded).
5. If succeeded is false and IsStrictReference(V) is true, then throw a TypeError exception.
6. Return.
Else base must be a reference whose base is an environment record. So,
1. Return the result of calling the SetMutableBinding (10.2.1) concrete method of base, passing GetReferencedName(V), W, and IsStrictReference(V) as arguments.
Return.

NOTE The object that may be created in step 6.a.ii is not accessible outside of the above algorithm. An implementation might choose to avoid the actual creation of that transient object.

8.2.4.3 GetThisValue (V)

ReturnIfAbrupt(V).
If Type(V) is not Reference, return V.
If IsUnresolvableReference(V), throw a ReferenceError exception.
If IsSuperReference(V), then
1. Return the value of the thisValue component of the reference V.
Return GetBase(V).

8.2.5 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.1.6.1. In addition, any field may be present or absent.

Property Descriptor values may be further classified as data property descriptors and accessor property descriptors based upon the existence or use of certain fields. A data property descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor property descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data property descriptor and an accessor property descriptor; however, it may be neither. A generic property descriptor is a Property Descriptor value that is neither a data property descriptor nor an accessor property descriptor. A fully populated property descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of the fields that correspond to the property attributes defined in either 8.1.6.1 Table 5 or Table 6.

A Property Descriptor may be derived from an ECMAScript object that has properties that directly correspond to the fields of a Property Descriptor. Such a derived Property Descriptor has an additional field named [[Origin]] whose value is the object from which the Property Descriptor was derived.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

8.2.5.1 IsAccessorDescriptor ( Desc )

When the abstract operation IsAccessorDescriptor is called with property descriptor Desc, the following steps are taken:

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

8.2.5.2 IsDataDescriptor ( Desc )

When the abstract operation IsDataDescriptor is called with property descriptor Desc, the following steps are taken:

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

8.2.5.3 IsGenericDescriptor ( Desc )

When the abstract operation IsGenericDescriptor is called with property descriptor Desc, the following steps are taken:

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

8.2.5.4 FromPropertyDescriptor ( Desc )

When the abstract operation FromPropertyDescriptor is called with property descriptor Desc, the following steps are taken:

The following algorithm assumes that Desc is a fully populated Property Descriptor, such as that returned from [[GetOwnProperty]] (see 8.12.1).

If Desc is undefined, then return undefined.
If Desc has an [[Origin]] field, then return Desc.[[Origin]].
Let obj be the result of the abstract operation ObjectCreate with the intrinsic object %ObjectPrototype% as its argument.
Assert: obj is an extensible ordinary object with no own properties.
If Desc has a [[Value]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj, "value", and Property Descriptor {[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}
If Desc has a [[Writable]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj, "writable", and Property Descriptor {[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
If Desc has a [[Get]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj, "get", and Property Descriptor {[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
If Desc has a [[S]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj, "set", and Property Descriptor {[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
If Desc has a [[Enumerable]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj, "enumerable", and Property Descriptor {[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
If Desc has a [[Configurable]] field, then
1. Call OrdinaryDefineOwnProperty with arguments obj , "configurable", and Property Descriptor {[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
Return obj.

8.2.5.5 ToPropertyDescriptor ( Obj )

When the abstract operation ToPropertyDescriptor is called with object Obj, the following steps are taken:

ReturnIfAbrupt(Obj).
If Type(Obj) is not Object throw a TypeError exception.
Let desc be the result of creating a new Property Descriptor that initially has no fields.
If the result of HasProperty(Obj, "enumerable") is true, then
1. Let enum be the result of Get(Obj, "enumerable").
2. ReturnIfAbrupt(enum).
3. Set the [[Enumerable]] field of desc to ToBoolean(enum).
If the result of HasProperty(Obj, "configurable") is true, then
1. Let conf be the result of Get(Obj, "configurable").
2. ReturnIfAbrupt(conf).
3. Set the [[Configurable]] field of desc to ToBoolean(conf).
If the result of HasProperty(Obj, "value") is true, then
1. Let value be the result of Get(Obj, "value").
2. ReturnIfAbrupt(value).
3. Set the [[Value]] field of desc to value.
If the result of HasProperty(Obj, "writable") is true, then
1. Let writable be the result of Get(Obj, "writable").
2. ReturnIfAbrupt(writable).
3. Set the [[Writable]] field of desc to ToBoolean(writable).
If the result of HasProperty(Obj, "get") is true, then
1. Let getter be the result of Get(Obj, "get").
2. ReturnIfAbrupt(getter).
3. If IsCallable(getter) is false and getter is not undefined, then throw a TypeError exception.
4. Set the [[Get]] field of desc to getter.
If the result of HasProperty(Obj, "set") is true, then
1. Let setter be the result of Get(Obj, "set").
2. ReturnIfAbrupt(setter).
3. If IsCallable(setter) is false and setter is not undefined, then throw a TypeError exception.
4. Set the [[Set]] field of desc to setter.
If either desc.[[Get]] or desc.[[Set]] are present, then
1. If either desc.[[Value]] or desc.[[Writable]] are present, then throw a TypeError exception.
Set the [[Origin]] field of desc to Obj.
Return desc.

8.2.5.6 CompletePropertyDescriptor ( Desc, LikeDesc )

When the abstract operation CompletePropertyDescriptor is called with Property Descriptor Desc, the following steps are taken:

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

8.2.6 The Lexical Environment and Environment Record Specification Types

The Lexical Environment and Environment Record types are used to explain the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined in Clause 10.

8.3 Ordinary Object Internal Methods and Internal Data Properties

Sections 8.3-8.5 will eventually be subsectons of a new toplevel section that follow the current section 10

All ordinary objects have an internal data property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (are 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 ECMAScript object has a Boolean-valued [[Extensible]] internal data property that controls whether or not properties may be added to the object. If the value of the [[Extensible]] internal data property is false then additional properties may not be added to the object. In addition, if [[Extensible]] is false the value of [[Prototype]] internal data properties of the object may not be modified. Once the value of an object’s [[Extensible]] internal data property has been set to false it may not be subsequently changed to true.

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

8.3.1 [[GetInheritance]] ( )

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

Return the value of the [[Prototype]] internal data property of O.

8.3.2 [[SetInheritance]] (V)

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

Assert: Either Type(V) is Object or Type(V) is Null.
Let extensible be the value of the [[Extensible]] internal data property of O.
If extensible is false, then return false.
Set the value of the [[Prototype]] internal data property of O to V.
Return true.

8.3.3 [[HasIntegrity]] ( Level )

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

Assert: Level is one of "nonextensible", "sealed", or "frozen".
If Level is "nonextensible", then
1. Return the value of the [[Extensible]] internal data property of O
Return the result of TestIntegrityLevel(O, Level).

8.3.4 [[SetIntegrity]] ( Level )

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

Assert: Level is one of "nonextensible", "sealed", or "frozen".
Set the value of the [[Extensible]] internal data property of O to false.
If Level is not "nonextensible", then
1. Return the result of SetIntegrityLevel(O, Level).
Return true.

8.3.5 [[HasOwnProperty]] (P)

When the [[HasOwnProperty]] internal method of O is called 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 false
Return true.

8.3.6 [[GetOwnProperty]] (P)

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

Return the result of OrdinaryGetOwnProperty with arguments O and P.

8.3.6.1 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, so
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.

8.3.7 [[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 the result of OrdinaryDefineOwnProperty with arguments O, P, and Desc.

8.3.7.1 OrdinaryDefineOwnProperty (O, P, Desc)

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

Let current be the result of calling OrdinaryGetOwnProperty with arguments O and P.
Let extensible be the value of the [[Extensible]] internal data property of O.
Return the result of ValidateAndApplyPropertyDescriptor with arguments O, P, extensible, Desc, and current.

8.3.7.2 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 the result of ValidateAndApplyPropertyDescriptor with arguments undefined, undefined, Extensible, Desc, and Current.

8.3.7.3 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:

This algorithm contains steps that test various fields of the Property Descriptor Desc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.

NOTE If undefined is passed as the O argument only validation is performed and no object updates are preformed.

Assert: If O is not undefined then P is a valid property key.
If current is undefined, then
1. If extensible is false, then return false.
2. Assert: extensible is true.
3. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
  1. If O is not undefined, then 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, then 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.
Return true, if every field in Desc is absent.
Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm (9.12).
If the [[Configurable]] field of current is false then
1. Return false, if the [[Configurable]] field of Desc is true.
2. Return false, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
If IsGenericDescriptor(Desc) is true, then no further validation is required.
Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
1. Return false, if the [[Configurable]] field of current is false.
2. If IsDataDescriptor(current) is true, then
  1. If O is not undefined, then 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, then 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 the [[Configurable]] field of current is false, then
  1. Return false, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
  2. If the [[Writable]] field of current is false, then
    1. Return false, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]], current.[[Value]]) is false.
2. else the [[Configurable]] field of current is true, so any change is acceptable.
Else IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true,
1. If the [[Configurable]] field of current is false, then
  1. Return false, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
  2. Return false, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
If O is not undefined, then
1. For each attribute field of Desc that is present, set the correspondingly named attribute of the property named P of object O to the value of the field.
Return true.

NOTE Step 8.b allows any field of Desc to be different from the corresponding field of current if current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]] is set to false.

8.3.8 [[HasProperty]](P)

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

Assert: IsPropertyKey(P) is true.
Let hasOwn be the result of calling [[HasOwnProperty]] internal method of O with argument P.
ReturnIfAbrupt(hasOwn).
If hasOwn is false, then
1. Let parent be the result of calling the [[GetInheritance]] internal method of O.
2. ReturnIfAbrupt(parent).
3. If parent is not null, then
  1. Return the result of calling the [[HasProperty]] internal method of parent with argument P.
Return hasOwn.

8.3.9 [[Get]] (P, Receiver)

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

Assert: IsPropertyKey(P) is true.
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
ReturnIfAbrupt(desc).
If desc is undefined, then
1. Let parent be the result of calling the [[GetInheritance]] internal method of O.
2. ReturnIfAbrupt(parent).
3. If parent is null, then return undefined.
4. Return the result of calling the [[Get]] internal method of parent with arguments P and Receiver.
If IsDataDescriptor(desc) is true, return desc.[[Value]].
Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
If getter is undefined, return undefined.
Return the result of calling the [[Call]] internal method of getter with Receiver as the thisArgument and an empty List as argumentsList.

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

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

Assert: IsPropertyKey(P) is true.
Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
ReturnIfAbrupt(ownDesc).
If ownDesc is undefined, then
1. Let parent be the result of calling the [[GetInheritance]] internal method of O.
2. ReturnIfAbrupt(parent).
3. If parent is not null, then
  1. Return the result of calling the [[Set]] internal method of parent with arguments P, V, and Receiver.
4. Else,
  1. If Type(Receiver) is not Object, return false.
5. 1. Return the result of performing CreateOwnDataProperty(Receiver, P, V).
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 be the result of calling the [[GetOwnProperty]] internal method of Receiver with argument P.
4. ReturnIfAbrupt(existingDescriptor).
5. If existingDescriptor is not undefined, then
  1. Let valueDesc be the Property Descriptor {[[Value]]: V}.
  2. Return the result of calling the [[DefineOwnProperty]] internal method of Receiver with arguments P and valueDesc.
6. Else Receiver does not currently have a property P,
  1. Return the result of performing CreateOwnDataProperty(Receiver, P, V).
If IsAccessorDescriptor(ownDesc) is true, then
1. Let setter be ownDesc.[[Set]].
2. If setter is undefined, return false.
3. Let setterResult be the result of calling the [[Call]] internal method of setter providing Receiver as thisArgument and a new List containing V as argumentsList.
4. ReturnIfAbrupt(setterResult).
5. Return true.

8.3.11 [[Delete]] (P)

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

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

8.3.12 [[Enumerate]] ()

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

Return an Iterator object (reference xxxx) whose next method iterates over all the keys of enumerable property keys of O. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

Enumerated properties do not include properties whose property key is a Symbol. Properties of the object being enumerated may be deleted during enumeration. If a property that has not yet been visited during enumeration is deleted, then it will not be visited. If new properties are added to the object being enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A property name must not be visited more than once in any enumeration.

Enumerating the properties of an 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 enumerated if it is “shadowed” because some previous object in the prototype chain has a property with the same name. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object is shadowed by a previous object on the prototype chain.

The following is an informative algorithm that conforms to these rules

Let obj be O.
Let proto be the result of calling the [[GetInheritance]] internal method of O with no arguments.
ReturnIfAbrupt(proto).
If proto is the value null, then
1. Let propList be a new empty List.
Else
1. Let propList be the result of calling the [[Enumerate]] internal method of proto.
ReturnIfAbrupt(propList).
For each name that is the property key of an own property of O
1. If Type(name) is String, then
  1. Let desc be the result of calling OrdinaryGetOwnProperty with arguments O and name.
  2. If name is an element of propList, then remove name as an element of propList.
  3. If desc.[[Enumerable]] is true, then add name as an element of propList.
Order the elements of propList in an implementation defined order.
Return propList.

8.3.13 [[OwnPropertyKeys]] ( )

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

Let keys be a new empty List.
For each own property key P of O
1. If P is not a private Symbol, then
  1. Add P as the last element of keys.
Return MakeListIterator(list).

8.3.14 ObjectCreate(proto, internalDataList) Abstract Operation

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 internalDataList is a List of the names of internal data property names that should be defined as part of the object. If the list is not provided, an empty List is used. It performs the following steps:

If proto was not provided, let proto be the intrinsic %ObjectPrototype%.
Let obj be a newly created ECMAScript object with an internal data property for each name in internalDataList.
Set obj’s essential internal methods to the default ordinary object definitions specified in 8.3.
Set the [[Prototype]] internal data property of obj to proto.
Set the [[Extensible]] internal data property of obj to true.
Return obj.

8.3.15 Ordinary Function Objects

Ordinary function objects encapsulate parameterised ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ordinary function object is an ordinary object and has the same internal data properties and (except as noted below) the same internal methods as other ordinary objects.

Ordinary function objects have the additional internal data properties listed in Table 13. They also have a [[BuiltinBrand]] internal data property whose value is BuiltinFunction.

Ordinary function objects provide alternative definitions for the [[Get]] and [[GetOwnProperty]] internal methods. These alternatives prevent the value of strict mode function from being revealed as the value of a function object property named "caller". These alternative definitions exist sole to preclude a non-standard legacy feature of some ECMAScript implementations from revealing information about strict mode callers. If an implementation does not provide such a feature, it need not implement these alternative internal methods for ordinary function objects.

Table 13 -- Internal Data Properties of Ordinary Function Objects

Internal Data Property	Type	Description
[[Scope]]	Lexical Environment	The Lexical Environment that the function was closed over. Is used as the outer environment when evaluating the code of the function.
[[FormalParameters]]	Parse Node	The root parse node of the source code that defines the function’s formal parameter list.
[[Code]]	Parse Node	The root parse node of the source code that defines the function’s body.
[[Realm]]	Realm Record	The Code Realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[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 mode function, false this is not a strict mode function.
[[Home]]	Object	If the function uses `super`, this is the object whose [[GetInheritance]] provides the object where super property lookups begin. Not present for functions that don’t reference `super`.
[[MethodName]]	String or Symbol	If the function uses `super`, this is the property keys that is used for unqualified references to `super`. Not present for functions that don’t reference `super`.

Ordinary function objects all have the [[Call]], [[Get]] and [[GetOwnProperty]] internal methods defined here. Oridinary functions that are also constructors in addition have the [[Construct]] internal method.

8.3.15.1 [[Call]] Internal Method

The [[Call]] internal method for an ordinary 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, then Suspend callerContext.
Let calleeContext be a new ECMAScript Code execution context.
Let calleeRealm be the value of F’s [[Realm]] internal data property.
Set calleeContext’s Realm calleeRealm.
Let thisMode be the value of F’s [[ThisMode]] internal data property.
If thisMode is lexical, then
1. Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the [[Scope]] internal data property of F as the argument.
Else,
1. If thisMode is strict, set thisValue to thisArgument.
2. Else
  1. if thisArgument is null or undefined, then
    1. Set thisValue to calleeRealm.[[globalThis]].
  2. Else if Type(thisArgument) is not Object, set the thisValue to ToObject(thisArgument).
  3. Else set the thisValue to thisArgument.
3. Let localEnv be the result of calling NewFunctionEnvironment passing F and thisValue as the arguments.
Set the LexicalEnvironment of calleeContext to localEnv.
Set the VariableEnvironment of calleeContext to localEnv.
Push calleeContext on to the execution context stack; calleeContext is now the running execution context.
Let status be the result of performing Function Declaration Instantiation using the function F, argumentsList , and localEnv as described in 10.5.3.
If status is an abrupt completion, then
1. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
2. Return status.
Let result be the result of evaluating the FunctionBody that is the value of F's [[Code]] internal data property.
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
Return result.

NOTE Most ordinary functions use a Function Environment Record as their LexicalEnvironment. Ordinary functions that are arrow functions use a Declarative Environment Record as their LexicalEnvironment.

8.3.15.2 [[Construct]] Internal Method

The [[Construct]] internal method for an ordinary Function object F is called with a single parameter argumentsList which is a possibly empty List of ECMAScript language values. The following steps are taken:

Return the result of OrdinaryConstruct(F, argumentsList).

8.3.15.2.1 OrdinaryConstruct (F, argumentsList)

When the abstract operation OrdinaryConstruct is called with Object F and List argumentsList the following steps are taken:

Let creator be the result of Get(F, @@create).
ReturnIfAbrupt(creator).
If creator is not undefined, then
1. If IsCallable(creator) is false, then throw a TypeError exception.
2. Let obj be the result of calling the [[Call]] internal method of creator with arguments F and an empty List.
Else creator is undefined so fall back to object creation defaults
1. Let obj be the result of calling OrdinaryCreateFromConstructor(F, "%ObjectPrototype%").
ReturnIfAbrupt(obj).
Let result be the result of calling the [[Call]] internal method of F, providing obj and argumentsList as the arguments.
ReturnIfAbrupt(result).
If Type(result) is Object then return result.
Return obj.

8.3.15.3 [[Get]] (P, Receiver)

When the [[Get]] internal method of ordinary function object F is called with property key P and ECMAScipt language value Receiver the following steps are taken:

Let v be the result of calling the default ordinary object [[Get]] internal method (8.3.7) on F passing P and Receiver as arguments.
ReturnIfAbrupt(v).
If P is "caller" and v is a strict mode Function object, return null.
Return v.

8.3.19.4 [[GetOwnProperty]] (P)

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

Let v be the result of calling the default ordinary object [[GetOwnProperty]] internal method (8.3.6) on F passing P as the argument.
ReturnIfAbrupt(v).
If IsDataDescriptor(v) is true, then
1. If P is "caller" and v.[[Value]] is a strict mode Function object, then
  1. Set v.[[Value]] to null.
Return v.

8.4 Built-in Exotic Object Internal Methods and Data Fields

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:

8.4.1 Bound Function Exotic Objects

A bound function is an exotic object that wrappers another function object. A bound function is callable (it has [[Call]] and [[Construct]] internal methods). Calling a bound function generally results in a call of its wrappered function.

Bound function objects do not have the internal data properties of ordinary function objects defined in Table 13. Instead they have the internal data properties defined in Table 14. They also have a [[BuiltinBrand]] internal data property whose value is BuiltinFunction.

Table 14 -- Internal Data Properties of Exotic Bound Function Objects

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

Unlike ordinary function objects, bound function objects do not use alternative definitions of the [[Get]] and [[GetOwnPropety]] internal methods. Bound function objects provide all of the essential internal methods as specified in 8.3. However, they use the following definitions for the essential internal methods of function objects.

8.4.1.1 [[Call]]

When the [[Call]] internal method of an exotic bound function 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 boundArgs be the value of F’s [[BoundArguments]] internal data property.
Let boundThis be the value of F’s [[BoundThis]] internal data property.
Let target be the value of F’s [[BoundTargetFunction]] internal data property.
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 the result of calling the [[Call]] internal method of target providing boundThis as thisArgument and providing args as argumentsList.

8.4.1.2 [[Construct]]

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

Let target be the value of F’s [[BoundTargetFunction]] internal data property.
If target has no [[Construct]] internal method, a TypeError exception is thrown.
Let boundArgs be the value of F’s [[BoundArguments]] internal data property.
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 ExtraArgs in the same order.
Return the result of calling the [[Construct]] internal method of target providing args as the arguments.

8.4.1.3 BoundFunctionCreate Abstract Operation

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

Let proto be the intrinsic %FunctionPrototype%.
Let obj be a newly created ECMAScript object.
Set obj’s essential internal methods to the default ordinary object definitions specified in 8.3.
Set the [[Call]] internal method of obj as described in 8.4.1.1.
Set the [[Construct]] internal method of obj as described in 8.4.1.2.
Set the [[Prototype]] internal data property of obj to proto.
Set the [[Extensible]] internal data property of obj to true.
Set the [[BoundTargetFunction]] internal data property of obj to targetFunction.
Set the [[BoundThis]] internal data property of obj to the value of boundThis.
Set the [[BoundArguments]] internal data property of obj to boundArgs.
Add the [[BuiltinBrand]] internal data property with value BuiltinFunction to obj.
Return obj.

8.4.2 Array Exotic Objects

An Array object is an exotic object that gives special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³²−1. 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 property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically 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.

Exotic Array objects have the same internal data properties as ordinary objects. They also have an [[ArrayInitializationState]] internal data property.

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

Exotic Array objects provide alternative definitions for the [[Set]] and [[DefineOwnProperty]] internal methods. Except for these two internal methods, exotic Array objects provide all of the other essential internal methods as specified in 8.3.

8.4.2.1 [[DefineOwnProperty]] ( P, Desc)

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

Assert: IsPropertyKey(P) is truet.
If P is "length", then
1. Return the result of calling ArraySetLength with arguments A, and Desc.
Else if P is an array index, then
1. Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument. The result will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
2. Let oldLen be oldLenDesc.[[Value]].
3. Let index be ToUint32(P).
4. ReturnIfAbrupt(index).
5. If index ≥ oldLen and oldLenDesc.[[Writable]] is false, then return false.
6. Let succeeded be the result of calling OrdinaryDefineOwnProperty passing A, P, and Desc as arguments.
7. ReturnIfAbrupt(succeeded).
8. If succeeded is false, then return false.
9. If index ≥ oldLen
  1. Set oldLenDesc.[[Value]] to index + 1.
  2. Let succeeded be the result of calling OrdinaryDefineOwnProperty passing A, "length", and oldLenDesc as arguments.
  3. ReturnIfAbrupt(succeeded).
10. Return true.
Return the result of calling OrdinaryDefineOwnProperty passing A, P, and Desc as arguments.

8.4.2.3 ArrayCreate Abstract Operation

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

If the proto argument was not passed, then let proto be the intrinsic object %ArrayPrototype%.
Let A be a newly created Array exotic object.
Set A’s essential internal methods to the default ordinary object definitions specified in 8.3.
Set the [[DefineOwnProperty]] internal method of A as specified in 8.4.2.1.
Set the [[Prototype]] internal data property of A to proto.
Set the [[Extensible]] internal data property of A to true.
If lenth is not undefined, then
1. Set the [[ArrayInitializationState]] internal data property of A to true.
Else
1. Set the [[ArrayInitializationState]] internal data property of A to false.
2. Let length be 0.
Call OrdinaryDefineOwnProperty with arguments A, "length" and Property Descriptor {[[Value]]: length, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}.
Return A.

8.4.2.4 ArraySetLength Abstract Operation

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

If the [[Value]] field of Desc is absent, then
1. Return the result of calling OrdinaryDefineOwnProperty passing A, "length", and Desc as arguments.
Let newLenDesc be a copy of Desc.
Let newLen be ToUint32(Desc.[[Value]]).
If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
Set newLenDesc.[[Value]] to newLen.
Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument. The result 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 the result of calling OrdinaryDefineOwnProperty passing A, "length", and newLenDesc as arguments.
If oldLenDesc.[[Writable]] is false, then 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 the result of calling OrdinaryDefineOwnProperty passing A, "length", and newLenDesc as arguments.
ReturnIfAbrupt(succeeded).
If succeeded is false, return false.
While newLen < oldLen repeat,
1. Set oldLen to oldLen – 1.
2. Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing ToString(oldLen).
3. ReturnIfAbrupt(succeeded).
4. If deleteSucceeded is false, then
  1. Set newLenDesc.[[Value]] to oldLen+1.
  2. If newWritable is false, set newLenDesc.[[Writable]] to false.
  3. Let succeeded be the result of calling OrdinaryDefineOwnProperty passing A, "length", and newLenDesc as arguments.
  4. ReturnIfAbrupt(succeeded).
  5. Return false.
If newWritable is false, then
1. Call OrdinaryDefineOwnProperty passing A, "length", and Property Descriptor{[[Writable]]: false} as arguments. This call will always return true.
Return true.

8.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual array index data properties corresponding to the individual code unit elements of the string value. Exotic String 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.

Exotic String objects have the same internal data properties as ordinary objects. They also have a [[StringData]] internal data property.

Exotic String objects provide alternative definitions for the following internal methods. All of the other exotic String object essential internal methods that are not defined below are as specified in 8.3.

8.4.3.1 [[HasOwnProperty]] (P)

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

Assert: IsPropertyKey(P) is true.
Let has be the result of calling the ordinary object [[HasOwnProperty]] internal method (8.3.5) on O with argument P.
ReturnIfAbrupt(has).
If has is true, then return true.
Let index be ToInteger(P).
ReturnIfAbrupt(index).
Let absIntIndex be ToString(abs(index)).
ReturnIfAbrupt(absIntIndex).
If SameValue(absIntIndex, P) is false return false.
Let str be the String value of the [[StringData]] internal property of O.
Let len be the number of elements in str.
If len ≤ index, return false.
Return true.

8.4.3.2 [[GetOwnProperty]] ( P )

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

Assert: IsPropertyKey(P) is true.
Let desc be the result of OrdinaryGetOwnProperty(S, P).
ReturnIfAbrupt(desc).
If desc is not undefined return desc.
Let index be ToInteger(P).
ReturnIfAbrupt(index).
Let absIntIndex be ToString(abs(index)).
ReturnIfAbrupt(absIntIndex).
If SameValue(absIntIndex, P) is false return undefined.
Let str be the String value of the [[StringData]] internal data property of S.
Let len be the number of elements in str.
If len ≤ index, return undefined.
Let resultStr be a String value of length 1, containing one code unit from str, specifically the code unit at position index, where the first (leftmost) element in str is considered to be at position 0, the next one at position 1, and so on.
Return a Property Descriptor { [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false }.

8.4.3.3 [[DefineOwnProperty]] ( P, Desc)

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

Let current be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
Let extensible be the value of the [[Extensible]] internal data property of O.
Return the result of ValidateAndApplyPropertyDescriptor with arguments O, P, extensible, Desc, and current.

NOTE This algorithm differs from the ordinary object OrdinaryDefineOwnProperty abstract operation algorithm only in invocation of [[GetOwnProperty]] in step 1.

8.4.3.4 [[Enumerate]] ()

When the [[Enumerate]] internal method of an exotic String object O is called the following steps are taken:

8.4.3.5 [[OwnPropertyKeys]] ( )

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

8.4.6.6 StringCreate Abstract Operation

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

Let A be a newly created String exotic object.
Set A’s essential internal methods to the default ordinary object definitions specified in 8.3.
Set the [[HasOwnProperty]] internal method of A as specified in 8.4.3.1.
Set the [[GetOwnProperty]] internal method of A as specified in 8.4.3.2.
Set the [[DefineOwnProperty]] internal method of A as specified in 8.4.3.3.
Set the [[Enumerate]] internal method of A as specified in 8.4.3.4.
Set the [[OwnPropertyKeys]] internal method of A as specified in 8.4.3.5.
Set the [[Prototype]] internal data property of A to prototype.
Set the [[Extensible]] internal data property of A to true.
Return A.

8.4.4 Exotic Symbol Objects

A Symbol object is an exotic object that may be used as a property key. Symbol exotic objects are unique in that they are always immutable and never observably reference any other object.

Exotic String objects have a single internal data properties named [[Private]] that is set when the object is created and never modified.

Exotic Symbol objects provide alternative definitions for all of the essential internal methods.

8.4.4.1 [[GetInheritance]] ( )

When the [[GetInheritance]] internal method of an exotic Symbol object O is called the following steps are taken:

Return null.

8.4.4.2 [[SetInheritance]] (V)

When the [[SetInheritance]] internal method of an exotic Symbol object O is called with argument V the following steps are taken:

Assert: Either Type(V) is Object or Type(V) is Null.
Return false.

8.4.4.3 [[HasIntegrity]] ( Level )

When the [[HasIntegrity]] internal method of an exotic Symbol object O is called the following steps are taken:

Assert: Level is one of "nonextensible", "sealed", or "frozen".
Return true.

8.4.4.4 [[Setntegrity]] ( Level )

When the [[Setntegrity]] internal method of an exotic Symbol object an exotic Symbol object O is called the following steps are taken:

Assert: Level is one of "nonextensible", "sealed", or "frozen".
Return true.

8.4.4.5 [[HasOwnProperty]] (P)

When the [[HasOwnProperty]] internal method of an exotic Symbol object O is called with property key P, the following steps are taken:

Return false.

8.4.4.6 [[GetOwnProperty]] (P)

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

Return undefined.

8.4.4.7 [[DefineOwnProperty]] (P, Desc)

When the [[DefineOwnProperty]] internal method of an exotic Symbol object O is called with property key P and property descriptor Desc, the following steps are taken:

Return false.

8.4.4.8 [[HasProperty]] (P)

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

Return false.

8.4.4.9 [[Get]] (P, Receiver)

When the [[Get]] internal method of an exotic Symbol object O is called with property key P and ECMAScipt language value Receiver the following steps are taken:

Assert: IsPropertyKey(P) is true.
If P is "toString", then
1. Let ctx be the running execution context.
2. Let ctxRealm be ctx’s Realm component.
3. Return ctxRealm.[[intrinsics]].% ObjProto_toString %.
Return undefined.

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

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

Return false.

8.4.4.11 [[Delete]] (P)

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

Assert: IsPropertyKey(P) is true.
Return true.

8.4.4.12 [[Enumerate]] ()

When the [[Enumerate]] internal method of an exotic Symbol object O is called the following steps are taken:

Return an Iterator object (reference xxxx) whose next method immediately throws %StopIteration% and forms no other action..

8.4.4.13 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of an exotic Symbol object O is called the following steps are taken:

Return an Iterator object (reference xxxx) whose next method immediately throws %StopIteration% and forms no other action.

8.4.5 Exotic Arguments Objects

An arguments object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of a non-strict function.

Exotic arguments objects have the same internal data properties as ordinary objects. They also have a [[ParameterMap]] internal data property and a [[BuiltinBrand]] internal data property whose value is BuiltinArguments.

Exotic arguments objects provide alternative definitions for the following internal methods. All of the other exotic arguments object essential internal methods that are not defined below are as specified in 8.3.

8.4.6 Indexed Delegation Exotic Objects

A Indexed Delegation object is an exotic object that that delegates [[Get]] and [[Set]] handling of array index property keys to methods of the object.

Indexed Delegation objects initially have the same internal data properties as ordinary objects.

Exotic Indexed Delegation objects provide alternative definitions for the following internal methods. All of the other exotic Indexed Delegation object essential internal methods that are not defined below are as specified in 8.3.

8.4.6.1 [[Get]] (P, Receiver)

When the [[Get]] internal method of an exotic indexed delegation object O is called with property key P and ECMAScipt language value Receiver the following steps are taken:

If SameValue(O, Receiver) is true and P is an array index, then
1. Let args be a new List containing P.
2. Return the result of Invoke(O, @@elementGet, args).
Return the result of calling the default ordinary object [[Get]] internal method (8.3.7) on O passing P and Receiver as arguments.

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

When the [[Set]] internal method of an an exotic indexed delegation object O is called with property key P, value V, and ECMAScipt language value Receiver, the following steps are taken:

If SameValue(O, Receiver) is true and P is an array index, then
1. Let args be a new List containing P and V.
2. Return the result of ToBoolean(Invoke(O, @@elementSet, args)).
Return the result of calling the default ordinary object [[Set]] internal method (8.3.7) on O passing P, V, and Receiver as arguments.

8.4.6.3 IndexedDelegatorCreate Abstract Operation

The abstract operation IndexedDelegatorCreate with argument prototype is used to specify the creation of new exotic Indexed Delegation objects. It performs the following steps:

Let A be a newly created ECMAScript object.
Set A’s essential internal methods to the default ordinary object definitions specified in 8.3.
Set the [[Get]] internal method of A as specified in 8.4.6.1.
Set the [[Set]] internal method of A as specified in 8.4.6.2.
Set the [[Prototype]] internal data property of A to prototype.
Set the [[Extensible]] internal data property of A to true.
Return A.

8.4.7 Built-in Function Objects

The function objects specified in Clause 15 may be implemented as either ordinary function objects whose behaviour is provided using ECMAScript code or as implementation provided exotic function objects whose behaviour is provide in some other manner. In either case, the effect of calling such functions must be that specified for each one in Clause 15.

If an implementation provided exotic object is used, the object must have the ordinary object behaviour specified in 8.3 except for [[Get]] and [[GetOwnProperty]] which must be as specified in 8.3.19. All such exotic function objects also have [[Prototype]] and [[Extensible]] internal data properties and a [[BuiltinBrand]] internal data property whose value is BuiltinFunction.

[[Calll]] and [[Construct]]

8.5 Proxy Object Internal Methods and Internal Data Properties

A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy objects has an internal data property called [[ProxyHandler]]. The value of [[ProxyHandler]] is always an object, called the proxy’s handler object. Methods 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 data property called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy’s target object.

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 data properties 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 [[ProxyHander]] internal data property is set to a special revoked proxy handler object and its [[ProxyTarget]] internal data property is set to null.

Because proxy permit arbitrary ECMAScript code to be used to in the implementation of internal methods, it is possible to define a proxy object whose handler methods violates the invariants defined in 8.1.6.2. Some of the internal method invariants defined in 8.1.6.2 are essential integrity invariants. These invariants are explicitly enforced by the proxy 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, Desc is a Property Description record, and B is a Boolean flag.

8.5.1 [[GetInheritance]] ( )

When the [[GetInheritance]] internal method of an exotic Proxy object O is called the following steps are taken:

Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "getPrototypeOf").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[GetInheritance]] internal method of target.
Let handlerProto be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
ReturnIfAbrupt(handlerProto).
Let targetProto be the result of calling the [[GetInheritance]] internal method of target.
ReturnIfAbrupt(targetProto).
If SameValue(handlerProto, targetProto) is false, then throw a TypeError exception.
Return handlerProto.

NOTE [[GetInheritance]] for proxy objects enforces the following invariant:

[[GetInheritance]] applied to the proxy object must return the same value as [[GetInheritance] applied to the proxy object’s target object.

8.5.2 [[SetInheritance]] (V)

When the [[SetInheritance]] internal method of an exotic Proxy 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 the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "setPrototypeOf").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[SetInheritance]] internal method of target with argument V.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and V.
ReturnIfAbrupt(trapResult).
Let trapResult be ToBoolean(trapResult).
Let targetProto be the result of calling the [[GetInheritance]] internal method of target.
ReturnIfAbrupt(targetProto).
If trapResult is true and SameValue(V, targetProto) is false, then throw a TypeError exception.
Return trapResult.

NOTE [[SetInheritance]] for proxy objects enforces the following invariant:

[[SetInheritance]] may only return true if the argument value is now the valued return for [[GetInheritance] applied to the proxy object’s target object.

8.5.3 [[HasIntegrity]] ( Level )

When the [[HasIntegrity]] internal method of an exotic Proxy object O is called the following steps are taken:

Assert: Level is one of "nonextensible", "sealed", or "frozen".
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
If Level is "nonextensible", then let trapName be "isExtensible".
Else if Level is "sealed ", then let trapName be "isSealed".
Else Level is "frozen ", so let trapName be "isFrozen".
Let trap be the result of GetMethod(handler, trapName).
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[HasIntegrity]] internal method of target with argument Level.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
ReturnIfAbrupt(trapResult).
Let booleanTrapResult be ToBoolean(trapResult).
Let targetResult be the result of calling the [[HasIntegrity]] internal method of target with argument Level.
ReturnIfAbrupt(targetResult).
If SameValue(booleanTrapResult, targetResult) is false, then throw a TypeError exception.
Return booleanTrapResult.

NOTE [[HasIntegrity]] for proxy objects enforces the following invariant:

[[HasIntegrity]] applied to the proxy object must return the same value as [[HasIntegrity]] applied to the proxy object’s target object with the same argument.

8.5.4 [[SetIntegrity]] ( Level )

When the [[Setntegrity]] internal method of an exotic Proxy object O is the following steps are taken:

Assert: Level is one of "nonextensible", "sealed", or "frozen".
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
If Level is "nonextensible", then let trapName be "preventExtensions".
Else if Level is "sealed ", then let trapName be "seal".
Else Level is "frozen ", so let trapName be "freeze".
Let trap be the result of GetMethod(handler, trapName).
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[SetIntegrity]] internal method of target with argument Level.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
ReturnIfAbrupt(trapResult).
Let booleanTrapResult be ToBoolean(trapResult).
Let targetResult be the result of calling the [[HasIntegrity]] internal method of target with argument Level.
ReturnIfAbrupt(targetResult).
If SameValue(booleanTrapResult, targetResult) is false , then throw a TypeError exception.
Return booleanTrapResult.

NOTE [[SetIntegrity]] for proxy objects enforces the following invariant:

[[SetIntegrity]] applied to the proxy object must return the same value as [[HasIntegrity]] applied to the proxy object’s target object with the same argument

8.5.5 [[HasOwnProperty]] (P)

When the [[HasOwnProperty]] internal method of an exotic Proxy object O is called with property key P, the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "hasOwn").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[HasOwnProperty]] internal method of target with argument P.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
ReturnIfAbrupt(trapResult).
Let success be ToBoolean(trapResult).
If success is false, then
1. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
2. ReturnIfAbrupt(targetDesc).
3. If targetDesc is not undefined, then
  1. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
  2. Let extensibleTarget be the result of calling the [[IsExtensible]] internal method of target.
  3. ReturnIfAbrupt(extensibleTarget).
  4. If ToBoolean(extensibleTarget) is false, then throw a TypeError exception.
Else success is true,
1. Let extensibleTarget be the result of IsExtensible(target).
2. ReturnIfAbrupt(extensibleTarget).
3. If ToBoolean(extensibleTarget) is true, then return success.
4. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
5. ReturnIfAbrupt(targetDesc).
6. If targetDesc is undefined, then throw a TypeError exception.
Return success.

NOTE [[HasOwnProperty]] for proxy objects enforces the following invariants:

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 a own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exists as a own property of the target object and the target object is not extensible.

8.5.6 [[GetOwnProperty]] (P)

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

Assert: IsPropertyKey(P) is true.
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "getOwnPropertyDescriptor").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[GetOwnProperty]] internal method of target with argument P.
Let trapResultObj be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
ReturnIfAbrupt(trapResultObj).
If Type(trapResultObj) is neither Object or Undefined, then throw a TypeError exception.
Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
ReturnIfAbrupt(targetDesc).
If trapResultObj is undefined, then
1. If targetDesc is undefined, then return undefined.
2. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
3. Let extensibleTarget be the result of IsExtensible(target).
4. ReturnIfAbrupt(extensibleTarget).
5. If ToBoolean(extensibleTarget) is false, then throw a TypeError exception.
6. Return undefined.
Let extensibleTarget be the result of IsExtensible(target).
ReturnIfAbrupt(extensibleTarget).
Set extensibleTarget to ToBoolean(extensibleTarget),
Let resultDesc be ToPropertyDescriptor(trapResultObj).
ReturnIfAbrupt(resultDesc).
Call CompletePropertyDescriptor(resultDesc, targetDesc).
Let valid be the result of IsCompatiblePropertyDescriptor (extensibleTarget, resultDesc, targetDesc).
If valid is false, then 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 a own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exists as a 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 exists as a own property of the target object or if it exists as a configurable own property of the target object.
The result of [[GetOwnProperty]] can be applied to the target object using [[DefineOwnPropery]] and will not throw an exception.

8.5.7 [[DefineOwnProperty]] (P, Desc)

When the [[DefineOwnProperty]] internal method of an exotic Proxy 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 the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "defineProperty").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[DefineOwnProperty]] internal method of target with arguments P and Desc.
Let descObj be FromPropertyDescriptor(Desc).
NOTE If Desc was originally generated from an object using ToPropertyDescriptor, then descObj will be that original object.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, and descObj.
ReturnIfAbrupt(trapResult).
If ToBoolean(trapResult) is false, then return false.
Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
ReturnIfAbrupt(targetDesc).
Let extensibleTarget be the result of IsExtensible(target).
ReturnIfAbrupt(extensibleTarget).
Set extensibleTarget to ToBoolean(extensibleTarget),
If targetDesc is undefined, then
1. If extensibleTarget is false, then throw a TypeError exception.
2. If Desc.[[Configurable]] is false, then throw a TypeError exception.
Else targetDesc is not undefined,
1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc , targetDesc) is false, then throw a TypeError exception.
2. If Desc.[[Configurable]] is false and targetDesc.[[Configurable]] is true, then throw a TypeError exception.
Return true.

NOTE [[GetOwnProperty]] for proxy objects enforces the following invariants:

A property cannot be added, if the target object is not extensible.
A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.
A property may not be non-configurable, if is corresponding configurable property of the target object exists.
If a property has a corresponding target object property then apply the property descriptor of the property to the target object using [[DefineOwnPropery]] will not throw an exception.

8.5.8 [[HasProperty]] (P)

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

Assert: IsPropertyKey(P) is true.
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "has").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[HasProperty]] internal method of target with argument P.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
ReturnIfAbrupt(trapResult).
Let success be ToBoolean(trapResult).
If success is false, then
1. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
2. ReturnIfAbrupt(targetDesc).
3. If targetDesc is not undefined, then
  1. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
  2. Let extensibleTarget be the result of IsExtensible(target).
  3. ReturnIfAbrupt(extensibleTarget).
  4. If ToBoolean(extensibleTarget) is false, then throw a TypeError exception.
Return success.

NOTE [[HasProperty] for proxy objects enforces the following invariants:

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 a own property of the target object and the target object is not extensible.

8.5.9 [[Get]] (P, Receiver)

When the [[Get]] internal method of an exotic Proxy object O is called with property key P and ECMAScipt language value Receiver the following steps are taken:

Assert: IsPropertyKey(P) is true..
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "get").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Get]] internal method of target with arguments P and Receiver.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, and Receiver.
ReturnIfAbrupt(trapResult).
Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
ReturnIfAbrupt(targetDesc).
If targetDesc is not undefined, then
1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
  1. If SameValue(trapResult, targetDesc.[[Value]]) is false, then throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Get]] is undefined, then
  1. If trapResult is not undefined, then throw a TypeError exception.
Return trapResult.

NOTE [[GetP] 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 data property.
The value reported for a property must be undefined if the corresponding corresponding target object property is non-configurable accessor property that has undefined as its [[Get]] attribute.

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

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

Assert: IsPropertyKey(P) is true.
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "set").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Set]] internal method of target with arguments P, V, and Receiver.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, V, and Receiver.
ReturnIfAbrupt(trapResult).
If ToBoolean(trapResult) is false, then return false.
Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
ReturnIfAbrupt(targetDesc).
If targetDesc is not undefined, then
1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
  1. If SameValue(V, targetDesc.[[Value]]) is false, then throw a TypeError exception.
2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false, then
  1. If targetDesc.[[Set]] is undefined, then throw a TypeError exception.
Return true.

NOTE [[Set]] for proxy objects enforces the following invariants:

Cannnot 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 data property.
Cannot set the value of a property if the corresponding corresponding target object property is a non-configurable accessor property that has undefined as its [[Set]] attribute.

8.5.11 [[Delete]] (P)

When the [[Delete]] internal method of an exotic Proxy object O is called with property name P the following steps are taken:

Assert: IsPropertyKey(P) is true.
Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "deleteProperty").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Delete]] internal method of target with argument P.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
ReturnIfAbrupt(trapResult).
If ToBoolean(trapResult) is false, then return false.
Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
ReturnIfAbrupt(targetDesc).
If targetDesc is undefined, then return true.
If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
Return true.

NOTE [[Delete]] for proxy objects enforces the following invariant:

A property cannot be deleted, if it exists as a non-configurable own property of the target object.

8.5.12 [[Enumerate]] ()

When the [[Enumerate]] internal method of an exotic Proxy object O is called the following steps are taken:

Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "enumerate").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Enumerate]] internal method of target.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
ReturnIfAbrupt(trapResult).
If Type(trapResult) is not Object, then throw a TypeError exception.

TODO: we may need to add a lot of additional invariant checking here according to the wiki spec. But maybe it really isn’t necessary. Tomvc response: I think it may be possible to waive the extra invariant checks for [[Enumerate]]. It's not a crucial primitive. My reasoning is that [[Enumerate]] deals with both own and inherited properties, and we don't really enforce any invariants on inherited properties. So I guess it's ok if the invariants for [[Enumerate]] are weakened.

Do note that this is a bit inconsistent with the way we treat internal methods like [[HasProperty]], [[GetP]] and [[SetP]]: these also deal with own and inherited properties, but still enforce invariants on own properties.

Return trapResult.

NOTE [[Enumerate] for proxy objects enforces the following invariants:

The result of [[Enumerate]] must be an Object.

8.5.13 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of an exotic Proxy object O is called the following steps are taken:

Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "ownKeys").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[OwnPropertyKeys]] internal method of target.
Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
ReturnIfAbrupt(trapResult).
If Type(trapResult) is not Object, then throw a TypeError exception.
TODO: we may need to add a lot of additional invariant checking here according to the wiki spec. But maybe it really isn’t necessary
Return trapResult.

NOTE [[OwnPropertyKeys] for proxy objects enforces the following invariants:

The result of [[OwnPropertyKeys]] must be an Object.

8.5.14 [[Call]] (thisArgument, argumentsList)

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

Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "apply").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Call]] internal method of target with arguments thisArgument and argumentsList.
Let argArray be the result of CreateArrayFromList(argumentsList).
Return the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, thisArgument, and argArray.

NOTE An Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal data property is an object that has a [[Call]] internal method.

8.5.15 [[Construct]] Internal Method

The [[Construct]] internal method of an exotic Proxy object O is called with a single parameter argumentsList which is a possibly empty List of ECMAScript language values. The following steps are taken:

Let handler be the value of the [[ProxyHandler]] internal data property of O.
Let target be the value of the [[ProxyTarget]] internal data property of O.
Let trap be the result of GetMethod(handler, "construct").
ReturnIfAbrupt(trap).
If trap is undefined, then
1. Return the result of calling the [[Construct]] internal method of target with argument argumentsList.
Let argArray be the result of CreateArrayFromList(argumentsList).
Return the result of calling trap with handler as the this value and a new List containing target and argArray.

NOTE An Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal data property is an object that has a [[Construct]] internal method.

9 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here to solely to aid the specification of the semantics of the ECMAScript Language. Other, more specialized abstract operations are defined throughout this specification.

9.1 Type Conversion and Testing

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations.. The conversion abstract operations are polymorphic; that is, they can accept a value of any ECMAScript language type, but not of specification types.

9.1.1 ToPrimitive

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 15:

Table 15 — ToPrimitive Conversions

Input Type	Result
Completion Record	If argument is an abrupt completion, return argument. Otherwise return ToPrimitive(argument.[[value]]) also passing the optional hint PreferredType.
Undefined	Return argument (no conversion).
Null	Return argument (no conversion).
Boolean	Return argument (no conversion).
Number	Return argument (no conversion).
String	Return argument (no conversion).
Object	Perform the steps following this table.

When the InputType is Object, the following steps are taken:

If PreferredType was not passed, let hint be "default".
Else if PreferredType is hint String, let hint be "string".
Else PreferredType is hint Number, let hint be "number".
Let exoticToPrim be the result of Get(argument, @@ToPrimitive).
ReturnIfAbrupt(exoticToPrim).
If exoticToPrim is not undefined, then
1. If IsCallable(exoticToPrim) is false, then throw a TypeError exception.
2. Let result be the result of calling the [[Call]] internal method of exoticToPrim, with argument as thisArgument and a List containing hint as argumentsList.
3. ReturnIfAbrupt(result).
4. If result is an ECMAScript language value and Type(result) is not Object, then return result.
5. Else, throw a TypeError exception.
If hint is "default" then, let hint be "number".
Return the result of OrdinaryToPrimitive(argument,hint).

When the OrdinaryToPrimitive is called with arguments O and hint, the following steps are taken:

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 tryFirst be "toString".
2. Let trySecond be "valueOf".
Else,
1. Let tryFirst be "valueOf".
2. Let trySecond be "toString".
Let first be the result of Get(O, tryFirst).
ReturnIfAbrupt(first).
If IsCallable(first) is true then,
1. Let result be the result of calling the [[Call]] internal method of first, with O as thisArgument and an empty List as argumentsList.
2. ReturnIfAbrupt(result).
3. If result is an ECMAScript language value and Type(result) is not Object, then return result.
4. Else, throw a TypeError exception.
Let second be the result of Get(O, trySecond ).
ReturnIfAbrupt(second).
If IsCallable(second) is true then,
1. Let result be the result of calling the [[Call]] internal method of second, with O as thisArgument and an empty argument list.
2. ReturnIfAbrupt(result).
3. If result is an ECMAScript language value and Type(result) is not Object, then return result.
Throw a TypeError exception.

NOTE When ToPrimitive is called with no hint, then it generally behaves as if the hint were Number. However, objects may over-ride this behaviour by defining a @@ToPrimitve method. Of the objects defined in this specification only Date objects (see 15.9.6) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.

9.1.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 16:

Table 16 — ToBoolean Conversions

Argument Type	Result
Completion Record	If argument is an abrupt completion, return the argument. Otherwise return ToBoolean(argument.[[value]])
Undefined	Return false
Null	Return false
Boolean	Return the input argument (no conversion).
Number	Return false if the argument is +0, −0, or NaN; otherwise return true.
String	Return false if the argument is the empty String (its length is zero); otherwise return true.
Object	Return true

9.1.3 ToNumber

The abstract operation ToNumber converts its argument to a value of type Number according to Table 17:

Table 17 — ToNumber Conversions

Argument Type	Result
Completion Record	If argument is an abrupt completion, return argument. Otherwise return ToNumber(argument.[[value]])
Undefined	Return NaN
Null	Return +0
Boolean	Return 1 if argument is true. Return +0 if argument is false.
Number	Return argument (no conversion).
String	See grammar and note below.
Object	Apply the following steps: Let primValue be ToPrimitive(argument, hint Number). Return ToNumber(primValue).

9.1.3.1 ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

Syntax

StringNumericLiteral :::

StrWhiteSpace_opt

StrWhiteSpace_opt StrNumericLiteral StrWhiteSpace_opt

StrWhiteSpace :::

StrWhiteSpaceChar StrWhiteSpace_opt

StrWhiteSpaceChar :::

WhiteSpace

LineTerminator

StrNumericLiteral :::

StrDecimalLiteral

HexIntegerLiteral

StrDecimalLiteral :::

StrUnsignedDecimalLiteral

+ StrUnsignedDecimalLiteral

- StrUnsignedDecimalLiteral

StrUnsignedDecimalLiteral :::

Infinity

DecimalDigits . DecimalDigits_opt ExponentPart_opt

. DecimalDigits ExponentPart_opt

DecimalDigits ExponentPart_opt

DecimalDigits :::

DecimalDigit

DecimalDigits DecimalDigit

DecimalDigit ::: one of

0 1 2 3 4 5 6 7 8 9

ExponentPart :::

ExponentIndicator SignedInteger

ExponentIndicator ::: one of

e E

SignedInteger :::

DecimalDigits

+ DecimalDigits

- DecimalDigits

HexIntegerLiteral :::

0x HexDigit

0X HexDigit

HexIntegerLiteral HexDigit

HexDigit ::: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

NOTE Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 7.8.3):

A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0 digits.
A StringNumericLiteral that is decimal may be preceded by + or - to indicate its sign.
A StringNumericLiteral that is empty or contains only white space is converted to +0.
Infinity and –Infinity are recognized as a StringNumericLiteral but not as a NumericLiteral.

Runtime Semantics

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 7.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below.

The MV of StringNumericLiteral ::: [empty] is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace_opt StrNumericLiteral StrWhiteSpace_opt is the MV of StrNumericLiteral, no matter whether white space is present or not.
The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.
The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: + StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: - StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point +0 or −0 as appropriate.)
The MV of StrUnsignedDecimalLiteral ::: Infinity is 10¹⁰⁰⁰⁰ (a value so large that it will round to +∞).
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits is the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10⁻ⁿ), where n is the number of characters in the second DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . ExponentPart is the MV of DecimalDigits times 10^e, where e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits ExponentPart is (the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10⁻ⁿ)) times 10^e, where n is the number of characters in the second DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits is the MV of DecimalDigits times 10⁻ⁿ, where n is the number of characters in DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10^e−n, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10^e, where e is the MV of ExponentPart.
The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.
The MV of ExponentPart ::: ExponentIndicator SignedInteger 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 is 0.
The MV of DecimalDigit ::: 1 or of HexDigit ::: 1 is 1.
The MV of DecimalDigit ::: 2 or of HexDigit ::: 2 is 2.
The MV of DecimalDigit ::: 3 or of HexDigit ::: 3 is 3.
The MV of DecimalDigit ::: 4 or of HexDigit ::: 4 is 4.
The MV of DecimalDigit ::: 5 or of HexDigit ::: 5 is 5.
The MV of DecimalDigit ::: 6 or of HexDigit ::: 6 is 6.
The MV of DecimalDigit ::: 7 or of HexDigit ::: 7 is 7.
The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.
The MV of DecimalDigit ::: 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 HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a String 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 unless the first non white space character in the String numeric literal is ‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteral 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 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.

9.1.4 ToInteger

The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:

Let number be the result of calling ToNumber on the input argument.
ReturnIfAbrupt(number).
If number is NaN, return +0.
If number is +0, −0, +∞, or −∞, return number.
Return the result of computing sign(number) × floor(abs(number)).

9.1.5 ToInt32: (Signed 32 Bit Integer)

The abstract operation ToInt32 converts its argument to one of 2³² integer values in the range −2³¹ through 2³¹−1, inclusive. This abstract operation functions as follows:

Let number be the result of calling ToNumber on the input argument.
ReturnIfAbrupt(number).
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let int be sign(number) × floor(abs(number)).
Let int32bit be int modulo 2³².
If int32bit is greater than or equal to 2³¹, return int32bit − 2³², otherwise return int32bit.

NOTE Given the above definition of ToInt32:

The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToInt32 maps −0 to +0.

9.1.6 ToUint32: (Unsigned 32 Bit Integer)

The abstract operation ToUint32 converts its argument to one of 2³² integer values in the range 0 through 2³²−1, inclusive. This abstract operation functions as follows:

Let number be the result of calling ToNumber on the input argument.
ReturnIfAbrupt(number).
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let int be sign(number) × floor(abs(number)).
Let int32bit be int modulo 2³².
Return int32bit.

NOTE Given the above definition of ToUInt32:

Step 6 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToUint32 maps −0 to +0.

9.1.7 ToUint16: (Unsigned 16 Bit Integer)

The abstract operation ToUint16 converts its argument to one of 2¹⁶ integer values in the range 0 through 2¹⁶−1, inclusive. This abstract operation functions as follows:

Let number be the result of calling ToNumber on the input argument.
ReturnIfAbrupt(number).
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let int be sign(number) × floor(abs(number)).
Let int16bit be int modulo 2¹⁶.
Return int16bit.

NOTE Given the above definition of ToUint16:

The substitution of 2¹⁶ for 2³² in step 4 is the only difference between ToUint32 and ToUint16.
ToUint16 maps −0 to +0.

9.1.8 ToString

The abstract operation ToString converts its argument to a value of type String according to Table 18:

Table 18 — ToString Conversions

Argument Type	Result
Completion Record	If argument is an abrupt completion, return argument. Otherwise return ToString(argument.[[value]])
Undefined	`"undefined"`
Null	`"null"`
Boolean	If argument is true, then return `"true"`. If argument is false, then return "false".
Number	See 9.8.1.
String	Return argument (no conversion)
Object	Apply the following steps: 1. Let primValue be ToPrimitive(argument, hint String). 2. Return ToString(primValue).

9.1.8.1 ToString Applied to the Number Type

The abstract operation ToString converts a Number m to String format as follows:

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 the String "-" and ToString(−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 consisting of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n−k occurrences of the character ‘0’.
If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−n digits of the decimal representation of s.
If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point ‘.’, followed by −n occurrences of the character ‘0’, followed by the k digits of the decimal representation of s.
Otherwise, if k = 1, return the String consisting of the single digit of s, followed by lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).
Return the String consisting of the most significant digit of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−1 digits of the decimal representation of s, followed by the lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).

NOTE 1 The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

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 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

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. Available as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.

9.1.9 ToObject

The abstract operation ToObject converts its argument to a value of type Object according to Table 19:

Table 19 — ToObject Conversions

Argument Type	Result
Completion Record	If argument is an abrupt completion, return argument. Otherwise return ToObject(argument.[[value]])
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return a new Boolean object whose [[BooleanData]] internal data property is set to the value of argument. See 15.6 for a description of Boolean objects.
Number	Return a new Number object whose [[NumberData]] internal data property is set to the value of argument. See 15.7 for a description of Number objects.
String	Return a new String object whose [[StringData]] internal data property is set to the value of argument. See 15.5 for a description of String objects.
Object	Return argument (no conversion).

9.1.10 ToPropertyKey

The abstract operation ToPropertyKey converts its argument to a value that can be used as a property key by performing the following steps:

ReturnIfAbrupt(argument).
If Type(argument) is Object, then
1. If argument is an exotic Symbol object, then
  1. Return argument.
Return ToString(argument).

9.1.11 ToPositiveInteger

The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:

Let number be the result of calling ToNumber on the input argument.
ReturnIfAbrupt(number).
If number is NaN, return +0.
If number is +∞, or −∞, return number.
If number ≤ 0, return +0.
Return the result of computing floor(number).

9.2 Testing and Comparison Operations

9.2.1 CheckObjectCoercible

The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 20:

Table 20 — CheckObjectCoercible Results

Argument Type	Result
Completion Record	If argument is an abrupt completion, return argument. Otherwise return CheckObjectCoercible(argument.[[value]])
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return argument
Number	Return argument
String	Return argument
Object	Return argument

9.2.2 IsCallable

The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value or a Completion Record, is a callable function Object according to Table 21:

Table 21 — IsCallable Results

Argument Type	Result
Completion Record	If `argument` is an abrupt completion, return `argument`. Otherwise return IsCallable(`argument`.[[value]])
Undefined	Return false.
Null	Return false.
Boolean	Return false.
Number	Return false.
String	Return false.
Object	If `argument` has a [[Call]] internal method, then return true, otherwise return false.

9.2.3 SameValue(x, y)

The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:

ReturnIfAbrupt(x).
ReturnIfAbrupt(y).
If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
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.
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 in corresponding positions) return true; otherwise, return false.
If Type(x) is Boolean, then
1. If x and y are both true or both false, then return true; otherwise, return false.
Return true if x and y are the same Object value. Otherwise, return false.

9.2.4 SameValueZero(x, y)

The internal comparison abstract operation SameValueZero(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:

ReturnIfAbrupt(x).
ReturnIfAbrupt(y).
If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
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.
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 in corresponding positions) return true; otherwise, return false.
If Type(x) is Boolean, then
1. If x and y are both true or both false, then return true; otherwise, return false.
Return true if x and y are the same Object value. Otherwise, return false.

NOTE SameValueZero differs from SameValue only in its treatment of +0 and -0.

9.2.5 IsConstructor

The abstract operation IsConstructor determines if its argument, which must be an ECMAScript language value or a Completion Record, is a function object with a [[Construct]] internal method.

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

9.2.6 IsPropertyKey

The abstract operation IsPropertyKey determine if its argument, which must be an ECMAScript language value or a Completion Record, is a value that may be used as a property key.

ReturnIfAbrupt(argument).
If Type(argument) is String, return true.
If Type(argument) is Object and argument is an exotic Symbol object, return true.
Return false.

9.2.7 IsExtensible (O)

The abstract operation IsExtensible is used to determine whether additional properties an be added to an object. A Boolean value is return. The operation is called with argument O where O is the object. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Let notExtensible be the result of calling the [[HasIntegrity]] internal method of globalObject with argument "nonextensible".
ReturnIfAbrupt(notExtensible).
If notExtensib le is true, then return false; else return true.

9.3 Operations on Objects

9.3.1 Get (O, P)

The abstract operation Get is used to retrieve the value of an specific property of an object. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return the result of calling the [[Get]] internal method of O passing P and O as the arguments.

9.3.2 Put (O, P, V, Throw)

The abstract operation Put is used to set the value of a specific property of an object. The operation is called with arguments O, P, V, and Throw where O is the object, P is the property key, V is the new value for the property and Throw is a Boolean flag. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Assert: Type(Throw) is Boolean.
Let success be the result of calling the [[Set]] internal method of O passing P, V, and O as the arguments.
ReturnIfAbrupt(success).
If success is false and Throw is true, then throw a TypeError exception.
Return success.

9.3.3 CreateOwnDataProperty (O, P, V)

The abstract operation CreateOwnProperty is used to create a new own property of an object. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the new value for the property. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Assert: O does not have an own property whose key is P..
Let notExtensible be the result of calling the [[HasIntegrity]] internal method of O with argument "nonextensible".
ReturnIfAbrupt(notExtensible).
If notExtensible is true, then return false.
Let newDesc be the Property Descriptor.
{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
Return the result of calling the [[DefineOwnProperty]] internal method of O passing P and newDesc as arguments.

9.3.4 DefinePropertyOrThrow (O, P, desc)

The abstract operation DefinePropertyOrThrow is used to call the [[DefineOwnProperlty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update can not be performed. The operation is called with arguments O, P, and desc where O is the object, P is the property key, and desc is the Property Descriptor for the property. This abstract operation perform, the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be the result of calling the [[DefineOwnProperty]] internal method of O passing P and desc as arguments.
ReturnIfAbrupt(success).
If success is false, then throw a TypeError exception.
Return success.

9.3.5 DeletePropertyOrThrow (O, P)

The abstract operation Put is used to remove a specific own property of an object. It throws an exception is the property is not configurable. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let success be the result of calling the [[Delete]] internal method of O passing P as the argument.
ReturnIfAbrupt(success).
If success is false, then throw a TypeError exception.
Return success.

9.3.6 HasProperty (O, P)

The abstract operation HasProperty is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. A Boolean value is return. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Return the result of calling the [[HasProperty]] internal method of O argument P.

9.3.7 GetMethod (O, P)

The abstract operation GetMethod is used to get the value of an specific property of an object when the value of the property is expected to be a function. The operation is called with arguments O and P where O is the object, P is the property key. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: IsPropertyKey(P) is true.
Let func be the result of calling the [[Get]] internal method of O passing P and O as the arguments.
ReturnIfAbrupt(func).
If func is undefined, then return undefined.
If IsCallable(func) is false, then throw a TypeError exception.
Return func.

9.3.8 Invoke(O,P, [args])

The abstract operation Invoke is used to call a method property of an object. The operation is called with arguments P, O, and optionally args where P is the property key, O serves as both the lookup point for the property and the this value of the call, and args is the list of arguments values passed to the method. If args is not present, an empty List is used as its value. This abstract operation performs the following steps:

Assert: P is a valid property key.
If args was not passed, then let args be a new empty List.
Let obj be ToObject(O).
ReturnIfAbrupt(obj).
Let func be the result of GetMethod(obj, P).
ReturnIfAbrupt(func).
If func is undefined, throw a TypeError exception.
Return the result of calling the [[Call]] internal method of func passing O as thisArgument and args as argumentsList.

9.3.9 SetIntegrityLevel (O, level)

The abstract operation SetIntegrityLevel is used to fix the set of own properties of an object. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: level is either "sealed" or "frozen".
Let keys be the result of calling [[OwnPropertyKeys]] internal method of O.
ReturnIfAbrupt(keys).
Let pendingException be undefined.
If level is "sealed", then
1. Repeat for each element k of keys,
  1. Let status be the result of DefinePropertyOrThrow(O, k, PropertyDescriptor{ [[Configurable]]: false}).
  2. If status is an Abrupt Completion, then
    1. If pendingException is undefined, then set pendingException to status.
Else level is "frozen",
1. Repeat for each element k of keys,
  1. Let status be the result of calling the [[GetOwnProperty]] internal method of O with k.
  2. If status is an Abrupt Completion, then
    1. If pendingException is undefined, then set pendingException to status.
  3. Else,
    1. Let currentDesc be status.[[value]].
    2. If currentDesc is not undefined, then
      1. If IsAccessorDescriptor(currentDesc) is true, then
        
        Let desc be PropertyDescriptor{[[Configurable]]: false}.
      2. Else,
        
        Let desc be PropertyDescriptor [[Configurable]]: false, [[Writable]]: false }.
      3. Let status be the result of DefinePropertyOrThrow(O, k, desc).
      4. If status is an Abrupt Completion, then
        
        If pendingException is undefined, then set pendingException to status.
If pendingException is not undefined, then return pendingException.
Return the result of calling the[[PreventExtensions]] internal method of O.

9.3.10 TestIntegrityLevel (O, level)

The abstract operation TestIntegrityLevel is used to determine if the set of own properties of an object are fixed. This abstract operation performs the following steps:

Assert: Type(O) is Object.
Assert: level is either "sealed" or "frozen".
Let status be the result of IsExtensible(O).
ReturnIfAbrupt(status).
If status is true, then return false
NOTE If the object is extensible, none of its properties are examined.
Let keys be the result of calling [[OwnPropertyKeys]] internal method of O.
ReturnIfAbrupt(keys).
Let pendingException be undefined.
Let configurable be false.
Let writable be false.
Repeat for each element k of keys,
1. Let status be the result of calling the [[GetOwnProperty]] internal method of O with k.
2. If status is an Abrupt Completion, then
  1. If pendingException is undefined, then set pendingException to status.
  2. Let configurable be true.
3. Else,
  1. Let currentDesc be status.[[value]].
  2. If currentDesc is not undefined, then
    1. Set configurable to configurable logically ored with currentDesc.[[Configurable]].
    2. If IsDataDescriptor(currentDesc) is true, then
      1. Set writable to writable logically ored with currentDesc.[[Writable]].
If pendingException is not undefined, then return pendingException.
If level is "frozen"and writable is true, then return false.
If configurable is true, then return false.
Return true.

9.3.11 CreateArrayFromList (elements)

The abstract operation CreateArrayFromList is used to create an Array object whose elements are provided by an internal List. This abstract operation performs the following steps:

Assert: elements is a List whose elements are all ECMAScript language values.
Let array be the result of the abstract operation ArrayCreate with argument 0.
Let n be 0.
For each element e of elements
1. Call CreateOwnDataProperty(array, ToString(n), e).
2. Assert: the call in step 4.a will never result in an abrupt completion.
3. Increment n by 1.
Return array.

9.3.12 OrdinaryHasInstance (C, O)

The abstract operation OrdinaryHasInstance implements the default algorithm for determining if an object O inherits from the inheritance path used by constructor C. This abstract operation performs the following steps:

If IsCallable(C) is false, return false.
If C has a [[BoundTargetFunction]] internal data property, then
1. Ler BC be the value of C’s a [[BoundTargetFunction]] internal data property.
2. Return the result of instanceofOperator(O,BC) (see 11.8).
If Type(O) is not Object, return false.
Let P be the result of Get(C, "prototype").
ReturnIfAbrupt(P).
If Type(P) is not Object, throw a TypeError exception.
Repeat
1. Set O to the result of calling the [[GetInheritance]] internal method of O with no arguments.
2. ReturnIfAbrupt(O).
3. If O is null, return false.
4. If SameValue(P, O) is true, return true.

9.3.13 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 theconstructor’s prototype property, if it exists. Otherwise the supplied default is used for [[Prototype]]. This abstract operation performs the following steps:

Assert: intrinsicProto 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.
If IsConstructor (constructor) is false, then throw a TypeError exception.
Let proto be the result of Get(construtor, "prototype").
ReturnIfAbrupt(proto).
If Type(proto) is not Object, then
1. If constructor has a [[Realm]] internal data property, let realm be F’s [[Realm]].
2. Else,
  1. Let ctx be the running execution context.
  2. Let realm be be ctx’s Realm.
3. Let proto be 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 Code Realm of the constructor function rather than from the running execution context. This accounts for the possibility that a built-in @@create method from a different Code Realm might be installed on constructor.

9.3.13 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto, internalDataList )

The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]] value is retrieved from a constructor’s prototype property, if it exists. Otherwise the supplied default is used for [[Prototype]]. The optional internalDataList is a List of the names of internal data property names that should be defined as part of the object. If the list is not provided, an empty List is used. This abstract operation performs the following steps:

Assert: intrinsicProto 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 the result of GetPrototypeFromConstructor(construtor, intrinsicDefaultProto).
ReturnIfAbrupt(proto).
Return the result of the abstract operation ObjectCreate(proto, internalDataList).

10 Executable Code and Execution Contexts

10.1 Types of Executable Code

There are five types of ECMAScript executable 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 FunctionBody, ConciseBody, ClassBody, or ModuleBody.
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 [[Code]] internal data property (see 13.6) of function and generator objects. The function code of a particular function or generator does not include any source text that is parsed as the function code of a nested FunctionBody, ConciseBody, or ClassBody..
Generator code is source text that is parsed to supply the value of the [[Code]] internal data property (see 13.5) of generator objects. The generator code of a particular generator does not include any source text that is parsed as the function code of a nested FunctionBody, ConciseBody, or ClassBody. All generator code is also considered to be function code, but only function code that is defined within a generator is generator code.
Module code is source text that is parse 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 FunctionBody, ConciseBody, ClassBody, or ModuleBody..

NOTE Function code is generally provided as the bodies of Function Definitions (13.1), Arrow Function Definditions (13.2), Method Definitions (13.3) and Generator Definitions (13.4). Function code is also derived from the last argument to the Function constructor (15.3). Generator code is provided as the bodies of Generator Definitions 13.4 and Generator Expressions (11.????).

10.1.1 Strict Mode Code

An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. When processed using strict mode the three types of ECMAScript code are referred to as strict global code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:

Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1).
Module code is always strict code.
All code contained in a ClassBody is strict code.
Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is contained in strict mode code.
Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyDefinition is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyDefinition is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the last argument to the built-in Function constructor is strict function code if the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

10.1.2 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function 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.

10.2 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.

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 environment record may be prepopulated with identifier bindings and includes an associated global object whose properties provide some of the global environment’s identifier bindings. This global object is the value of a global environment’s this binding. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

A method environment is a Lexical Environment that corresponds to the invocation of an ECMAScript function object that establishes a new this binding. A method 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.

10.2.1 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 funtions.

For specification purposes Environment Record values 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 are a subclass of declarative environment record. The abstract class includes the abstract specification methods defined in Table 22. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 22 — Abstract Methods of Environment Records

Method	Purpose
HasBinding(N)	Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value `N` is the text of the identifier.
CreateMutableBinding(N, D)	Create a new but uninitialised mutable binding in an environment record. The String value `N` is the text of the bound name. If the optional Boolean argument `D` is true the binding is may be subsequently deleted.
CreateImmutableBinding(N)	Create a new but uninitialised immutable binding in an environment record. The String value N is the text of the bound name.
InitializeBinding(N,V)	Set the value of an already existing but uninitialised 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. `S` is used to identify strict mode references.
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 strict mode references. If `S` is true and the binding does not exist or is uninitialised throw a ReferenceError exception.
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.

10.2.1.1 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.

10.2.1.1.1 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.
If it does not have such a binding, return false.

10.2.1.1.2 CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the name N that is uninitialised. A binding must not already exist in this Environment Record for N. If Boolean argument D is provided and 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 uninitialised. If D is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return NormalCompletion(empty)

10.2.1.1.3 CreateImmutableBinding (N)

The concrete Environment Record method CreateImmutableBinding for declarative environment records creates a new immutable binding for the name N that is uninitialised. A binding must not already exist in this environment record for N.

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 uninitialised.

10.2.1.1.4 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 uninitialised binding for N must already exist.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec must have an uninitialised binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialised.

10.2.1.1.5 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 must already exist. 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.
Assert: envRec must have a binding for N.
If the binding for N in envRec is a mutable binding, change its bound value to V.
Else if binding for N in envRec has not yet been initialized throw a ReferenceError exception.
Else this must be an attempt to change the value of an immutable binding so if S is true throw a TypeError exception.
Return NormalCompletion(empty).

10.2.1.1.6 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. The binding must already exist. If S is true and the binding is an uninitialised immutable binding throw a ReferenceError exception.

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 uninitialised binding, then
1. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Else,
1. Return the value currently bound to N in envRec.

10.2.1.1.7 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.
If envRec does not have a binding for the name that is the value of N, return true.
If the binding for N in envRec cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.

10.2.1.1.8 HasThisBinding ()

Regular Declarative Environment Records do not provide a this binding.

Return false.

10.2.1.1.9 HasSuperBinding ()

Regular Declarative Environment Records do not provide a super binding.

Return false.

10.2.1.1.10 WithBaseObject()

Declarative Environment Records always return undefined as their WithBaseObject.

Return undefined.

10.2.1.2 Object Environment Records

Each object environment record is associated with an object called its binding object. An object environment record binds the set of identifier names that directly correspond to the property names of its binding object. Property names that are not 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 (12.10) 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.

10.2.1.2.1 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.
Return the result of HasProperty(bindings, N).

10.2.1.2.2 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 initialises it to the value undefined. A property named N must not already exist in the binding object. If Boolean argument D is provided and 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.
Assert: The result of HasProperty(bindings, N) is false.
If D is true then let configValue be true otherwise let configValue be false.
Return the result of DefinePropertyOrThrow(bindings, N, Property Descriptor {[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}).

10.2.1.2.3 CreateImmutableBinding (N)

The concrete Environment Record method CreateImmutableBinding is never used within this specification in association with Object environment records.

10.2.1.2.4 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 uninitialised binding for N must already exist.

Let envRec be the object environment record for which the method was invoked.
Assert: envRec must have an uninitialised binding for N.
Record that the binding for N in envRec has been initialised.
Call the SetMutableBinding concrete method of envRec with N, V, and false as arguments.

10.2.1.2.5 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 the result of Put(bindings, N, V, and S).

10.2.1.2.6 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 the result of HasProperty(bindings, N).
ReturnIfAbrupt(value).
If value is false, then
1. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Return the result of Get(bindings, N).

10.2.1.2.7 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 the result of calling the [[Delete]] internal method of bindings passing N as the argument.

10.2.1.2.8 HasThisBinding ()

Regular Object Environment Records do not provide a this binding.

Return false.

10.2.1.2.9 HasSuperBinding ()

Regular Object Environment Records do not provide a super binding.

Return false.

10.2.1.2.10 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.

10.2.1.3 Function Environment Records

A function environment record is a declarative environment record that is used to represent the outer most scope of a function that provides a this binding. In addition to its identifier bindings, a function environment record contains the this value used within its scope. If such a function 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 store their this binding as the value of their thisValue. If the associated function references super, the environment record stores in HomeObject the object that the function is bound to as a method and in MethodName the property key used for unqualified super invocations from within the function. The default value for HomeObject and MethodName is undefined.

Methods environment records support all of Declarative Environment Record methods listed in Table 22 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, declarative environment records support the methods listed in Table 23:

Table 23 — Additional Methods of Function Environment Records

Method	Purpose
GetThisBinding()	Return the value of this environment record’s `this` binding.
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 binding. If the value is Empty, return undefined.
GetMethodName()	Return the value of this environment record’s MethodName binding.

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

10.2.1.3.1 HasThisBinding ()

Function Environment Records always provide a this binding.

Return true.

10.2.1.3.2 HasSuperBinding ()

If this environment record’s HomeObject has the value Empty, then return false. Otherwise, return true.

10.2.1.3.3 GetThisBinding ()

Return the value of this environment record’s thisValue.

10.2.1.3.4 GetSuperBase ()

Let home be the value of this environment record’s HomeObject.
If home has the value Empty, then return undefined.
Assert Type(home) is Object.
Return the result of calling home’s [[GetInheritance]] internal method..

10.2.1.3.5 GetMethodName ()

Return the value of this environment record’s MethodName.

10.2.1.4 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 (10.3). A global environment provides the bindings for built-in globals (15.1), properties of the global object, and for all declarations that are not function code and that occur within Script productions.

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. This global object is also the value of the global environment record’s thisValue. The object environment record component of a global environment record contains the bindings for all built-in globals (15.1) and all bindings introduced by a FunctionDeclaration 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 or VariableStatement declarations and binding 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 CreateGlobalVarBindings and CreateGlobalFunctionBindings concrete methods.

Global environment records have the additional state components listed in Table 24 and the additional methods listed in Table 25.

Table 24 -- Components of Global Environment Records

Component	Purpose
ObjectEnvironment	A Object Environment Record whose base object is the global object. Contains global built-in bindings as well as bindings for FunctionDeclaration or VariableStatement declarations in global code for the associated Realm.
DeclarativeEnvironment	A Declarative Environment Record that contains bindings for all declarations in global for the associated Realm code except for FunctionDeclaration and VariableStatement declarations.
VarNames	A List containing the string names bound by FunctionDeclaration or VariableStatement declarations in global code for the associated Realm.

Table 25 — 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 VariableStatement or a FunctionDeclaration.
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 :
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 global `var` bindings in the ObjectEnvironmentComponent of the environment record. The binding will be a mutable binding. The corresponding global object property will have attribute values approate for a `var`. The String value `N` is the text of the bound name. V is the initial value of the binding If the optional Boolean argument `D` is true the binding is may be subsequently deleted. This is logically equivalent to CreateMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)	Used to create and initialize global `function` bindings in the ObjectEnvironmentComponent of the environment record. The binding will be a mutable binding. The corresponding global object property will have attribute values approate for a `function`.The String value `N` is the text of the bound name. If the optional Boolean argument `D` is true the binding is may be subsequently deleted. This is 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.

10.2.1.4.1 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’s DeclarativeEnvironment.
If the result of calling DclRec’s HasBinding concrete method with argument N is true, return true.
Let ObjRec be envRec’s ObjectEnvironment.
Return the result of calling ObjRec’s HasBinding concrete method with argument N.

10.2.1.4.2 CreateMutableBinding (N, D)

The concrete environment record method CreateMutableBinding for global environment records creates a new mutable binding for the name N that is uninitialised. The binding is created in the associated DeclarativeEnvironment. A binding for N must not already exist in the DeclarativeEnvironment. If Boolean argument D is provided and 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’s DeclarativeEnvironment.
Assert: DclRec does not already have a binding for N.
Create a mutable binding in DclRec for N and record that it is uninitialised. If D is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return NormalCompletion(empty)

10.2.1.4.3 CreateImmutableBinding (N)

Let envRec be the global 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 uninitialised.

10.2.1.4.4 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 uninitialised binding for N must already exist.

Let envRec be the global environment record for which the method was invoked.
Let DclRec be envRec’s DeclarativeEnvironment.
If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
1. Return the result of calling DclRec’s InitializeBinding concrete method with arguments N and V.
Let ObjRec be envRec’s ObjectEnvironment.
If the result of calling ObjRec’s HasBinding concrete method with argument N is true, then
1. Set the bound value for N in envRec to V.
2. Record that the binding for N in envRec has been initialised.

10.2.1.4.5 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 declarative environment record for which the method was invoked.
Let DclRec be envRec’s DeclarativeEnvironment.
If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
1. Return the result of calling the SetMutableBinding concrete method of DclRec with arguments N, V, and S.
Let ObjRec be envRec’s ObjectEnvironment.
Return the result of calling the SetMutableBinding concrete method of ObjRec with arguments N, V, and S.

10.2.1.4.6 GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for global environment records simply returns the value of its bound identifier whose name is the value of the argument N. If S is true and the binding is an uninitialised 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 declarative environment record for which the method was invoked.
Let DclRec be envRec’s DeclarativeEnvironment.
If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
1. Return the result of calling the GetBindingValue concrete method of DclRec with arguments N, and S.
Let ObjRec be envRec’s ObjectEnvironment.
Return the result of calling the GetBindingValue concrete method of ObjRec with arguments N, and S.

10.2.1.4.7 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 declarative environment record for which the method was invoked.
Let DclRec be envRec’s DeclarativeEnvironment.
If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
1. Return the result of calling the DeleteBinding concrete method of DclRec with argument N.
Let ObjRec be envRec’s ObjectEnvironment.
If the result of calling ObjRec’s HasBinding concrete method with argument N is true, then
1. Let status be the result of calling the DeleteBinding concrete method of DclRec with argument N.
2. ReturnIfAbrupt(status).
3. If status is true, then
  1. Let varNames be envRec’s VarNames List.
  2. If N is an element of varNames, then remove that element from the varNames.
4. Return status.
Return true.

10.2.1.4.8 HasThisBinding ()

Global Environment Records always provide a this binding whose value is the associated global object.

Return true.

10.2.1.4.9 HasSuperBinding ()

Return false.

10.2.1.4.10 WithBaseObject()

Global Environment Records always return undefined as their WithBaseObject.

Return undefined.

10.2.1.4.11 GetThisBinding ()

Let envRec be the global environment record for which the method was invoked.
Let ObjRec be envRec’s ObjectEnvironment.
Let bindings be the binding object for ObjRec.
Return bindings.

10.2.1.4.12 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’s VarNames List.
If varDeclaredNames contains the value of N, return true.
Return false.

10.2.1.4.13 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’s DeclarativeEnvironment.
Return the result of calling DclRec’s HasBinding concrete method with argument N.

10.2.1.4.14 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. Redundent 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’s ObjectEnvironment.
If the result of calling ObjRec’s HasBinding concrete method with argument N is true, return true.
Let bindings be the binding object for ObjRec.
Let extensible be the result of IsExtensible(bindings).
Return extensible.

10.2.1.4.15 CanDeclareGlobalFunction (N)

The concrete environment record method CanDeclareGlobalVar 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’s ObjectEnvironment.
Let globalObject be the binding object for ObjRec.
Let extensible be the result of IsExtensible(globalObject).
ReturnIfAbrupt(extensible).
If the result of calling ObjRec’s HasBinding concrete method with argument N is false, then return extensible.
Let existingProp be the result of calling the [[GetOwnProperty]] internal method of globalObject with argument N.
If existingProp is undefined, then return extensible.
If existingProp.[[Configurable]] is true, then return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values {[[Writable]]: true, [[Enumerable]]: true}, then return true.
Return false.

10.2.1.4.16 CreateGlobalVarBinding (N, D)

The concrete Environment Record method CreateVarBinding for global environment records creates 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.

Let envRec be the declarative environment record for which the method was invoked.
Let ObjRec be envRec’s ObjectEnvironment.
Assert: The result of calling envRec’s CanDeclareGlobalVar concrete method with argument N is true.
If the result of calling ObjRec’s HasBinding concrete method with argument N is false, then
1. Call the CreateMutableBinding concrete method of ObjRec with arguments N and D.
Let varDeclaredNames be envRec’s VarNames List.
If varDeclaredNames does not contain the value of N, then
1. Append N to varDeclaredNames.
Return.

10.2.1.4.17 CreateGlobalFunctionBinding (N, V, D)

The concrete Environment Record method CreateFunctionBinding for global environment records creates 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 declarative environment record for which the method was invoked.
Let ObjRec be envRec’s ObjectEnvironment.
Assert: The result of calling envRec’s CanDeclareGlobalFunction concrete method with argument N is true.
Let globalObject be the binding object for ObjRec.
Let existingProp be the result of calling the [[GetOwnProperty]] internal method of globalObject with argument N.
If existingProp is undefined or existingProp.[[Configurable]] is true, then
1. Call the [[DefineOwnProperty]] internal method of globalObject passing N and Property Descriptor {[[Value]]:V, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: D} as arguments.
Else,
1. Call the [[DefineOwnProperty]] internal method of globalObject passing N and Property Descriptor {[[Value]]:V } as arguments.
NOTE The assertion in step 3 means that the above [[DefineOwnProperty]] calls will never return false or an abrupt completion.
Let varDeclaredNames be envRec’s VarNames List.
If varDeclaredNames does not contain the value of N, then
1. Append N to varDeclaredNames.
Return.

NOTE Global unction declarations are always represented as a own property of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values.

10.2.2 Lexical Environment Operations

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

10.2.2.1 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 is undefined, whose referenced name is name, and whose strict reference flag is strict.
Let envRec be lex’s environment record.
Let exists be the result of calling the HasBinding(N) concrete method of envRec passing name as the argument N.
If exists is true, then
1. Return a value of type Reference whose base value is envRec, whose referenced name is name, and whose strict reference flag is strict.
Else
1. Let outer be the value of lex’s outer environment reference.
2. Return the result of calling GetIdentifierReference passing outer, name, and strict as arguments.

10.2.2.2 NewDeclarativeEnvironment (E)

When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or null 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 environment record to be envRec.
Set the outer lexical environment reference of env to E.
Return env.

10.2.2.3 NewObjectEnvironment (O, E)

When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical Environment E (or null) 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 environment record to be envRec.
Set the outer lexical environment reference of env to E.
Return env.

10.2.2.4 NewFunctionEnvironment (F, T)

When the abstract operation NewFunctionEnvironment is called with an ECMAScript function Object F and a ECMAScript value T as arguments, the following steps are performed:

Assert: F’s thisMode is not lexical.
Let env be a new Lexical Environment.
Let envRec be a new Function environment record containing containing no bindings.
Set envRec’s thisValue to T.
If F has a [[HomeObject]] internal data property, then
1. Set envRec’s HomeObject to the value of F’s [[HomeObject]] internal data property.
2. Set envRec’s MethodName to the value of F’s [[MethodName]] internal data property.
Else,
1. Set envRec’s HomeObject to Empty.
Set env’s environment record to be envRec.
Set the outer lexical environment reference of env to the value of F’s [[Scope]] internal data property.
Return env.

10.3 Code Realms

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

A Realm is specified as a Record with the fields specified in Table 26:

Table 26 — Realm Record Fields

Field Name	Value	Meaning
[[intrinsics]]	A record whose field names are intrinsic keys and whose values are objects	These are the intrinsic values used by code associated with this Realm
[[globalThis]]	An ECMAScript object	The global object for this Realm
[[globalEnv]]	A ECMAScript environment	The global environment for this Realm
[[loader]]	any ECMAScript identifier or empty	The Loader object that can associate ECMAScript code with this Realm

The intrinsic objects associated with a code Realm include the well-known intrinsics listed in Table 11 and additional intrinsics specified by Table 27.

Table 27 — Additional Intrinsic Objects with Realm Specific Bindings

Intrinsic Name	ECMAScript Language Association
???	???
???	???

10.4 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 that is actually executing code. This is known as the running execution context. A 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 the state components listed in Table 28.

Table 28 —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.

Realm	The Realm from which associated code accesses ECMAScript resources.

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 latter 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.

Execution contexts for ECMAScript code have the additional state components listed in Table 29.

Table 29 —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 environment record holds bindings created by VariableStatements within this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.

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 an ECMAScript program to directly access or observe an execution context.

10.4.1 Identifier Resolution

Identifier resolution is the process of determining the binding of an IdentifierName using the LexicalEnvironment of the running execution context. During execution of ECMAScript code, Identifier Resolution is performed using the following algorithm:

Let env be the running execution context’s LexicalEnvironment.
If the syntactic production that is being evaluated is contained in strict mode code, then let strict be true, else let strict be false.
Return the result of calling GetIdentifierReference abstract operation passing env, the StringValue of IdentifierName, and strict as arguments.

The result of evaluating an identifier is always a value of type Reference with its referenced name component equal to the Identifier String.

10.4.2 GetThisEnvironment

The abstract operation GetThisEnviroment finds the lexical environment that currently supplies the binding of the keyword this. GetThisEnviroment performs the following steps:

Let lex be the running execution context’s LexicalEnvironment.
Repeat
1. Let envRec be lex’s environment record.
2. Let exists be the result of calling the HasThisBinding concrete method of envRec.
3. If exists is true, then return envRec.
4. Let outer be the value of lex’s outer environment reference.
5. Let lex be outer.

NOTE The loop in step 4 will always terminate because the llst of environment always end with the global environment which has a this binding.

10.4.3 This Resolution

The abstract operation ThisResolution is the process of determining the binding of the keyword this using the LexicalEnvironment of the running execution context. ThisResolution performs the following steps:

Let env be the result of performing the GetThisEnvironment abstract operation.
Return the result of calling the GetThisBinding concrete method of env.

10.4.4 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.[[globalThis]].

10.5 Declaration Binding Instantiation

10.5.1 Global Declaration Instantiation

NOTE When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declarated in the code is instantiated.

Global Declaration Instantiation is performed as follows using arguments script, env, and deletableBindings. script is the ScriptBody that for which the execution context is being established. env is the global environment record in which bindings are to be created. deletableBindings is true if the bindings that are created should be deletable.

Let strict be IsStrict of script.
Let lexNames be the LexicallyDeclaredNames of script.
Let varNames be the VarDeclaredNames of script.
For each name in lexNames, do
1. If the result of calling env’s HasVarDeclaration concrete method passing name as the argument is true, throw a SyntaxError exception.
2. If the result of calling env’s HasLexicalDeclaration concrete method passing name as the argument is true, throw a SyntaxError exception.
For each name in varNames, do
1. If the result of calling env’s HasLexicalDeclaration concrete method passing name as the argument is true, throw a SyntaxError exception.
Let varDeclarations be the VarScopedDeclarations of script.
Let functionsToInitialize be an empty List.
Let declaredFunctionNames be an empty List.
For each d in varDeclarations, in reverse list order do
1. If d is a FunctionDeclaration then
  1. NOTE If there are multiple FunctionDeclarations for the same name, the last declaration is used.
  2. Let fn be the sole element of the BoundNames of d.
  3. If fn is not an element of declaredFunctionNames, then
    1. Let fnDefinable be the result of calling env’s CanDeclareGlobalFunction concrete method passing fn as the argument.
    2. If fnDefinable is false, throw TypeError exception.
    3. Append fn to declaredFunctionNames.
    4. Append d to functionsToInitialize.
Let declaredVarNames be an empty List.
For each d in varDeclarations, do
1. If d is a VariableStatement 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 the result of calling env’s CanDeclareGlobalVar concrete method passing vn and deletableBindings as the arguments.
      2. If vnDefinable is false, throw 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.
For each FunctionDeclaration 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. Call env’s CreateGlobalFunctionBinding concrete method passing fn, fo, and deletableBindings as the arguments.
For each String vn in declaredVarNames, in list order do
1. Call env’s CreateGlobalVarBinding concrete method passing vn and deletableBindings as the argument.
Let lexDeclarations be the LexicallyScopedDeclarations of script.
For each element d in lexDeclarations do
1. NOTE Lexically declarated 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. Call env’s CreateImmutableBinding concrete method passing dn as the argument.
  2. Else,
    1. Call env’s CreateMutableBinding concrete method passing dn and false as the arguments.
Return NormalCompletion(empty)

NOTE Early errors specified in 14.1 prevent name conflicts between function/var declarations and let/const/class/module declarations as well as redeclaration of let/const/class/module 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 Global Declaration Instantiation. If any such errors are detected, no bindings are instantiated for the script.

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, and module declarations.

10.5.2 Module Declaration Instantiation

10.5.3 Function Declaration Instantiation

This version reflects the concensus as of the Sept. 2012 TC39 meeting. However, it now appears that the binding semantics of formal parameters is like to change again.

NOTE When an execution context is established for evaluating function code a new Declarative Environment Record is created and bindings for each formal parameter, and each function level variable, constant, or function declarated in the function are instantiated in the environment record. Formal parameters and functions are initialized as part of this process. All other bindings are initialized during execution of the function code.

Function Declaration Instantiation is performed as follows using arguments func, argumentsList, and env. func is the function object that for which the execution context is being established. env is the declarative environment record in which bindings are to be created.

Let code be the value of the [[Code]] internal data property of func.
Let strict be the value of the [[Strict]] internal data property of func.
Let formals be the value of the [[FormalParameters]] internal data property of func.
Let parameterNames be the BoundNames of formals.
Let varDeclarations be the VarScopedDeclarations of code.
Let functionsToInitialize be an emptyList.
Let argumentsObjectNotNeeded be false.
For each d in varDeclarations, in reverse list order do
1. If d is a FunctionDeclaration then
  1. NOTE If there are multiple FunctionDeclarations for the same name, the last declaration is used.
  2. Let fn be the sole element of the BoundNames of d.
  3. If fn is "arguments", then let argumentsObjectNotNeeded be true.
  4. Let alreadyDeclared be the result of calling env’s HasBinding concrete method passing fn as the argument.
  5. If alreadyDeclared is false, then
    1. Let status be the result of calling env’s CreateMutableBinding concrete method passing fn as the argument.
    2. Assert: status is never an Abrupt Completion.
    3. Append d to functionsToInitialize.
For each String paramName in parameterNames, do
1. Let alreadyDeclared be the result of calling env’s HasBinding concrete method passing paramName as the argument.
2. NOTE Duplicate parameter names can only occur in non-strict functions. Parameter names that are the same as function declaration names do not get initialized to undefined.
3. If alreadyDeclared is false, then
  1. If paramName is "arguments", then let argumentsObjectNotNeeded be true.
  2. Let status be the result of calling env’s CreateMutableBinding concrete method passing paramName as the argument.
  3. Assert: status is never an Abrupt Completion
  4. Call env’s InitializeBinding concrete method passing paramName, and undefined as the arguments.
NOTE If there is a function declaration or formal parameter with the name "arguments" then an argument object is not created.
If argumentsObjectNotNeeded is false, then
1. If strict is true, then
  1. Call env’s CreateImmutableBinding concrete method passing the String "arguments" as the argument.
2. Else,
  1. Call env’s CreateMutableBinding concrete method passing the String "arguments" as the argument.
Let varNames be the VarDeclaredNames of code.
For each String varName in varNames, in list order do
1. Let alreadyDeclared be the result of calling env’s HasBinding concrete method passing varName as the argument.
2. NOTE A VarDeclaredNames is only instantiated and initialied here if it is not also the name of a formal parameter or a FunctionDeclarations.
3. If alreadyDeclared is false, then
  1. Call env’s CreateMutableBinding concrete method passing varName as the argument.
Let lexDeclarations be the LexicalDeclarations of code.
For each element d in lexDeclarations do
1. NOTE A lexically declared name can not be the same as a function declaration, formal parameter, or a var name. Lexically declarated 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. Call env’s CreateImmutableBinding concrete method passing dn as the argument.
  2. Else,
    1. Call env’s CreateMutableBinding concrete method passing dn and false as the arguments.
For each FunctionDeclaration 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. Call env’s SetMutableBinding concrete method passing fn, fo, and false as the arguments.
NOTE Function declaration are initialised prior to parameter initialisation so that default value expressions may reference them. it is not extended code. "arguments" is not initialized until after parameter initialization.
Let ao be the result of InstantiateArgumentsObject with argument argumentsList.
NOTE If argumentsObjectNotNeeded is true then the value of ao is not directly observable to ECMAScript code and need not actually exist. In that case, its use in the above steps is strictly as a device for specifying formal parameter initialisation semantics.
Let formalStatus be the result of performing Binding Initialisation for formals with ao and undefined as arguments.
ReturnIfAbrupt(formalStatus).
If argumentsObjectNotNeeded is false, then
1. If strict is true, then
  1. Perform the abstract operation CompleteStrictArgumentsObject with argument ao.
2. Else,
  1. Perform the abstract operation CompleteMappedArgumentsObject with arguments ao, func, formals, and env.
3. Call env’s InitializeBinding concrete method passing "arguments" and ao as arguments.
Return NormalCompletion(empty).

10.5.4 Block Declaration Instantiation

NOTE When a Block or CaseBlock production is evaluated a new Declarative Environment Record is created and bindings for each block scoped variable, constant, or function declarated in the block are instantiated in the environment record.

Block Declaration Instantiation is performed as follows using arguments code and env. code is the grammar production corresponding to the body of the block. env is the declarative environment record in which bindings are to be created.

Let declarations be the LexicalDeclarations 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. Call env’s CreateImmutableBinding concrete method passing dn as the argument.
  2. Else,
    1. Call env’s CreateMutableBinding concrete method passing dn and false as the arguments.
For each FunctionDeclaration f in declarations, in list order 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. Call env’s InitializeBinding concrete method passing fn, and fo as the arguments.

10.5.5 Eval Declaration Instantiation

10.6 Arguments Object

When function code is evaluated, an arguments object is created unless (as specified in 10.5) the identifier arguments occurs as an Identifier in the function’s FormalParameterList or occurs as the BindingIdentifier of a FunctionDeclaration contained in the outermost StatementList of the function code.

The abstract operation InstantiateArgumentsObject called with an argument args performs the following steps:

Let len be the number of elements in args.
Let obj be the result of the abstract operation ObjectCreate with the intrinsic object %ObjectPrototype% as its argument.
Add the [[BuiltinBrand]] internal data property to obj with value BuiltinArguments.
Call the [[DefineOwnProperty]] internal method on obj passing "length" and the Property Descriptor {[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true} as arguments.
Let indx = len - 1.
Repeat while indx ≥ 0,
1. Let val be the element of args at 0-origined list position indx.
2. Call the [[DefineOwnProperty]] internal method on obj passing ToString(indx) and the Property Descriptor {[[Value]]: val, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true} as arguments.
3. Let indx = indx - 1
Return obj

The abstract operation CompleteStrictArgumentsObject called with argument obj which must have been previous created by the abstract operation InstantiateArgumentsObject. The following steps are performed:

Perform the AddRestrictedFunctionProperties abstract operation with argument obj.
Return.

The abstract operation CompleteMappedArgumentsObject is called with object obj, object func, grammar production formals, and environment record env. obj must have been previous created by the abstract operation InstantiateArgumentsObject.The following steps are performed:

Let len be the result of Get(obj, "length").
Let mappedNames be an empty List.
Let numberOfNonRestFormals be NumberOfParameters of formals.
Let map be the result of the abstract operation ObjectCreate with the intrinsic object %ObjectPrototype% as its argument.
Let indx = len - 1.
Repeat while indx ≥ 0,
1. If indx is less than the numberOfNonRestFormals, then
  1. Let param be getParameter of formals with argument indx.
  2. If param is a BindingIdentifier, then
    1. Let name be the sole element of BoundNames of param.
    2. If name is not an element of mappedNames, then
      1. Add name as an element of the list mappedNames.
      2. Let g be the result of calling the MakeArgGetter abstract operation with arguments name and env.
      3. Let p be the result of calling the MakeArgSetter abstract operation with arguments name and env.
      4. Call the [[DefineOwnProperty]] internal method of map passing ToString(indx) and the Property Descriptor {[[Set]]: p, [[Get]]: g, [[Configurable]]: true} as arguments.
2. Let indx = indx - 1
If mappedNames is not empty, then
1. Set the [[ParameterMap]] internal data property of obj to map.
2. Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods of obj to the definitions provided below.
Call the [[DefineOwnProperty]] internal method on obj passing "callee" and the Property Descriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true} as arguments.
Return obj

The abstract operation MakeArgGetter called with String name and environment record env creates a function object that when executed returns the value bound for name in env. It performs the following steps:

Let bodyText be the result of concatenating the Strings "return ", name, and ";".
Let body be the result of parsing bodyText using FunctionBody as the goal symbol.
Let parameters be a FormalParameterList : [empty] production.
Return the result of calling the abstract operation FunctionCreate using Normal as the kind, parameters as FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

The abstract operation MakeArgSetter called with String name and environment record env creates a function object that when executed sets the value bound for name in env. It performs the following steps:

Let paramText be the String name concatenated with the String "_arg".
Let parameters be the result of parsing paramText using FormalParameterList as the goal symbol.
Let bodyText be the String "<name> = <param>;" with <name> replaced by the value of name and <param> replaced by the value of paramText.
Let body be the result of parsing bodyText using FunctionBody as the goal symbol.
Return the result of calling the abstract operation FunctionCreate using Normal as the kind, parameters as FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P performs the following steps:

Let args be the arguments object.
Let map be the value of the [[ParameterMap]] internal data property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
If the value of isMapped is undefined, then
1. Let v be the result of calling the default ordinary object [[Get]] internal method (8.12.3) on args passing P and args as the arguments.
2. If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
3. Return v.
Else map contains a formal parameter mapping for P,
1. Return the result of calling Get(map, P).

The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P performs the following steps:

Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the arguments object passing P as the argument.
If desc is undefined then return desc.
Let map be the value of the [[ParameterMap]] internal data property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
If the value of isMapped is not undefined, then
1. Set desc.[[Value]] to the result of calling Get(map, P).
Return desc.

The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P and Property Descriptor Desc performs the following steps:

Let map be the value of the [[ParameterMap]] internal data property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.3.9) on the arguments object passing P and Desc as the arguments.
ReturnIfAbrupt(allowed).
If allowed is false, then
return false.
If the value of isMapped is not undefined, then
1. If IsAccessorDescriptor(Desc) is true, then
  1. Call the [[Delete]] internal method of map passing P as the argument.
2. Else
  1. If Desc.[[Value]] is present, then
    1. Assert: the follow Put call will always succeed because formal parameters mapped by argument objects are always writable.
    2. Call Put(map, P, Desc.[[Value]], false).
  2. If Desc.[[Writable]] is present and its value is false, then
    1. Call the [[Delete]] internal method of map passing P as the argument.
Return true.

The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property key P performs the following steps:

Let map be the value of the [[ParameterMap]] internal data property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
Let result be the result of calling the default [[Delete]] internal method for ordinary objects (8.3.10) on the arguments object passing P as the argument.
If result is true and the value of isMapped is not undefined, then
1. Call the [[Delete]] internal method of map passing P as the argument.
Return result.

NOTE 1 For non-strict mode functions the array index (defined in 15.4) data properties of an arguments 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. For strict mode functions, the values of the arguments object’s 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 accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access. The "callee" property has a more specific meaning for non-strict mode functions and a "caller" property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming ECMAScript implementations.

11 Expressions

11.1 Primary Expressions

Syntax

PrimaryExpression :

this

Identifier

Literal

ArrayInitialiser

ObjectLiteral

FunctionExpression

ClassExpression

GeneratorExpression

GeneratorComprehension

RegularExpressionLiteral

TemplateLiteral

CoverParenthesizedExpressionAndArrowParameterList

CoverParenthesizedExpressionAndArrowParameterList :

( Expression )

( )

( ... Identifier )

( Expression , ... Identifier )

Supplemental Syntax

When processing the production PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList the following grammar is used to refine the interpretation of CoverParenthesizedExpressionAndArrowParameterList.

ParenthesizedExpression :

( Expression )

Static Semantics

Static Semantics: CoveredParenthesizedExpression

CoverParenthesizedExpressionAndArrowParameterList : ( Expression )

Return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList using ParenthesizedExpression as the goal symbol.

Static Semantics: IsValidSimpleAssignmentTarget

PrimaryExpression :

this

Literal

ArrayInitialiser

ObjectLiteral

FunctionExpression

ClassExpression

GeneratorExpression

GeneratorComprehension

RegularExpressionLiteral

TemplateLiteral

Return false.

PrimaryExpression : Identifier

If this PrimaryExpression is contained in strict code and StringValue of Identifier is "eval" or "arguments", then return false.
Return true.

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return IsValidSimpleAssignmentTarget of expr.

11.1.1 The this Keyword

Runtime Semantics: Evaluation

PrimaryExpression : this

Return the result of calling the ThisResolution abstract operation.

11.1.2 Identifier Reference

Runtime Semantics: Evaluation

PrimaryExpression : Identifier

Let ref be the result of performing Identifier Resolution as specified in 10.4.1 using the IdentifierName corresponding to Identifier.
Return ref.

NOTE: The result of evaluating an Identifier is always a value of type Reference.

11.1.3 Literals

Syntax

Literal :

NullLiteral

ValueLiteral

ValueLiteral :

BooleanLiteral

NumericLiteral

StringLiteral

Runtime Semantics

Runtime Semantics: Evaluation

Literal : NullLiteral

Return null.

ValueLiteral : BooleanLiteral

Return false if BooleanLiteral is the token BooleanLiteral :: false
Return true if BooleanLiteral is the token BooleanLiteral :: true

ValueLiteral : NumericLiteral

Return the number whose value is MV of NumericLiteral as defined in 7.8.3.

ValueLiteral : StringLiteral

Return the string whose elements are the SV of StringLiteral as defined in 7.8.4.

11.1.4 Array Initialiser

Syntax

ArrayInitialiser :

ArrayLiteral

ArrayComprehension

11.1.4.1 Array Literal

NOTE An ArrayLiteral is an expression describing the initialisation 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 initialiser 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

ArrayLiteral :

[ Elision_opt ]

[ ElementList ]

[ ElementList , Elision_opt ]

ElementList :

Elision_opt AssignmentExpression

Elision_opt SpreadElement

ElementList , Elision_opt AssignmentExpression

ElementList , Elision_opt SpreadElement

Elision :

,

Elision ,

SpreadElement :

... AssignmentExpression

Static Semantics

Static Semantics: Elision Width

Elision : ,

Return the numeric value 1.

Elision : Elision ,

Let preceding be the Elision Width of Elision.
Return preceding+1.

Runtime Semantics

Runtime Semantics: Array Accumulation

With parameters array and nextIndex.

ElementList : Elision_opt AssignmentExpression

Let padding be the Elision Width 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).
ReturnIfAbrupt(initValue).
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32(nextIndex+padding)) and the Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
Assert: the above call to [[DefineOwnProperty]] will never return false or an abrupt completion value.
Return nextIndex+padding+1.

ElementList : Elision_opt SpreadElement

Let padding be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing Array Accumulation for SpreadElement with arguments array and nextIndex+padding.

ElementList : ElementList , Elision_opt AssignmentExpression

Let postIndex be the result of performing Array Accumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(postIndex).
Let padding be the Elision Width 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).
ReturnIfAbrupt(initValue).
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32(postIndex+padding)) and the Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
Assert: the above call to [[DefineOwnProperty]] will never return false or an abrupt completion value.
Return postIndex+padding+1.

ElementList : ElementList , Elision_opt SpreadElement

Let postIndex be the result of performing Array Accumulation for ElementList with arguments array and nextIndex.
ReturnIfAbrupt(postIndex).
Let padding be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing Array Accumulation for SpreadElement with arguments array and postIndex+padding.

SpreadElement : ... AssignmentExpression

Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadValue be GetValue(spreadRef).
Let spreadObj be ToObject(spreadValue).
ReturnIfAbrupt(spreadObj).
Let lenVal be the result of calling Get(spreadObj, "length").
Let spreadLen be ToUint32(lenVal).
ReturnIfAbrupt(spreadLen).
Let n=0;
Repeat, while n < spreadLen
1. Let exists be the result of HasProperty(spreadObj, ToString(n)).
2. ReturnIfAbrupt(exists).
3. If exists is true then,
  1. Let v be the result of calling the [[Get]] internal method of spreadObj passing ToString(n) as the argument.
  2. ReturnIfAbrupt(v).
  3. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32(nextIndex)) and Property Descriptor {[[Value]]: v, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  4. Assert: the above call to [[DefineOwnProperty]] will never return false or an abrupt completion value.
4. Let n = n+1.
5. Let nextIndex = nextIndex +1.
Return nextIndex.

NOTE [[DefineOwnProperty]] 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]].

Runtime Semantics: Evaluation

ArrayLiteral : [ Elision_opt ]

Let array be the result of the abstract operation ArrayCreate with argument 0.
Let pad be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Call Put(array, "length", pad, false).
Return array.

ArrayLiteral : [ ElementList ]

Let array be the result of the abstract operation ArrayCreate with argument 0.
Let len be the result of performing Array Accumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(len).
Call Put(array, "length", len, false).
Return array.

ArrayLiteral : [ ElementList , Elision_opt ]

Let array be the result of the abstract operation ArrayCreate with argument 0.
Let len be the result of performing Array Accumulation for ElementList with arguments array and 0.
ReturnIfAbrupt(len).
Let padding be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Call Put(array, "length", ToUint32(padding+len), false).
Return array.

11.1.4.2 Array Comprehension

Syntax

ArrayComprehension :

[Comprehension ]

Comprehension :

ComprehensionQualification AssignmentExpression

ComprehensionQualification :

ComprehensionFor ComprehensionQualifierList_opt

ComprehensionQualifierList :

ComprehensionQualifier

ComprehensionQualifierList ComprehensionQualifier

ComprehensionQualifier :

ComprehensionFor

ComprehensionIf

ComprehensionFor :

for ( ForBinding of AssignmentExpression )

ComprehensionIf :

if ( AssignmentExpression )

ForBinding :

BindingIdentifier

BindingPattern

Runtime Semantics

Runtime Semantics: Binding Initialisation

With arguments value and environment.

NOTE undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialisation value. This is the case for var statements formal parameter lists of non-strict functions. In those cases a lexical binding is hosted and preinitialized prior to evaluation of its initializer.

ForBinding : BindingPattern

Let obj be ToObject(value).
ReturnIfAbrupt(obj).
Return the result of performing Binding Initialisation for BindingPattern passing obj and environment as the arguments.

Runtime Semantics: Evaluation

ToDo

11.1.5 Object Initialiser

NOTE An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values need not be literals; they are evaluated each time the object initialiser is evaluated.

Syntax

ObjectLiteral :

{ }

{ PropertyDefinitionList }

{ PropertyDefinitionList , }

PropertyDefinitionList :

PropertyDefinition

PropertyDefinitionList , PropertyDefinition

PropertyDefinition :

IdentifierName

CoverInitialisedName

PropertyName : AssignmentExpression

MethodDefinition

PropertyName :

IdentifierName

StringLiteral

NumericLiteral

CoverInitialisedName :

IdentifierName Initialiser

Initialiser :

= AssignmentExpression

NOTE 1 MethodDefinition is defined in 13.3.

NOTE 2 In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitialisedName 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.

Static Semantics

Static Semantics: Early Errors

In addition to describe an actual object initialiser the ObjectLiteral productions are used as a cover grammar for ObjectAssignmentPattern (11.13.1). When ObjectLiteral appears in a context where ObjectAssignmentPattern is required, the following Early Error rules are not applied.

ObjectLiteral : { PropertyDefinitionList }

and

ObjectLiteral : { PropertyDefinitionList , }

It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains any duplicate entries, unless one of the following conditions are true for each duplicate entry:
1. The source code corresponding to PropertyDefinitionList is not strict code and all occurrences in the list of the duplicated entry were obtained from productions of the form PropertyDefinition : PropertyName : AssignmentExpression .
2. The duplicated entry occurs exactly twice in the list and one occurrence was obtained from a get accessor MethodDefinition and the other occurrence was obtained from a set accessor MethodDefinition.

PropertyDefinition : MethodDefinition

It is a Syntax Error if ReferencesSuper of MethodDefinition is true.

PropertyDefinition : IdentifierName

It is a Syntax Error if IdentifierName is a ReservedWord.

PropertyDefinition : CoverInitialisedName

Always throw a Syntax Error if this production is present

NOTE This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern (11.13.1). It can not occur in an actual object initialiser.

Static Semantics: Contains

With parameter symbol.

PropertyDefinition : MethodDefinition

If symbol is MethodDefinition, return true.
Return false.

NOTE Static semantic rules that depend upon substructure generally do not look into function definitions.

PropertyName : IdentifierName

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.

Static Semantics: IsValidSimpleAssignmentTarget

PrimaryExpression : Literal

Return false.

Static Semantics: PropName

PropertyDefinition : IdentifierName

Return StringValue of IdentifierName.

PropertyDefinition : PropertyName : AssignmentExpression

Return PropName of PropertyName.

PropertyName : StringLiteral

Return a String value whose characters are the SV of the StringLiteral.

PropertyName : NumericLiteral

Let nbr be the result of forming the value of the NumericLiteral.
Return ToString(nbr).

Static Semantics: PropertyNameList

PropertyDefinitionList : PropertyDefinition

Return a new List containing PropName of PropertyDefinition.

PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition

Let list be PropertyNameList of PropertyDefinitionList.
Append PropName of PropertyDefinition to the end of list.
Return list.

Runtime Semantics

Runtime Semantics: Evaluation

ObjectLiteral : { }

Return a new object created as if by the expression new Object() where Object is the standard built-in constructor with that name.

ObjectLiteral :

{ PropertyDefinitionList }
{ PropertyDefinitionList , }

Let obj be the result of the abstract operation ObjectCreate with the intrinsic object %ObjectPrototype% as its argument.
Let status be the result of performing Property Definition Evaluation of PropertyDefinitionList with argument obj.
ReturnIfAbrupt(status).
Return obj.

Runtime Semantics: Property Definition Evaluation

With parameter object.

PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition

Let status be the result of performing Property Definition Evaluation of PropertyDefinitionList with argument object.
ReturnIfAbrupt(status).
Return the result of performing Property Definition Evaluation of PropertyDefinition with argument object.

PropertyDefinition : IdentifierName

Let propName be StringValue of IdentifierName.
Let exprValue be the result of performing Identifier Resolution as specified in 10.3.1 using IdentifierName.
Let propValue be GetValue(exprValue).
ReturnIfAbrupt(propValue).
Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}
Return the result of DefinePropertyOrThrow(object, propName desc).

PropertyDefinition : PropertyName : AssignmentExpression

Let propName be PropName of PropertyName.
Let exprValue be the result of evaluating AssignmentExpression.
Let propValue be GetValue(exprValue).
ReturnIfAbrupt(propValue).
Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}
Return the result of DefinePropertyOrThrow(object, propName desc).

11.1.6 Function Defining Expressions

See 13.1 for PrimaryExpression : FunctionExpression .

See 13.4 for PrimaryExpression : GeneratorExpression .

See 13.5 for PrimaryExpression : ClassExpression .

11.1.7 Generator Comprehensions

Syntax

GeneratorComprehension :

( Comprehension )

11.1.8 Regular Expression Literals

Syntax

See 7.8.5.

Static Semantics

Static Semantics: Early Errors

PrimaryExpression : RegularExpressionLiteral

It is a Syntax Error if BodyText of RegularExpressionLiteral can not be recognized using the goal symbol Pattern of the ECMAScript RegExp grammar specified in 15.10.
It is a Syntax Error if FlagText of RegularExpressionLiteral contains any character other than "g", "i", "m", "u", or "y", or if it contains the same character more than once.

Runtime Semantics

Runtime Semantics: Evaluation

PrimaryExpression : RegularExpressionLiteral

A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new object is created as if by the expression new RegExp(Pattern, Flags) where RegExp is the standard built-in constructor with that name. The newly constructed object becomes the value of the RegularExpressionLiteral.

11.1.9 Template Literals

Syntax

TemplateLiteral :

NoSubstitutionTemplate

TemplateHead Expression [Lexical goal InputElementTemplateTail ] TemplateSpans

TemplateSpans :

TemplateTail

TemplateMiddleList [Lexical goal InputElementTemplateTail ] TemplateTail

TemplateMiddleList :

TemplateMiddle Expression

TemplateMiddleList [Lexical goal InputElementTemplateTail ] TemplateMiddle Expression

Static Semantics

Static Semantics: TemplateStrings

With parameter raw.

TemplateLiteral : NoSubstitutionTemplate

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.

TemplateLiteral : TemplateHead Expression [Lexical goal InputElementTemplateTail] TemplateSpans

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 element, in order of tail.

TemplateSpans : TemplateTail

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.

TemplateSpans : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateTail

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.

TemplateMiddleList : TemplateMiddle Expression

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.

TemplateMiddleList : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateMiddle Expression

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.

Runtime Semantics

Runtime Semantics: ArgumentListEvaluation

TemplateLiteral : NoSubstitutionTemplate

Let siteObj be the result of the abstract operation GetTemplateCallSite passing this TemplateLiteral production as the argument.
Return a List containing the one element which is siteObj.

TemplateLiteral : TemplateHead Expression [Lexical goal InputElementTemplateTail] TemplateSpans

Let siteObj be the result of the abstract operation GetTemplateCallSite passing this TemplateLiteral production as the argument.
Let firstSub be the result of evaluating Expression.
ReturnIfAbrupt(firstSub).
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.

Runtime Semantics: GetTemplateCallSite Abstract Operation

The abstract operation GetTemplateCallSite is called with a grammar production, templateLiteral, as an argument. It performs the following steps:

If a call site object for the source code corresponding to templateLiteral has already been created by a previous call to this abstract operation, then return that call site object.
Let cookedStrings be TemplateStrings of templateLiteral with argument false.
Let rawStrings be TemplateStrings of templateLiteral with argument true.
Let count be the number of elements in the List cookedStrings.
Let siteObj be the result of the abstract operation ArrayCreate with argument count.
Let rawObj be the result of the abstract operation ArrayCreate with argument count.
Let index be 0.
Repeat while index < count
1. Let prop be ToString(index).
2. Let cookedValue be the string value at 0-based position index of the List cookedStrings.
3. Call the [[DefineOwnProperty]] internal method of siteObj with arguments prop and Property Descriptor {[[Value]]: cookedValue, [[Writable]]: false, [[Configurable]]: false}.
4. Let rawValue be the string value at 0-based position index of the List rawStrings.
5. Call the [[DefineOwnProperty]] internal method of rawObj with arguments prop and Property Descriptor {[[Value]]: rawValue, [[Writable]]: false, [[Configurable]]: false}.
6. Let index be index+1.
Call the [[Freeze]] internal method of rawObj.
Call the [[DefineOwnProperty]] internal method of siteObj with arguments "raw" and Property Descriptor {[[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}.
Call the [[Freeze]] internal method of siteObj.
Remember an association between the source code corresponding to templateLiteral and siteObj such that siteObj can be retrieve in subsequent calls to this abstract operation.
Return siteObj.

NOTE 1 The creation of a call site object cannot result in an abrupt completion.

NOTE 2 Each TemplateLiteral in the program code is associated with a unique Template call site object that is used in the evaluation of tagged Templates (11.2.6). The same call site object is used each time a specific tagged Template is evaluated. Whether call site 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.

Runtime Semantics: SubstitutionEvaluation

TemplateSpans : TemplateTail

Return an empty List.

TemplateSpans : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateTail

Return the result of SubstitutionEvaluation of TemplateMiddleList.

TemplateMiddleList : TemplateMiddle Expression

Let sub be the result of evaluating Expression.
ReturnIfAbrupt(sub).
Return a List containing only sub.

TemplateMiddleList : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateMiddle Expression

Let preceeding be the result of SubstitutionEvaluation of TemplateMiddleList .
ReturnIfAbrupt(preceeding).
Let next be the result of evaluating Expression.
ReturnIfAbrupt(next).
Append next as the last element of the List preceeding.
Return preceeding.

Runtime Semantics: Evaluation

TemplateLiteral : NoSubstitutionTemplate

Return the string value whose elements are the TV of NoSubstitutionTemplate as defined in 7.8.6.

TemplateLiteral : TemplateHead Expression [Lexical goal InputElementTemplateTail] TemplateSpans

Let head be the TV of TemplateHead as defined in 7.8.6.
Let sub be the result of evaluating Expression.
Let middle be ToString(sub).
ReturnIfAbrupt(middle).
Let tail be the result of evaluating TemplateSpans .
ReturnIfAbrupt(tail).
Return the string value whose elements are the code units of head followed by the code units of tail.

TemplateSpans : TemplateTail

Let tail be the TV of TemplateTail as defined in 7.8.6.
Return the string whose elements are the code units of tail.

TemplateSpans : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateTail

Let head be the result of evaluating TemplateMiddleList.
ReturnIfAbrupt(head).
Let tail be the TV of TemplateTail as defined in 7.8.6.
Return the string whose elements are the elements of head followed by the elements of tail.

TemplateMiddleList : TemplateMiddle Expression

Let head be the TV of TemplateMiddle as defined in 7.8.6.
Let sub be the result of evaluating Expression.
Let middle be ToString(sub).
ReturnIfAbrupt(middle).
Return the sequence of characters consisting of the code units of head followed by the elements of middle.

TemplateMiddleList : TemplateMiddleList [Lexical goal InputElementTemplateTail] TemplateMiddle Expression

Let rest be the result of evaluating TemplateMiddleList .
ReturnIfAbrupt(rest).
Let middle be the TV of TemplateMiddle as defined in 7.8.6.
Let sub be the result of evaluating Expression.
Let last be ToString(sub).
ReturnIfAbrupt(last).
Return the sequence of characters consisting of the elements of rest followed by the code units of middle followed by the elements of last.

11.1.10 The Grouping Operator

Static Semantics: Early Errors

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

It is a Syntax Error if the lexical token sequence matched by CoverParenthesizedExpressionAndArrowParameterList cannot be parsed with no tokens left over using ParenthesizedExpression as the goal symbol.
All Early Errors rules for ParenthesizedExpression and its derived productions also apply to the CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.

Static Semantics: IsValidSimpleAssignmentTarget

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return IsValidSimpleAssignmentTarget of expr.

ParenthesizedExpression : ( Expression )

Return IsValidSimpleAssignmentTarget of Expression.

Runtime Semantics: Evaluation

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
Return the result of evaluating expr.

ParenthesizedExpression : ( Expression )

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 parenthesised expressions.

11.2 Left-Hand-Side Expressions

Syntax

MemberExpression :

[Lexical goal InputElementRegExp ] PrimaryExpression

MemberExpression [ Expression ]

MemberExpression . IdentifierName

MemberExpression TemplateLiteral

super [ Expression ]

super . IdentifierName

new super Arguments_opt

new MemberExpression Arguments

NewExpression :

MemberExpression

new NewExpression

CallExpression :

MemberExpression Arguments

super Arguments

CallExpression Arguments

CallExpression [ Expression ]

CallExpression . IdentifierName

CallExpression TemplateLiteral

Arguments :

( )

( ArgumentList )

ArgumentList :

AssignmentExpression

... AssignmentExpression

ArgumentList , AssignmentExpression

ArgumentList , ... AssignmentExpression

LeftHandSideExpression :

NewExpression

CallExpression

Static Semantics

Static Semantics: Contains

With parameter symbol.

MemberExpression : MemberExpression . IdentifierName

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.

MemberExpression : super . IdentifierName

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.

CallExpression : CallExpression . IdentifierName

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.

MemberExpression : new super

If symbol is the ReservedWord super, return true.
If symbol is the ReservedWord new, return true.
Return false.

MemberExpression : new super Arguments

If symbol is the ReservedWord super, return true.
If symbol is the ReservedWord new, return true.
Return the result of Arguments Contains symbol.

Static Semantics: IsValidSimpleAssignmentTarget

CallExpression :

CallExpression [ Expression ]

CallExpression . IdentifierName

MemberExpression :

MemberExpression [ Expression ]

MemberExpression . IdentifierName

super [ Expression ]

super . IdentifierName

Return true.

CallExpression :

MemberExpression Arguments

super Arguments

CallExpression Arguments

CallExpression TemplateLiteral

NewExpression : new NewExpression

MemberExpression :

new super Arguments_opt

new MemberExpression Arguments

Return false.

11.2.1 Property Accessors

Properties are accessed by name, using either the dot notation:

MemberExpression . IdentifierName
CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]

where <identifier-name-string> is a string literal containing the same sequence of characters after processing of Unicode escape sequences as the IdentifierName.

Runtime Semantics: Evaluation

MemberExpression : MemberExpression [ Expression ]

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be GetValue(baseReference).
ReturnIfAbrupt(baseValue).
Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be GetValue(propertyNameReference).
ReturnIfAbrupt(propertyNameValue).
Let bv be CheckObjectCoercible(baseValue).
ReturnIfAbrupt(bv).
Let propertyNameString be ToString(propertyNameValue).
If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
Return a value of type Reference whose base value is bv and whose referenced name is propertyNameString, and whose strict reference flag is strict.

CallExpression : CallExpression [ Expression ]

Is evaluated in exactly the same manner as MemberExpression : MemberExpression [ Expression ] except that the contained CallExpression is evaluated in step 1.

11.2.2 The new Operator

Runtime Semantics: Evaluation

NewExpression : new NewExpression

Let ref be the result of evaluating NewExpression.
Let constructor be GetValue(ref).
ReturnIfAbrupt(constructor).
If Type(constructor) is not Object, throw a TypeError exception.
If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
Return the result of calling the [[Construct]] internal method on constructor with an empty List as the argument.

MemberExpression : new MemberExpression Arguments

Let ref be the result of evaluating MemberExpression.
Let constructor be GetValue(ref).
ReturnIfAbrupt(constructor).
Let argList be the result of evaluating Arguments, producing an internal List of argument values (11.2.4).
ReturnIfAbrupt(argList).
If Type(constructor) is not Object, throw a TypeError exception.
If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
Return the result of calling the [[Construct]] internal method on constructor, passing argList as the argument.

11.2.3 Function Calls

Runtime Semantics: Evaluation

CallExpression : MemberExpression Arguments

Let ref be the result of evaluating MemberExpression.
If this CallExpression is in a tail position (13.7) then let tailCall be true, otherwise let tailCall be false.
Return the result of the abstract operation EvaluateCall with arguments ref, Arguments, and tailCall.

CallExpression : CallExpression Arguments

Let ref be the result of evaluating CallExpression.
If this CallExpression is in a tail position (13.7) then let tailCall be true, otherwise let tailCall be false.
Return the result of the abstract operation EvaluateCall with arguments ref, Arguments, and tailCall.

Runtime Semantics: EvaluateCall Abstract Operation

The abstract operation EvaluateCall takes as arguments a value ref, and a syntactic grammar production arguments, and a Boolean argument tailPosition. It performs the following steps:

Let func be GetValue(ref).
ReturnIfAbrupt(func).
Let argList be the result of performing 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 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 thisValue be the result of calling the WithBaseObject concrete method of GetBase(ref).
Else Type(ref) is not Reference,
1. Let thisValue be undefined.
If tailPosition is true, then
1. Let leafContext be the running execution context.
2. Suspend leafContext.
3. Pop leafContext from the execution context context stack. The execution context now on the top of the stack becomes the running execution context, however it remains in its suspended state.
4. Assert: leafContext has no further use. It will never be activated as the running execution context.
Let result be the result of calling the [[Call]] internal method on func, passing thisValue as the thisArgument and argList as the argumentsList.
Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue with the resumption of leafCallerContext as the running execution context.
Assert: Type(result) is an ECMAScript language type
Return result.

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 1 For example, a tail position call should only grow an implementation’s activication 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.

11.2.4 The super Keyword

Static Semantics

Static Semantics: Early Errors

MemberExpression :

super [ Expression ]

super . IdentifierName

new super Arguments_opt

CallExpression : super Arguments

It is a Syntax Error if the source code parsed with this production is global code that is not eval code.
It is a Syntax Error if the source code parsed with this production is eval code and the source code is not being processed by a direct call to eval that is contained in function code.

Runtime Semantics: Evaluation

MemberExpression : super [ Expression ]

Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be GetValue(propertyNameReference).
Let propertyKey be ToPropertyKey(propertyNameValue).
If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
Return the result of MakeSuperReference(propertyKey, strict).

MemberExpression : super . IdentifierName

Let propertyKey be StringValue of IdentifierName.
If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
Return the result of MakeSuperReference(propertyKey, strict).

MemberExpression : new super Arguments_opt

If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
Let ref be the result of MakeSuperReference(undefined, strict)..
Let constructor be GetValue(ref).
ReturnIfAbrupt(constructor).
If Arguments is present, then
1. Let argList be the result of evaluating Arguments, producing an internal List of argument values (11.2.4).
2. ReturnIfAbrupt(argList).
Else,
1. Let argList be a new empty List.
If Type(constructor) is not Object, throw a TypeError exception.
If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
Return the result of calling the [[Construct]] internal method on constructor, passing argList as the argument.

CallExpression : super Arguments

If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
Let ref be the result of MakeSuperReference(undefined, strict).
ReturnIfAbrupt(ref).
If this CallExpression is in a tail position (13.7) then let tailCall be true, otherwise let tailCall be false.
Return the result of the abstract operation EvaluateCall with arguments ref, Arguments, and tailCall.

Runtime Semantics: Abstract Operation MakeSuperReference(propertyKey, strict)

Let env be the result of performing the GetThisEnvironment abstract operation.
If the result of calling the HasSuperBinding concrete method of env is false, then throw a ReferenceError exception.
Let actualThis be the result of calling the GetThisBinding concrete method of env.
Let baseValue be the result of calling the GetSuperBase concrete method of env.
Let bv be CheckObjectCoercible(baseValue).
ReturnIfAbrupt(bv).
If propertyKey is undefined, then
1. Let propertyKey be the result of calling the GetMethodName concrete method of env.
Return a value of type Reference that is a Super Reference whose base value is bv, whose referenced name is propertyKey, whose thisValue is actualThis, and whose strict reference flag is strict.

11.2.5 Argument Lists

The evaluation of an argument list produces a List of values (see 8.7).

Runtime Semantics

Runtime Semantics: ArgumentListEvaluation

Arguments : ( )

Return an empty List.

ArgumentList : AssignmentExpression

Let ref be the result of evaluating AssignmentExpression.
Let arg be GetValue(ref).
ReturnIfAbrupt(arg).
Return a List whose sole item is arg.

ArgumentList : ... AssignmentExpression

Let list be an empty List.
Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadValue be GetValue(spreadRef).
Let spreadObj be ToObject(spreadValue).
ReturnIfAbrupt(spreadObj).
Let lenVal be the result of calling Get(spreadObj, "length").
Let spreadLen be ToUint32(lenVal).
ReturnIfAbrupt(spreadLen).
Let n = 0.
Repeat, while n < spreadLen
1. Let nextArg be the result of calling Get(spreadObj, ToString(n)).
2. ReturnIfAbrupt(nextArg).
3. Append nextArg as the last element of list.
4. Let n = n+1.
Return list.

ArgumentList : ArgumentList , AssignmentExpression

Let precedingArgs be the result of evaluating ArgumentList.
ReturnIfAbrupt(precedingArgs).
Let ref be the result of evaluating AssignmentExpression.
Let arg be GetValue(ref).
ReturnIfAbrupt(arg).
Return a List whose length is one greater than the length of precedingArgs and whose items are the items of precedingArgs, in order, followed at the end by arg which is the last item of the new list.

ArgumentList : ArgumentList , ... AssignmentExpression

Let precedingArgs be an empty List.
Let spreadRef be the result of evaluating AssignmentExpression.
Let spreadValue be GetValue(spreadRef).
Let spreadObj be ToObject(spreadValue).
ReturnIfAbrupt(spreadObj).
Let lenVal be the result of calling Get(spreadObj, "length").
Let spreadLen be ToUint32(lenVal).
ReturnIfAbrupt(spreadLen).
Let n = 0.
Repeat, while n < spreadLen
1. Let nextArg be the result of calling Get(spreadObj, ToString(n)).
2. ReturnIfAbrupt(nextArg).
3. Append nextArg as the last element of precedingArgs.
4. Let n = n+1.
Return precedingArgs.

11.2.6 Tagged Templates

Runtime Semantics

Runtime Semantics: Evaluation

MemberExpression : MemberExpression TemplateLiteral

Let tagRef be the result of evaluating MemberExpression.
If this MemberExpression is in a tail position (13.7) then let tailCall be true, otherwise let tailCall be false.
Return the result of the abstract operation EvaluateCall with arguments tagRef, TemplateLiteral, and tailCall.

CallExpression : CallExpression TemplateLiteral

Let tagRef be the result of evaluating CallExpression.
If this CallExpression is in a tail position (13.7) then let tailCall be true, otherwise let tailCall be false.
Return the result of the abstract operation EvaluateCall with arguments tagRef, TemplateLiteral, and tailCall.

11.3 Postfix Expressions

Syntax

PostfixExpression :

LeftHandSideExpression

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

Static Semantics

Static Semantics: Early Errors

PostfixExpression :

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

It is an early Reference Error if IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

Static Semantics: IsValidSimpleAssignmentTarget

PostfixExpression :

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

Return false.

11.3.1 Postfix Increment Operator

Runtime Semantics: Evaluation

PostfixExpression : LeftHandSideExpression [no LineTerminator here] ++

Let lhs be the result of evaluating LeftHandSideExpression.
Let oldValue be ToNumber(GetValue(lhs)).
ReturnIfAbrupt(oldValue).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 11.6.3).
Let status be PutValue(lhs, newValue).
ReturnIfAbrupt(status).
Return oldValue.

11.3.2 Postfix Decrement Operator

Runtime Semantics: Evaluation

PostfixExpression : LeftHandSideExpression [no LineTerminator here] --

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 (11.6.3).
Let status be PutValue(lhs, newValue).
ReturnIfAbrupt(status).
Return oldValue.

11.4 Unary Operators

Syntax

UnaryExpression :

PostfixExpression

delete UnaryExpression

void UnaryExpression

typeof UnaryExpression

++ UnaryExpression

-- UnaryExpression

+ UnaryExpression

- UnaryExpression

~ UnaryExpression

! UnaryExpression

Static Semantics

Static Semantics: Early Errors

UnaryExpression :

++ UnaryExpression

-- UnaryExpression

It is an early Reference Error if IsValidSimpleAssignmentTarget of UnaryExpression is false.

Static Semantics: IsValidSimpleAssignmentTarget

UnaryExpression :

delete UnaryExpression

void UnaryExpression

typeof UnaryExpression

++ UnaryExpression

-- UnaryExpression

+ UnaryExpression

- UnaryExpression

~ UnaryExpression

! UnaryExpression

Return false.

11.4.1 The delete Operator

Static Semantics: Early Errors

UnaryExpression : delete UnaryExpression

It is a Syntax Error if the UnaryExpression is contained in strict code and the derived UnaryExpression is PrimaryExpression : Identifier.
It is a Syntax Error if the derived UnaryExpression is
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
and derives a production that is used in place of UnaryExpression would produce a Syntax Error according to these rules. This rule is recursively applied.

Runtime Semantics: Evaluation

UnaryExpression : delete UnaryExpression

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. If IsStrictReference(ref) is true, then throw a SyntaxError exception.
2. Return true.
If IsPropertyReference(ref) is true, then
1. If IsSuperReference(ref), then throw a ReferenceError exception.
2. Let deleteStatus be the result of calling the [[Delete]] internal method on ToObject(GetBase(ref)), providing GetReferencedName(ref) as the argument.
3. ReturnIfAbrupt(deleteStatus).
4. If deleteStatus is false and IsStrictReference(ref) is true, then throw a TypeError exception.
5. Return true.
Else ref is a Reference to an Environment Record binding,
1. Let bindings be GetBase(ref).
2. Return the result of calling the DeleteBinding concrete method of bindings, providing GetReferencedName(ref) as the argument.

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.

11.4.2 The void Operator

Runtime Semantics: Evaluation

UnaryExpression : void UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let status be GetValue(expr).
ReturnIfAbrupt(status).
Return undefined.

NOTE GetValue must be called even though its value is not used because it may have observable side-effects.

11.4.3 The typeof Operator

Runtime Semantics: Evaluation

UnaryExpression : typeof UnaryExpression

Let val be the result of evaluating UnaryExpression.
If Type(val) is Reference, then
1. If IsUnresolvableReference(val) is true, return "undefined".
2. Let val be GetValue(val).
ReturnIfAbrupt(val).
Return a String determined by Type(val) according to { REF _Ref327355399 \h }Table 30 .

Table { SEQ Table \* ARABIC }30 — typeof Operator Results

Type of val	Result
Undefined	`"undefined"`
Null	`"object"`
Boolean	`"boolean"`
Number	`"number"`
String	`"string"`
Object (ordinary and does not implement [[Call]])	`"object"`
Object (implements [[Call]])	`"function"`
Object (exotic and does not implement [[Call]])	Implementation-defined unless explicitly specified. May not be `"undefined"`, `"boolean"`, `"number`", or `"string".`

11.4.4 Prefix Increment Operator

Runtime Semantics: Evaluation

UnaryExpression : ++ UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToNumber(GetValue(expr)).
ReturnIfAbrupt(oldValue).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 11.6.3).
Let status be PutValue(expr, newValue).
ReturnIfAbrupt(status).
Return newValue.

11.4.5 Prefix Decrement Operator

Runtime Semantics: Evaluation

UnaryExpression : -- UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToNumber(GetValue(expr)).
ReturnIfAbrupt(oldValue).
Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the - operator (see 11.6.3).
Let status be PutValue(expr, newValue).
ReturnIfAbrupt(status).
Return newValue.

11.4.6 Unary + Operator

NOTE The unary + operator converts its operand to Number type.

Runtime Semantics: Evaluation

UnaryExpression : + UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Return ToNumber(GetValue(expr)).

11.4.7 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.

Runtime Semantics: Evaluation

UnaryExpression : - UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToNumber(GetValue(expr)).
ReturnIfAbrupt(oldValue).
If oldValue is NaN, return NaN.
Return the result of negating oldValue; that is, compute a Number with the same magnitude but opposite sign.

11.4.8 Bitwise NOT Operator ( ~ )

Runtime Semantics: Evaluation

UnaryExpression : ~ UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToInt32(GetValue(expr)).
ReturnIfAbrupt(oldValue).
Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit integer.

11.4.9 Logical NOT Operator ( ! )

Runtime Semantics: Evaluation

UnaryExpression : ! UnaryExpression

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToBoolean(GetValue(expr)).
ReturnIfAbrupt(oldValue).
If oldValue is true, return false.
Return true.

11.5 Multiplicative Operators

Syntax

MultiplicativeExpression :

UnaryExpression

MultiplicativeExpression * UnaryExpression

MultiplicativeExpression / UnaryExpression

MultiplicativeExpression % UnaryExpression

Static Semantics: IsValidSimpleAssignmentTarget

MultiplicativeExpression :

MultiplicativeExpression * UnaryExpression

MultiplicativeExpression / UnaryExpression

MultiplicativeExpression % UnaryExpression

Return false.

Runtime Semantics: Evaluation

The production MultiplicativeExpression : MultiplicativeExpression @ UnaryExpression , where @ stands for one of the operators in the above definitions, is evaluated as follows:

Let left be the result of evaluating MultiplicativeExpression.
Let leftValue be GetValue(left).
ReturnIfAbrupt(leftValue).
Let right be the result of evaluating UnaryExpression.
Let rightValue be GetValue(right).
Let lnum be ToNumber(leftValue).
ReturnIfAbrupt(lnum).
Let rnum be ToNumber(rightValue).
ReturnIfAbrupt(rnum).
Return the result of applying the specified operation (*, /, or %) to lnum and rnum. See the Notes below 11.5.1, 11.5.2, 11.5.3.

11.5.1 Applying the * Operator

The * operator 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 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 or NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest 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.

11.5.2 Applying the / Operator

The / operator 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 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 round-to-nearest 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.

11.5.3 Applying the % Operator

The % operator 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. The IEEE 754 “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 round-to-nearest mode.

11.6 Additive Operators

Syntax

AdditiveExpression :

MultiplicativeExpression

AdditiveExpression + MultiplicativeExpression

AdditiveExpression - MultiplicativeExpression

Static Semantics: IsValidSimpleAssignmentTarget

AdditiveExpression :

AdditiveExpression + MultiplicativeExpression

AdditiveExpression - MultiplicativeExpression

Return false.

11.6.1 The Addition operator ( + )

NOTE The addition operator either performs string concatenation or numeric addition.

Runtime Semantics: Evaluation

AdditiveExpression : AdditiveExpression + MultiplicativeExpression

Let lref be the result of evaluating AdditiveExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lprim be ToPrimitive(lval).
ReturnIfAbrupt(lprim).
Let rprim be ToPrimitive(rval).
ReturnIfAbrupt(rprim).
If Type(lprim) is String or Type(rprim) is String, then
1. Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)
Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All standard ECMAScript 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 comparison algorithm for the relational operators (11.8.1), by using the logical-or operation instead of the logical-and operation.

11.6.2 The Subtraction Operator ( - )

Runtime Semantics: Evaluation

AdditiveExpression : AdditiveExpression - MultiplicativeExpression

Let lref be the result of evaluating AdditiveExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lnum be ToNumber(lval).
ReturnIfAbrupt(lnum).
Let rnum be ToNumber(rval).
ReturnIfAbrupt(rnum).
Return the result of applying the subtraction operation to lnum and rnum. See the note below 11.6.3.

11.6.3 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 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 round-to-nearest 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.

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).

11.7 Bitwise Shift Operators

Syntax

ShiftExpression :

AdditiveExpression

ShiftExpression << AdditiveExpression

ShiftExpression >> AdditiveExpression

ShiftExpression >>> AdditiveExpression

Static Semantics: IsValidSimpleAssignmentTarget

ShiftExpression :

ShiftExpression << AdditiveExpression

ShiftExpression >> AdditiveExpression

ShiftExpression >>> AdditiveExpression

Return false.

11.7.1 The Left Shift Operator ( << )

NOTE Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression << AdditiveExpression

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lnum be ToInt32(lval).
ReturnIfAbrupt(lnum).
Let rnum be ToUint32(rval).
ReturnIfAbrupt(rnum).
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.

11.7.2 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.

Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression >> AdditiveExpression

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lnum be ToInt32(lval).
ReturnIfAbrupt(lnum).
Let rnum be ToUint32(rval).
ReturnIfAbrupt(rnum).
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.

11.7.3 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.

Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression >>> AdditiveExpression

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lnum be ToUint32(lval).
ReturnIfAbrupt(lnum).
Let rnum be ToUint32(rval).
ReturnIfAbrupt(rnum).
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.

11.8 Relational Operators

NOTE 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

RelationalExpression :

ShiftExpression

RelationalExpression < ShiftExpression

RelationalExpression > ShiftExpression

RelationalExpression <= ShiftExpression

RelationalExpression >= ShiftExpression

RelationalExpression instanceof ShiftExpression

RelationalExpression in ShiftExpression

RelationalExpressionNoIn :

ShiftExpression

RelationalExpressionNoIn < ShiftExpression

RelationalExpressionNoIn > ShiftExpression

RelationalExpressionNoIn <= ShiftExpression

RelationalExpressionNoIn >= ShiftExpression

RelationalExpressionNoIn instanceof ShiftExpression

The semantics of the RelationalExpressionNoIn productions are the same as the RelationalExpression productions except that the contained RelationalExpressionNoIn is used in place of the contained RelationalExpression.

NOTE The “NoIn” variants are needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

Static Semantics: IsValidSimpleAssignmentTarget

RelationalExpression :

RelationalExpression < ShiftExpression

RelationalExpression > ShiftExpression

RelationalExpression <= ShiftExpression

RelationalExpression >= ShiftExpression

RelationalExpression instanceof ShiftExpression

RelationalExpression in ShiftExpression

Return false.

11.8.1 Runtime Semantics

Runtime Semantics: The Abstract Relational Comparison Algorithm

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:

ReturnIfAbrupt(x).
ReturnIfAbrupt(y).
If the LeftFirst flag is true, then
1. Let px be the result of calling ToPrimitive(x, hint Number).
2. ReturnIfAbrupt(px).
3. Let py be the result of calling ToPrimitive(y, hint Number).
4. ReturnIfAbrupt(py).
Else the order of evaluation needs to be reversed to preserve left to right evaluation
1. Let py be the result of calling ToPrimitive(y, hint Number).
2. ReturnIfAbrupt(py).
3. Let px be the result of calling ToPrimitive(x, hint Number).
4. ReturnIfAbrupt(px).
If both px and py are Strings, then
1. If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
2. If px is a prefix of py, return true.
3. Let k be the smallest nonnegative integer such that the character at position k within px is different from the character at position 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 code unit value for the character at position k within px.
5. Let n be the integer that is the code unit value for the character at position k within py.
6. If m < n, return true. Otherwise, return false.
Else,
1. Let nx be the result of calling ToNumber(px). Because px and py are primitive values evaluation order is not important.
2. Let ny be the result of calling ToNumber(py).
3. If nx is NaN, return undefined.
4. If ny is NaN, return undefined.
5. If nx and ny are the same Number value, return false.
6. If nx is +0 and ny is −0, return false.
7. If nx is −0 and ny is +0, return false.
8. If nx is +∞, return false.
9. If ny is +∞, return true.
10. If ny is −∞, return false.
11. If nx is −∞, return true.
12. 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 Step 3 differs from step 7 in the algorithm for the addition operator + (11.6.1) in using and instead of or.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering 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 String 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 normalised form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

Runtime Semantics: Evaluation

RelationalExpression : RelationalExpression < ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
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. (see 11.8.5)
ReturnIfAbrupt(r).
If r is undefined, return false. Otherwise, return r.

RelationalExpression : RelationalExpression > ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
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.

RelationalExpression : RelationalExpression <= ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
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.

RelationalExpression : RelationalExpression >= ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
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.

RelationalExpression : RelationalExpression instanceof ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Return the result of instanceofOperator(lval, rval).

The abstract operation instanceofOperator(O, C) implements the generic algorithm for determining if an object O inherits from the inheritance path defined by constructor C. This abstract operation performs the following steps:

If Type(C) is not Object, throw a TypeError exception.
Let instOfHandler be the result of GetMethod(C,@@hasInstance).
ReturnIfAbrupt(instOfHandler).
If instOfHandler is not undefined, then
1. Return the result of calling the [[Call]] internal method of instOfHandler passing C as thisArgument and a new List containing O as argumentsList.
If IsCallable(C) is false, then throw a TypeError exception.
Return the result of OrdinaryHasInstance(C, O).

NOTE Steps 5 and 6 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If a function object does not define @@hasInstance it is assumed that it implements the default instanceof semantics.

RelationalExpression : RelationalExpression in ShiftExpression

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
If Type(rval) is not Object, throw a TypeError exception.
Return the result of HasProperty(rval, ToPropertyKey(lval)).

11.9 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

EqualityExpression :

RelationalExpression

EqualityExpression == RelationalExpression

EqualityExpression != RelationalExpression

EqualityExpression === RelationalExpression

EqualityExpression !== RelationalExpression

EqualityExpressionNoIn :

RelationalExpressionNoIn

EqualityExpressionNoIn == RelationalExpressionNoIn

EqualityExpressionNoIn != RelationalExpressionNoIn

EqualityExpressionNoIn === RelationalExpressionNoIn

EqualityExpressionNoIn !== RelationalExpressionNoIn

The semantics of the EqualityExpressionNoIn productions are the same as the EqualityExpression productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are used in place of the contained EqualityExpression and RelationalExpression, respectively.

Static Semantics: IsValidSimpleAssignmentTarget

EqualityExpression :

EqualityExpression == RelationalExpression

EqualityExpression != RelationalExpression

EqualityExpression === RelationalExpression

EqualityExpression !== RelationalExpression

Return false.

11.9.1 Runtime Semantics

Runtime Semantics: The Abstract Equality Comparison Algorithm

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is the same as Type(y), then
1. Return the result of performing strict equality comparison algorithm x === y.
3. 1. −
  2. −
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 or Number 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 or Number,
return the result of the comparison ToPrimitive(x) == y.
Return false.

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 normalised form.

Runtime Semantics: The Strict Equality Comparison Algorithm

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
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.
If Type(x) is String, then
1. If x and y are exactly the same sequence of characters (same length and same characters in corresponding positions), return true.
2. Else, return false.
If Type(x) is Boolean, then
1. If x and y are both true or both false, return true.
2. Else, return false.
If x and y are the same Object value, return true.
Return false.

NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

Runtime Semantics: Evaluation

EqualityExpression : EqualityExpression == RelationalExpression

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Return the result of performing abstract equality comparison algorithm rval == lval.

EqualityExpression : EqualityExpression != RelationalExpression

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let r be the result of performing abstract equality comparison algorithm rval == lval.
If r is true, return false. Otherwise, return true.

EqualityExpression : EqualityExpression === RelationalExpression

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval)
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Return the result of performing the strict equality comparison algorithm rval === lval.

EqualityExpression : EqualityExpression !== RelationalExpression

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let r be the result of performing strict equality comparison algorithm rval === lval.
If r is true, return false. Otherwise, return true.

11.10 Binary Bitwise Operators

Syntax

BitwiseANDExpression :

EqualityExpression

BitwiseANDExpression & EqualityExpression

BitwiseANDExpressionNoIn :

EqualityExpressionNoIn

BitwiseANDExpressionNoIn & EqualityExpressionNoIn

BitwiseXORExpression :

BitwiseANDExpression

BitwiseXORExpression ^ BitwiseANDExpression

BitwiseXORExpressionNoIn :

BitwiseANDExpressionNoIn

BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn

BitwiseORExpression :

BitwiseXORExpression

BitwiseORExpression | BitwiseXORExpression

BitwiseORExpressionNoIn :

BitwiseXORExpressionNoIn

BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

Static Semantics: IsValidSimpleAssignmentTarget

BitwiseANDExpression : BitwiseANDExpression & EqualityExpression

BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression

BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression

Return false.

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).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating B.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let lnum be ToInt32(lval).
ReturnIfAbrupt(lnum).
Let rnum be ToInt32(rval).
ReturnIfAbrupt(rnum).
Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32 bit integer.

11.11 Binary Logical Operators

Syntax

LogicalANDExpression :

BitwiseORExpression

LogicalANDExpression && BitwiseORExpression

LogicalANDExpressionNoIn :

BitwiseORExpressionNoIn

LogicalANDExpressionNoIn && BitwiseORExpressionNoIn

LogicalORExpression :

LogicalANDExpression

LogicalORExpression || LogicalANDExpression

LogicalORExpressionNoIn :

LogicalANDExpressionNoIn

LogicalORExpressionNoIn || LogicalANDExpressionNoIn

The semantics of the LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are the same manner as the LogicalANDExpression and LogicalORExpression productions except that the contained LogicalANDExpressionNoIn, BitwiseORExpressionNoIn and LogicalORExpressionNoIn are used in place of the contained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.

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.

Static Semantics: IsValidSimpleAssignmentTarget

LogicalANDExpression : LogicalANDExpression && BitwiseORExpression

LogicalORExpression : LogicalORExpression || LogicalANDExpression

Return false.

Runtime Semantics: Evaluation

LogicalANDExpression : LogicalANDExpression && BitwiseORExpression

Let lref be the result of evaluating LogicalANDExpression.
Let lval be GetValue(lref).
Let lbool be ToBoolean(lval).
ReturnIfAbrupt(lbool).
If lbool is false, return lval.
Let rref be the result of evaluating BitwiseORExpression.
Return GetValue(rref).

LogicalORExpression : LogicalORExpression || LogicalANDExpression

Let lref be the result of evaluating LogicalORExpression.
Let lval be GetValue(lref).
Let lbool be ToBoolean(lval).
ReturnIfAbrupt(lbool).
If lbool is true, return lval.
Let rref be the result of evaluating LogicalANDExpression.
Return GetValue(rref).

11.12 Conditional Operator ( ? : )

Syntax

ConditionalExpression :

LogicalORExpression

LogicalORExpression ? AssignmentExpression : AssignmentExpression

ConditionalExpressionNoIn :

LogicalORExpressionNoIn

LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

The semantics of the ConditionalExpressionNoIn production is the same as the ConditionalExpression production except that the contained LogicalORExpressionNoIn, AssignmentExpression and AssignmentExpressionNoIn are used in place of the contained LogicalORExpression, first AssignmentExpression and second AssignmentExpression, respectively.

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit 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.

Static Semantics: IsValidSimpleAssignmentTarget

ConditionalExpression : LogicalORExpression ? AssignmentExpression : AssignmentExpression

Return false.

Runtime Semantics: Evaluation

ConditionalExpression : LogicalORExpression ? AssignmentExpression : AssignmentExpression

Let lref be the result of evaluating LogicalORExpression.
Let lval be ToBoolean(GetValue(lref)).
ReturnIfAbrupt(lval).
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).

11.13 Assignment Operators

Syntax

AssignmentExpression :

ConditionalExpression

YieldExpression

ArrowFunction

LeftHandSideExpression = AssignmentExpression

LeftHandSideExpression AssignmentOperator AssignmentExpression

AssignmentExpressionNoIn :

ConditionalExpressionNoIn

YieldExpression

ArrowFunction

LeftHandSideExpression = AssignmentExpressionNoIn

LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn

AssignmentOperator : one of

<<=

>>=

>>>=

The semantics of the AssignmentExpressionNoIn productions are the same manner as the AssignmentExpression productions except that the contained ConditionalExpressionNoIn and AssignmentExpressionNoIn are used in place of the contained ConditionalExpression and AssignmentExpression, respectively.

Static Semantics

Static Semantics: Early Errors

AssignmentExpression : LeftHandSideExpression = AssignmentExpression

It is a Syntax Error if LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if the lexical token sequence matched by LeftHandSideExpression cannot be parsed with no tokens left over using AssignmentPattern as the goal symbol.
If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if the lexical token sequence matched by LeftHandSideExpression can be parsed with no tokens left over using AssignmentPattern as the goal symbol then the following rules are not applied. Instead, the Early Error rules for AssignmentPattern are used.
It is a Syntax Error if LeftHandSideExpression is an Identifier that can be statically determined to always resolve to a declarative environment record binding and the resolved binding is an immutable binding.
It is an early Reference Error if LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral and IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression

It is a Syntax Error if the LeftHandSideExpression is an Identifier that can be statically determined to always resolve to a declarative environment record binding and the resolved binding is an immutable binding.
It is an early Reference Error if IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.

Static Semantics: IsValidSimpleAssignmentTarget

AssignmentExpression :

YieldExpression

ArrowFunction

LeftHandSideExpression = AssignmentExpression

LeftHandSideExpression AssignmentOperator AssignmentExpression

Return false.

Runtime Semantics

Runtime Semantics: Evaluation

AssignmentExpression : LeftHandSideExpression = AssignmentExpression

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. Let status be PutValue(lref, rval).
6. ReturnIfAbrupt(status).
7. Return rval.
Let AssignmentPattern be the parse of the source code corresponding to LeftHandSideExpression using AssignmentPattern as the goal symbol.
Let rref be the result of evaluating AssignmentExpression.
Let rval be ToObject(GetValue(rref)).
ReturnIfAbrupt(rval).
Let status be the result of performing Destructuring Assignment Evaluation of AssignmentPattern using rval as the argument.
ReturnIfAbrupt(status).
Return rval.

AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression

Let lref be the result of evaluating LeftHandSideExpression.
Let lval be GetValue(lref).
ReturnIfAbrupt(lval).
Let rref be the result of evaluating AssignmentExpression.
Let rval be GetValue(rref).
ReturnIfAbrupt(rval).
Let operator be the @ where AssignmentOperator is @=
Let r be the result of applying operator @ to lval and rval.
Let status be PutValue(lref, r).
ReturnIfAbrupt(status).
Return r.

NOTE When an assignment occurs within strict mode code, it is an runtime error if lref in step 1.e of the first algorithm or step 9 of the second algorithm it is an unresolvable reference. If it is, a ReferenceError exception is thrown. The LeftHandSide 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.

11.13.1 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing the production AssignmentExpression : LeftHandSideExpression = AssignmentExpression the following grammar is used to refine the interpretation of LeftHandSideExpression.

AssignmentPattern :

ObjectAssignmentPattern

ArrayAssignmentPattern

ObjectAssignmentPattern :

{ }

{ AssignmentPropertyList }

{ AssignmentPropertyList , }

ArrayAssignmentPattern :

[ Elision_opt AssignmentRestElement_opt ]

[ AssignmentElementList ]

[ AssignmentElementList , Elision_opt AssignmentRestElement_opt ]

AssignmentPropertyList :

AssignmentProperty

AssignmentPropertyList , AssignmentProperty

AssignmentElementList :

Elision_opt AssignmentElement

AssignmentElementList , Elision_opt AssignmentElement

AssignmentProperty :

Identifier Initialiser_opt

PropertyName : AssignmentElement

AssignmentElement :

DestructuringAssignmentTarget Initialiser_opt

AssignmentRestElement :

... DestructuringAssignmentTarget

DestructuringAssignmentTarget :

LeftHandSideExpression

Static Semantics

Static Semantics: Early Errors

AssignmentProperty : Identifier Initialiser_opt

It is a Syntax Error if Identifier is the Identifier eval or the Identifier arguments.
It is a Syntax Error if Identifier does not statically resolve to a declarative environment record binding or if the resolved binding is an immutable binding.

DestructuringAssignmentTarget : LeftHandSideExpression

It is a Syntax Error LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral and if the lexical token sequence matched by LeftHandSideExpression cannot be parsed with no tokens left over using AssignmentPattern as the goal symbol.
It is a Syntax Error if LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral and IsValidSimpleAssignmentTarget of LeftHandSideExpression is false.
It is a Syntax Error if the LeftHandSideExpression is an Identifier that can be statically determined to always resolve to a declarative environment record binding and the resolved binding is an immutable binding.
It is a Syntax Error if LeftHandSideExpression is the Identifier eval or the Identifier arguments.
It is a Syntax Error if IsInvalidAssignmentPattern of LeftHandSideExpression is true.
It is a Syntax Error if the LeftHandSideExpression is CoverParenthesizedExpressionAndArrowParameterList : ( Expression ) and Expression derived a production that would produce a Syntax Error according to these rules. This rule is recursively applied.

Runtime Semantics

Runtime Semantics: Destructuring Assignment Evaluation

with parameter obj

ObjectAssignmentPattern : { }

and

ArrayAssignmentPattern :

[]

[Elision]

Return NormalCompletion(empty).

AssignmentPropertyList : AssignmentPropertyList , AssignmentProperty

Let status be

the result of performing Destructuring Assignment Evaluation for AssignmentPropertyList using obj as the argument.
ReturnIfAbrupt(status).
Return the result of performing Destructuring Assignment Evaluation for AssignmentProperty using obj as the argument.

AssignmentProperty : Identifier Initialiser_opt

Let P be StringValue of Identifier.
Let v be the result of calling Get(obj, P).
ReturnIfAbrupt(v).
If Initialiser_opt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initialiser.
2. Let v be ToObject(GetValue(defaultValue)).
ReturnIfAbrupt(v).
Let lref be the result of performing Identifier Resolution(10.3.1) with the IdentifierName corresponding to Identifier.
Return PutValue(lref,v).

AssignmentProperty : PropertyName : AssignmentElement

Let name be PropName of PropertyName.
Return the result of performing Keyed Destructuring Assignment Evaluation of AssignmentElement with obj and name as the arguments.

ArrayAssignmentPattern : [ Elision_opt AssignmentRestElement ]

Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing Indexed Destructuring Assignment Evaluation of AssignmentRestElement with obj and skip as the arguments.

ArrayAssignmentPattern : [ AssignmentElementList ]

Return the result of performing Indexed Destructuring Assignment Evaluation of AssignmentElementList using obj and 0 as the arguments.

ArrayAssignmentPattern : [ AssignmentElementList , Elision_opt AssignmentRestElement_opt ]

Let lastIndex be the result of performing Indexed Destructuring Assignment Evaluation of AssignmentElementList using obj and 0 as the arguments.
ReturnIfAbrupt(lastIndex).
Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
If AssignmentRestElement is present, then return the result of performing Indexed Destructuring Assignment Evaluation of AssignmentRestElement with obj and lastIndex+skip as the arguments.
Return lastIndex.

Runtime Semantics: Indexed Destructuring Assignment Evaluation

with parameters obj and index

AssignmentElementList : Elision _opt AssignmentElement

Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Let name be ToString(index+skip).
Let status be the result of performing Keyed Destructuring Assignment Evaluation of AssignmentElement with obj and name as the arguments.
ReturnIfAbrupt(status).
Return index+skip+1.

AssignmentElementList : AssignmentElementList , Elision _opt AssignmentElement

Let listNext be the result of performing Indexed Destructuring Assignment Evaluation of AssignmentElementList using obj as the obj parameter and index as the index parameter
Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
ReturnIfAbrupt(listNext).
Let name be ToString(listNext+skip).
Let status be the result of performing Keyed Destructuring Assignment Evaluation of AssignmentElement with obj and name as the arguments.
ReturnIfAbrupt(status).
Return listNext+skip+1.

AssignmentRestElement : ... DestructuringAssignmentTarget

Let lref be the result of evaluating DestructuringAssignmentTarget.
ReturnIfAbrupt(lref).
Let lenVal be the result of Get(obj, "length").
Let len be ToUint32(lenVal).
ReturnIfAbrupt(len).
Let A be the result of the abstract operation ArrayCreate with argument 0.
Let n=0;
Repeat, while index < len
1. Let P be ToString(index).
2. Let exists be the result of HasProperty(obj, P).
3. ReturnIfAbrupt(exists).
4. If exists is true, then
  1. Let v be the result of Get(obj, ToString(index)).
  2. ReturnIfAbrupt(len).
  3. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n) and Property Descriptor {[[Value]]: v, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
5. Let n = n+1.
6. Let index = index+1.
Return PutValue(lref,A).

Runtime Semantics: Keyed Destructuring Assignment Evaluation

with parameters obj and propertyName

AssignmentElement : DestructuringAssignmentTarget Initialiser_opt

Let v be the result of Get(obj, propertyName).
ReturnIfAbrupt(v).
If Initialiser_opt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initialiser.
2. Let v be GetValue(defaultValue)
If DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral then
1. Let AssignmentPattern be the parse of the source code corresponding to DestructuringAssignmentTarget using AssignmentPattern as the goal symbol
2. Let vObj be ToObject(v).
3. ReturnIfAbrupt(vObj).
4. Return the result of performing Destructuring Assignment Evaluation of AssignmentPattern with vObj as the argument.
ReturnIfAbrupt(v).
Let lref be the result of evaluating DestructuringAssignmentTarget.
Return PutValue(lref,v).

11.14 Comma Operator ( , )

Syntax

Expression :

AssignmentExpression

Expression , AssignmentExpression

ExpressionNoIn :

AssignmentExpressionNoIn

ExpressionNoIn , AssignmentExpressionNoIn

The semantics of the ExpressionNoIn production is the same manner as the Expression production except that the contained ExpressionNoIn and AssignmentExpressionNoIn are used in place of the contained Expression and AssignmentExpression, respectively.

Static Semantics: IsValidSimpleAssignmentTarget

Expression : Expression , AssignmentExpression

Return false.

Runtime Semantics: Evaluation

Expression : Expression , AssignmentExpression

Let lref be the result of evaluating Expression.
ReturnIfAbrupt(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.

12 Statements and Declarations

Syntax

Statement :

BlockStatement

VariableStatement

EmptyStatement

ExpressionStatement

IfStatement

BreakableStatement

ContinueStatement

BreakStatement

ReturnStatement

WithStatement

LabelledStatement

ThrowStatement

TryStatement

DebuggerStatement

Declaration :

FunctionDeclaration

GeneratorDeclaration

ClassDeclaration

LexicalDeclaration

BreakableStatement :

IterationStatement

SwitchStatement

Static Semantics

Static Semantics: VarDeclaredNames

Statement :

EmptyStatement

ExpressionStatement

ContinueStatement

BreakStatement

ReturnStatement

ThrowStatement

DebuggerStatement

Return a new empty List.

Runtime Semantics

Runtime Semantics: Labelled Evaluation

With argument labelSet.

BreakableStatement : IterationStatement

Let stmtResult be the result of performing Labelled Evaluation of IterationStatement with argument labelSet.
If stmtResult.[[type]] is break and stmtResult.[[target]] is empty, then
1. If stmtResult.[[value]] is empty, then let stmtResult be NormalCompletion(undefined).
2. Else, let stmtResult be NormalCompletion(stmtResult.[[value]])
Return stmtResult.

BreakableStatement : SwitchStatement

Let stmtResult be the result of evaluating SwitchStatement.
If stmtResult.[[type]] is break and stmtResult.[[target]] is empty, then
1. If stmtResult.[[value]] is empty, then let stmtResult be NormalCompletion(undefined).
2. Else, let stmtResult be NormalCompletion(stmtResult.[[value]])
Return stmtResult.

NOTE A BreakableStatement is one that can be exited via an unlabelled BreakStatement.

Runtime Semantics: Evaluation

BreakableStatement :

IterationStatement

SwitchStatement

Let newLabelSet be a new empty List.
Return the result of performing Labelled Evaluation of this BreakableStatement with argument newLabelSet.

12.1 Block

Syntax

BlockStatement :

Block

Block :

{ StatementList_opt }

StatementList :

StatementListItem

StatementList StatementListItem

StatementListItem :

Statement

Declaration

Static Semantics

Static Semantics: Early Errors

Block : { StatementList }

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.

Static Semantics: LexicalDeclarations

StatementList : StatementList StatementListItem

Let declarations be LexicalDeclarations of StatementList.
Append to declarations the elements of the LexicalDeclarations of StatementListItem.
Return declarations.

StatementListItem : Statement

Return a new empty List.

StatementListItem : Declaration

Return a new List containing Declaration.

Static Semantics: LexicallyDeclaredNames

Block : { }

Return a new empty List.

StatementList : StatementList StatementListItem

Let names be LexicallyDeclaredNames of StatementList.
Append to names the elements of the LexicallyDeclaredNames of StatementListItem.
Return names.

StatementListItem : Statement

Return a new empty List.

StatementListItem : Declaration

Return the BoundNames of Declaration.

Static Semantics: TopLevelLexicallyDeclaredNames

StatementList : StatementList StatementListItem

Let names be TopLevelLexicallyDeclaredNames of StatementList.
Append to names the elements of the TopLevelLexicallyDeclaredNames of StatementListItem.
Return names.

StatementListItem : Statement

Return a new empty List.

StatementListItem : Declaration

If Declaration is Declaration : FunctionDeclaration , then return a new empty List.
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.

Static Semantics: TopLevelLexicallyScopedDeclarations

StatementList : StatementList StatementListItem

Let declarations be TopLevelLexicallyScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelLexicallyScopedDeclarations of StatementListItem.
Return declarations.

StatementListItem : Statement

Return a new empty List.

StatementListItem : Declaration

If Declaration is Declaration : FunctionDeclaration , then return a new empty List.
Return a new List containing Declaration.

Static Semantics: TopLevelVarDeclaredNames

StatementList : StatementList StatementListItem

Let names be TopLevelVarDeclaredNames of StatementList.
Append to names the elements of the TopLevelVarDeclaredNames of StatementListItem.
Return names.

StatementListItem : Declaration

If Declaration is Declaration : FunctionDeclaration , then return the LexicallyDeclaredNames of Declaration.
Return a new empty List.

StatementListItem : Statement

Return VarDeclaredNames of Statement.

NOTE At the top level of a function or script, inner function declarations are treated like var declarations.

Static Semantics: TopLevelVarScopedDeclarations

StatementList : StatementList StatementListItem

Let declarations be TopLevelVarScopedDeclarations of StatementList.
Append to declarations the elements of the TopLevelVarScopedDeclarations of StatementListItem.
Return declarations.

StatementListItem : Statement

If Statement is Statement : VariableStatement , then return a new List containing VariableStatement.
Return a new empty List.

StatementListItem : Declaration

If Declaration is Declaration : FunctionDeclaration , then return a new List containing Declaration.
Return a new empty List.

Static Semantics: VarDeclaredNames

Block : { }

Return a new empty List.

StatementList : StatementList StatementListItem

Let names be VarDeclaredNames of StatementList.
Append to names the elements of the VarDeclaredNames of StatementListItem.
Return names.

StatementListItem : Declaration

Return a new empty List.

Runtime Semantics

Runtime Semantics: Evaluation

Block : { }

Return NormalCompletion(undefined).

Block : { StatementList }

Let oldEnv be the running execution context’s LexicalEnvironment.
Let blockEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
Perform Block Declaration Instantiation using StatementList and 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.
If blockValue.[[type]] is normal and blockValue.[[value]] is empty, then
1. Return NormalCompletion(undefined).
Return blockValue.

NOTE No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

StatementList : StatementList StatementListItem

Let sl be the result of evaluating StatementList.
ReturnIfAbrupt(sl).
Let s be the result of evaluating StatementListItem.
If s.[[type]] is throw, return s.
If s.[[value]] is empty, let V = sl.[[value]], otherwise let V = s.[[value]].
Return Completion {[[type]]: s.[[type]], [[value]]: V, [[target]]: s.[[target]]}.

NOTE Steps 4 and 5 of the above algoritm ensure that the value of a StatementList is the value of the last value producing Statement 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;")

12.2 Declarations and the Variable Statement

12.2.1 Let and Const Declarations

NOTE A 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 Initialiser is assigned the value of its Initialiser’s AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an an Initialiser the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

LexicalDeclaration :

LetOrConst BindingList ;

LexicalDeclarationNoIn :

LetOrConst BindingListNoIn

LetOrConst :

let

const

BindingList :

LexicalBinding

BindingList , LexicalBinding

BindingListNoIn :

LexicalBindingNoIn

BindingListNoIn , LexicalBindingNoIn

LexicalBinding :

BindingIdentifier Initialiseropt

BindingPattern Initialiser

LexicalBindingNoIn :

BindingIdentifier InitialiserNoInopt

BindingPattern InitialiserNoIn

BindingIdentifier :

Identifier

InitialiserNoIn :

= AssignmentExpressionNoIn

The semantics of the LexicalDeclarationNoIn, BindingListNoIn, LexicalBindingNoIn and InitialiserNoIn productions are the same as the LexicalDeclaration, BindingList, LexicalBinding and Initialiser productions except that the contained BindingListNoIn, LexicalBindingNoIn, InitialiserNoIn and AssignmentExpressionNoIn are used in place of the contained BindingList, LexicalBinding, Initialiser and AssignmentExpression, respectively.

Static Semantics

Static Semantics: Early Errors

LexicalBinding : BindingIdentifier

It is a Syntax Error if IsConstantDeclaration of the LexicalDeclaration containing this production is true.

BindingIdentifier : Identifier

It is a Syntax Error if the BindingIdentifier is contained in strict code and if the Identifier is eval or arguments.

Static Semantics: BoundNames

LexicalDeclaration : LetOrConst BindingList ;

Return the BoundNames of BindingList.

BindingList : BindingList , LexicalBinding

Let names be the BoundNames of BindingList.
Append to names the elements of the BoundNames of LexicalBinding.
Return names.

LexicalBinding : BindingIdentifier Initialiser_opt

Return the BoundNames of BindingIdentifier.

LexicalBinding : BindingPattern Initialiser

Return the BoundNames of BindingPattern.

BindingIdentifier : Identifier

Return a new List containing the StringValue of Identifier.

Static Semantics: IsConstantDeclaration

LexicalDeclaration : LetOrConst BindingList ;

Return IsConstantDeclaration of LetOrConst.

LetOrConst : let

Return false.

LetOrConst : const

Return true.

Runtime Semantics

Runtime Semantics: Binding Initialisation

With arguments value and environment.

BindingIdentifier : Identifier

If environment is not undefined, then
1. Let name be StringValue of Identifier.
2. Let env be the environment record component of environment.
3. Call the InitializeBinding concrete method of env passing name and value as the arguments.
4. Return NormalCompletion(undefined).
Else
1. Let lhs be the result of evaluating Identifier as described in 11.1.2.
2. Return PutValue(lhs, value).

Runtime Semantics: Evaluation

LexicalDeclaration : LetOrConst BindingList ;

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return NormalCompletion(empty).

BindingList : BindingList , LexicalBinding

Let next be the result of evaluating BindingList.
ReturnIfAbrupt(next).
Return the result of evaluating LexicalBinding.

LexicalBinding : BindingIdentifier

Let env be the running execution context’s LexicalEnvironment.
Return the result of performing Binding Initialisation for BindingIdentifier passing undefined and env as the arguments.

NOTE A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

LexicalBinding : BindingIdentifier Initialiser

Let rhs be the result of evaluating Initialiser.
Let value be GetValue(rhs).
ReturnIfAbrupt(value).
Let env be the running execution context’s LexicalEnvironment.
Return the result of performing Binding Initialisation for BindingIdentifier passing value and env as the arguments.

LexicalBinding : BindingPattern Initialiser

Let rhs be the result of evaluating Initialiser.
Let value be ToObject(GetValue(rhs)).
ReturnIfAbrupt(value).
Let env be the running execution context’s LexicalEnvironment.
Return the result of performing Binding Initialisation for BindingPattern using value and env as the arguments.

12.2.2 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 initialised to undefined when created. Within the scope of any VariableEnvironemnt a common Identifier may appear in more than one VariableDeclaration but those declarations collective define only one variable. A variable defined by a VariableDeclaration with an Initialiser is assigned the value of its Initialiser’s AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

VariableStatement :

var VariableDeclarationList ;

VariableDeclarationList :

VariableDeclaration

VariableDeclarationList , VariableDeclaration

VariableDeclarationListNoIn :

VariableDeclarationNoIn

VariableDeclarationListNoIn , VariableDeclarationNoIn

VariableDeclaration :

BindingIdentifier Initialiseropt

BindingPattern Initialiser

VariableDeclarationNoIn :

BindingIdentifier InitialiserNoInopt

BindingPattern InitialiserNoIn

The semantics of the VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are the same as the VariableDeclarationList, VariableDeclaration and Initialiser productions except that the contained VariableDeclarationListNoIn, VariableDeclarationNoIn, InitialiserNoIn and AssignmentExpressionNoIn are used in of the contained VariableDeclarationList, VariableDeclaration, Initialiser and AssignmentExpression, respectively.

Static Semantics

Static Semantics: BoundNames

VariableDeclarationList : VariableDeclarationList , VariableDeclaration

Let names be BoundNames of VariableDeclarationList.
Append to names the elements of BoundNames of VariableDeclaration.
Return names.

VariableDeclaration : BindingIdentifier Initialiser_opt

Return the BoundNames of BindingIdentifier.

VariableDeclaration : BindingPattern Initialiser

Return the BoundNames of BindingPattern.

Runtime Semantics

Runtime Semantics: Binding Initialisation

With arguments value and environment.

VariableDeclaration : BindingIdentifier

Return the result of performing Binding Initialisation for BindingIdentifier passing value and undefined as the arguments.

VariableDeclaration : BindingIdentifier Initialiser

Return the result of performing Binding Initialisation for BindingIdentifier passing value and undefined as the arguments.

VariableDeclaration : BindingPattern Initialiser

Return the result of performing Binding Initialisation for BindingPattern passing value and undefined as the arguments.

Runtime Semantics: Evaluation

VariableStatement : var VariableDeclarationList ;

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return NormalCompletion( empty).

VariableDeclarationList : VariableDeclarationList , VariableDeclaration

Let next be the result of evaluating VariableDeclarationList.
ReturnIfAbrupt(next).
Return the result of evaluating VariableDeclaration.

VariableDeclaration : BindingIdentifier

Return NormalCompletion(empty).

VariableDeclaration : BindingIdentifier Initialiser

Let rhs be the result of evaluating Initialiser.
Let value be GetValue(rhs).
ReturnIfAbrupt(value).
Return the result of performing Binding Initialisation for BindingIdentifier passing value and undefined as the arguments.

NOTE If a VariableDeclaration is nested within a with statement and the Identifier in the VariableDeclaration is the same as a property name of the binding object of the with statement’s object environment record, then step 3 will assign value to the property instead of to the VariableEnvironment binding of the Identifier.

VariableDeclaration : BindingPattern Initialiser

Let rhs be the result of evaluating Initialiser.
Let rval be ToObject(GetValue(rhs)).
ReturnIfAbrupt(rval).
Return the result of performing Binding Initialisation for BindingPattern passing rval and undefined as arguments.

12.2.4 Destructuring Binding Patterns

Syntax

BindingPattern :

ObjectBindingPattern

ArrayBindingPattern

ObjectBindingPattern :

{ }

{ BindingPropertyList }

{ BindingPropertyList , }

ArrayBindingPattern :

[ Elision_opt BindingRestElement_opt ]

[ BindingElementList ]

[ BindingElementList , Elision_opt BindingRestElement_opt ]

BindingPropertyList :

BindingProperty

BindingPropertyList , BindingProperty

BindingElementList :

Elision_opt BindingElement

BindingElementList , Elision_opt BindingElement

BindingProperty :

SingleNameBinding

PropertyName : BindingElement

BindingElement :

SingleNameBinding

BindingPattern Initialiser_opt

SingleNameBinding :

BindingIdentifier Initialiser_opt

BindingRestElement :

... BindingIdentifier

Static Semantics

Static Semantics: Early Errors

BindingPattern : ObjectBindingPattern

It is a Syntax Error if the BoundNames of ObjectBindingPattern contains the string “eval” or the string “arguments”.

BindingPattern : ArrayBindingPattern

It is a Syntax Error if the BoundNames of ArrayBindingPattern contains the string “eval” or the string “arguments”.

Static Semantics: BoundNames

ObjectBindingPattern : { }

Return an empty List.

ArrayBindingPattern : [ Elision_opt ]

Return an empty List.

ArrayBindingPattern : [ Elision_opt BindingRestElement ]

Return the BoundNames of BindingRestElement.

ArrayBindingPattern : [ BindingElementList , Elision_opt ]

Return the BoundNames of BindingElementList.

ArrayBindingPattern : [ BindingElementList , Elision_opt BindingRestElement ]

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingRestElement.
Return names.

BindingPropertyList : BindingPropertyList , BindingProperty

Let names be BoundNames of BindingPropertyList.
Append to names the elements of BoundNames of BindingProperty.
Return names.

BindingElementList : Elision_opt BindingElement

Return BoundNames of BindingElement.

BindingElementList : BindingElementList , Elision_opt BindingElement

Let names be BoundNames of BindingElementList.
Append to names the elements of BoundNames of BindingElement.
Return names.

BindingProperty : PropertyName : BindingElement

Return the BoundNames of BindingElement.

SingleNameBinding : BindingIdentifier Initialiser_opt

Return the BoundNames of BindingIdentifier.

BindingElement : BindingPattern Initialiser_opt

Return the BoundNames of BindingPattern.

Static Semantics: HasInitialiser

BindingElement : BindingPattern

Return false.

BindingElement : BindingPattern Initialiser

Return true.

SingleNameBinding : BindingIdentifier

Return false.

SingleNameBinding : BindingIdentifier Initialiser

Return true.

Runtime Semantics

Runtime Semantics: Binding Initialisation

With parameters value and environment.

NOTE When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialisation 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.

BindingPattern : ObjectBindingPattern

Assert: Type(value) is Object
Return the result of performing Binding Initialisation for ObjectBindingPattern using value and environment as arguments.

BindingPattern : ArrayBindingPattern

Assert: Type(value) is Object
Return the result of performing Indexed Binding Initialisation for ArrayBindingPattern using value, 0, and environment as arguments.

ObjectBindingPattern : { }

Return NormalCompletion(empty).

BindingPropertyList : BindingPropertyList , BindingProperty

Let status be the result of performing Binding Initialisation for BindingPropertyList using value and environment as arguments.
ReturnIfAbrupt(status).
Return the result of performing Binding Initialisation for BindingProperty using value and environment as arguments.

BindingProperty : SingleNameBinding

Let name be the string that is the only element of BoundNames of SingleNameBinding.
Return the result of performing Keyed Binding Initialisation for SingleNameBinding using value, environment, and name as the arguments.

BindingProperty : PropertyName : BindingElement

Let P be the PropName of PropertyName
Return the result of performing Keyed Binding Initialisation for BindingElement using value, environment, and P as arguments.

Runtime Semantics: Indexed Binding Initialisation

With parameters array, nextIndex, and environment.

ArrayBindingPattern : [ Elision_opt ]

Return NormalCompletion(empty).

ArrayBindingPattern : [ Elision_opt BindingRestElement ]

Let nextIndex be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing Indexed Binding Initialisation for BindingRestElement using array, nextIndex, and environment as arguments.

ArrayBindingPattern : [ BindingElementList ]

Return the result of performing Indexed Binding Initialisation for BindingElementList using array, nextIndex, and environment as arguments.

ArrayBindingPattern : [ BindingElementList , Elision_opt ]

Return the result of performing Indexed Binding Initialisation for BindingElementList using array, nextIndex, and environment as arguments.

ArrayBindingPattern : [ BindingElementList , Elision_opt BindingRestElement ]

Let next be the result of performing Indexed Binding Initialisation for BindingElementList using array , nextIndex, and environment as arguments.
ReturnIfAbrupt(next).
Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Return the result of performing Indexed Binding Initialisation for BindingRestElement using array, next+skip , and environment as arguments.

BindingElementList : Elision _opt BindingElement

Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Let status be the result of performing Indexed Binding Initialisation for BindingElement using array, nextIndex+skip , and environment as arguments.
ReturnIfAbrupt(status).
Return nextIndex +skip+1.

BindingElementList : BindingElementList , Elision _opt BindingElement

Let listNext be the result of performing Indexed Binding Initialisation for BindingElementList using array, nextIndex, and environment as arguments.
ReturnIfAbrupt(listNext).
Let skip be the Elision Width of Elision; if Elision is not present, use the numeric value zero.
Let status be the result of performing Indexed Binding Initialisation for BindingElement using array, listNext+skip , and environment as arguments.
ReturnIfAbrupt(status).
Return listNext +skip+1.

BindingElement : SingleNameBinding

Return the result of performing Keyed Binding Initialisation for SingleNameBinding using array, environment, and ToString(nextIndex) as the arguments.

BindingElement : BindingPattern Initialiser_opt

Let P be ToString(nextIndex).
Let v be the result of Get(array, P).
ReturnIfAbrupt(v).
If Initialiser_opt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initialiser.
2. Let v be ToObject(GetValue(defaultValue)).
ReturnIfAbrupt(v).
Return the result of performing Binding Initialisation for BindingPattern passing v and environment as arguments.

BindingRestElement : ... BindingIdentifier

Let A be the result of the abstract operation ArrayCreate with argument 0.
Let lenVal be the result of Get(array, "length").
Let arrayLength be ToUint32(lenVal).
ReturnIfAbrupt(arrayLength).
Let n=0.
Let index = nextIndex.
Repeat, while index < arrayLength
1. Let P be ToString(index).
2. Let exists be the result of HasProperty(array, P).
3. ReturnIfAbrupt(exists).
4. If exists is true, then
  1. Let v be the result of Get(array, P).
  2. ReturnIfAbrupt(v).
  3. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n) and Property Descriptor {[[Value]]: v, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
5. Let n = n+1.
6. Let index = index+1.
Return the result of performing Binding Initialisation for BindingIdentifier using A and environment as arguments.

Runtime Semantics: Keyed Binding Initialisation

With parameters obj, environment, and propertyName.

BindingElement : BindingPattern Initialiser_opt

Let v be the result of Get(obj, propertyName).
ReturnIfAbrupt(v).
If Initialiser_opt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initialiser.
2. Let v be ToObject(GetValue(defaultValue)).
ReturnIfAbrupt(v).
Return the result of performing Binding Initialisation for BindingPattern passing v and environment as arguments.

SingleNameBinding : BindingIdentifier Initialiser_opt

Let v be the result of Get(obj, propertyName).
ReturnIfAbrupt(v).
If Initialiser_opt is present and v is undefined, then
1. Let defaultValue be the result of evaluating Initialiser.
2. Let v be GetValue(defaultValue).
ReturnIfAbrupt(v).
Return the result of performing Binding Initialisation for BindingIdentifier passing v and environment as arguments.

12.3 Empty Statement

Syntax

EmptyStatement :

;

Runtime Semantics

Runtime Semantics: Evaluation

EmptyStatement : ;

Return NormalCompletion(empty).

12.4 Expression Statement

Syntax

ExpressionStatement :

[lookahead ∉ {{, function, class}] Expression ;

NOTE An ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration.

Runtime Semantics

Runtime Semantics: Evaluation

ExpressionStatement : [lookahead ∉ {{, function, class}] Expression ;

Let exprRef be the result of evaluating Expression.
Let value be GetValue(exprRef).
ReturnIfAbrupt(value).
Return NormalCompletion(value).

12.5 The if Statement

Syntax

IfStatement :

if ( Expression ) Statement else Statement

if ( Expression ) Statement

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.

Static Semantics: VarDeclaredNames

IfStatement : if ( Expression ) Statement else Statement

Let names be VarDeclaredNames of the first Statement.
Append to names the elements of the VarDeclaredNames of the second Statement.
Return names.

IfStatement : if ( Expression ) Statement

Return the VarDeclaredNames of Statement.

Runtime Semantics

Runtime Semantics: Evaluation

IfStatement : if ( Expression ) Statement else Statement

Let exprRef be the result of evaluating Expression.
Let exprValue be ToBoolean(GetValue(exprRef)).
ReturnIfAbrupt(exprValue).
If exprValue is true, then
1. Let stmtValue be the result of evaluating the first Statement.
Else,
1. Let stmtValue be the result of evaluating the second Statement.
If stmtValue.[[type]] is normal and stmtValue.[[value]] is empty, then
1. Return NormalCompletion(undefined).
Return stmtValue.

IfStatement : if ( Expression ) Statement

Let exprRef be the result of evaluating Expression.
Let exprValue be ToBoolean(GetValue(exprRef)).
ReturnIfAbrupt(exprValue).
If exprValue is false, then
1. Return NormalCompletion(undefined).
Else,
1. Let stmtValue be the result of evaluating Statement.
If stmtValue.[[type]] is normal and stmtValue.[[value]] is empty, then
1. Return NormalCompletion(undefined).
Return stmtValue.

12.6 Iteration Statements

Syntax

IterationStatement :

do Statement while ( Expression )

while ( Expression ) Statement

for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

for ( var VariableDeclarationListNoIn ; Expression_opt ; Expression_opt ) Statement

for ( LexicalDeclarationNoIn ; Expression_opt ; Expression_opt ) Statement

for ( LeftHandSideExpression in Expression ) Statement

for ( var ForBinding in Expression ) Statement

for ( ForDeclaration in Expression ) Statement

for ( LeftHandSideExpression of AssignmentExpression ) Statement

for ( var ForBinding of AssignmentExpressionExpression ) Statement

for ( ForDeclaration of AssignmentExpression ) Statement

ForDeclaration :

LetOrConst ForBinding

NOTE 1 ForBinding is defined in 11.1.4.2.

NOTE 2 A semicolon is not required after a do-while statement.

Runtime Semantics

Runtime Semantics: LoopContinues Abstract Operation

The abstract operation LoopContinues with arguments completion and labelSet is defined by the following step:

If completion.[[type]] is normal, then return true.
If completion.[[type]] is not continue, then return false.
If completion.[[target]] is empty, then return true.
If completion.[[target]] is an element of labelSet, then return true.
Return false.

NOTE Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

12.6.1 The do-while Statement

Static Semantics: VarDeclaredNames

IterationStatement : do Statement while ( Expression )

Return the VarDeclaredNames of Statement.

Runtime Semantics

Runtime Semantics: Labelled Evaluation

With argument labelSet.

IterationStatement : do Statement while ( Expression )

Let V = undefined.
Repeat
1. Let stmt be the result of evaluating Statement.
2. If stmt.[[value]] is not empty, let V = stmt.[[value]].
3. If stmt is an abrupt completion and LoopContinues (stmt,labelSet) is false, return stmt.
4. Let exprRef be the result of evaluating Expression.
5. Let exprValue be ToBoolean(GetValue(exprRef)).
6. If exprValue is false, Return NormalCompletion(V).
7. Else if exprValue is not true, then
  1. Assert: exprValue is an abrupt completion.
  2. If LoopContinues (exprValue,labelSet) is false, return exprValue.

12.6.2 The while Statement

Static Semantics: VarDeclaredNames

IterationStatement : while ( Expression ) Statement

Return the VarDeclaredNames of Statement.

Runtime Semantics

Runtime Semantics: Labelled Evaluation

With argument labelSet.

IterationStatement : while ( Expression ) Statement

Let V = undefined.
Repeat
1. Let exprRef be the result of evaluating Expression.
2. Let exprValue be ToBoolean(GetValue(exprRef)).
3. If exprValue is false, return NormalCompletion(V).
4. If exprValue is not true, then
  1. Assert: exprValue is an abrupt completion.
  2. If LoopContinues (exprValue,labelSet) is false, return exprValue.
5. Let stmt be the result of evaluating Statement.
6. If stmt.[[value]] is not empty, let V = stmt.[[value]].
7. If LoopContinues (stmt,labelSet) is false, return stmt.

12.6.3 The for Statement

Static Semantics

Static Semantics: VarDeclaredNames

IterationStatement : for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

Return the VarDeclaredNames of Statement.

IterationStatement : for ( var VariableDeclarationListNoIn ; Expression_opt ; Expression_opt ) Statement

Let names be BoundNames of VariableDeclarationListNoIn.
Append to names the elements of the VarDeclaredNames of Statement.
Return names.

IterationStatement : for ( LexicalDeclarationNoIn ; Expression_opt ; Expression_opt ) Statement

Return the VarDeclaredNames of Statement.

Runtime Semantics

Runtime Semantics: Labelled Evaluation

With argument labelSet.

IterationStatement : for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

If ExpressionNoIn is present, then
1. Let exprRef be the result of evaluating ExpressionNoIn.
2. Let exprValue be GetValue(exprRef).
3. If LoopContinues(exprValue,labelSet) is false, return exprValue.
Return the result of performing For Body Evaluation with the first Expression as the testExpr argument, the second Expression as the incrementExpr argument, Statement as the stmt argument, and with labelSet.

IterationStatement : for ( var VariableDeclarationListNoIn ; Expression_opt ; Expression_opt ) Statement

Let varDcl be the result of evaluating VariableDeclarationListNoIn.
If LoopContinues(varDcl,labelSet) is false, return varDcl.
Return the result of performing For Body Evaluation with the first Expression as the testExpr argument, the second Expression as the incrementExpr argument, Statement as the stmt argument, and with labelSet.

IterationStatement : for ( LexicalDeclarationNoIn ; Expression_opt ; Expression_opt ) Statement

Let oldEnv be the running execution context’s LexicalEnvironment.
Let loopEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
Let isConst be the result of performing IsConstantDeclaration of LexicalDeclarationNoIn.
For each element dn of the BoundNames of LexicalDeclarationNoIn do
1. If isConst is true, then
  1. Call loopEnv’s CreateImmutableBinding concrete method passing dn as the argument.
2. Else,
  1. Call loopEnv’s CreateMutableBinding concrete method passing dn and false as the arguments.
Set the running execution context’s LexicalEnvironment to loopEnv.
Let forDcl be the result of evaluating LexicalDeclarationNoIn.
If LoopContinues(forDcl,labelSet) is false, then
1. Set the running execution context’s LexicalEnvironment to oldEnv.
2. Return forDcl.
Let bodyResult be the result of performing For Body Evaluation with the first Expression as the testExpr argument, the second Expression as the incrementExpr argument, Statement as the stmt argument, and with labelSet.
Set the running execution context’s LexicalEnvironment to oldEnv.
Return bodyResult.

Runtime Semantics: For Body Evaluation Abstract Operation

The abstract operation For Body Evaluation with arguments testExpr, incrementExpr, stmt, and labelSet is performed as follows:

Let V = undefined.
Repeat
1. If testExpr is not [empty], then
  1. Let testExprRef be the result of evaluating testExpr.
  2. Let testExprValue be ToBoolean(GetValue(testExprRef))
  3. If testExprValue is false, return NormalCompletion(V).
  4. Else if LoopContinues (testExprValue,labelSet) is false, return testExprValue.

ECMA-262

6th Edition / Draft March 8, 2013

ECMAScript Language Specification

Contents

Introduction

1 Scope

2 Conformance

3 Normative references

4 Overview

4.1 Web Scripting

4.2 Language Overview

4.2.1 Objects

4.2.2 The Strict Variant of ECMAScript

4.3 Terms and definitions

4.3.1 type

4.3.2 primitive value

4.3.3 object

4.3.4 constructor

4.3.5 prototype

4.3.6 ordinary object

4.3.7 exotic object

4.3.8 standard object

4.3.9 built-in object

4.3.10 undefined value

4.3.11 Undefined type

4.3.12 null value

4.3.13 Null type

4.3.14 Boolean value

4.3.15 Boolean type

4.3.16 Boolean object

4.3.17 String value

4.3.18 String type

4.3.19 String object

4.3.20 Number value

4.3.21 Number type

4.3.22 Number object

4.3.23 Infinity

4.3.24 NaN

4.3.25 function

4.3.26 built-in function

4.3.27 property

4.3.28 method

4.3.29 built-in method

4.3.30 attribute

4.3.31 own property

4.3.32 inherited property

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

5.1.2 The Lexical and RegExp Grammars

5.1.3 The Numeric String Grammar

5.1.4 The Syntactic Grammar

5.1.5 The JSON Grammar

5.1.6 Grammar Notation

5.2 Algorithm Conventions

5.3 Static Semantic Rules

6 Source Text

Syntax

7 Lexical Conventions

Syntax

7.1 Unicode Format-Control Characters

7.2 White Space

Syntax

7.3 Line Terminators

Syntax

7.4 Comments

Syntax

7.5 Tokens

Syntax

7.6 Identifier Names and Identifiers

Syntax

7.6.1 Reserved Words

Syntax

7.6.1.1 Keywords

Syntax

7.6.1.2 Future Reserved Words

Syntax

7.7 Punctuators

Syntax

7.8 Literals

6^th Edition / Draft March 8, 2013