4.2.1 Objects

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initialises all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named “prototype” that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009,11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of its constructor’s “prototype” property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, and structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf₁, cf₂, cf₃, cf₄, and cf₅. Each of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example, cf₃’s prototype is CF_p. The constructor, CF, has two properties itself, named P1 and P2, which are not visible to CF_p, cf₁, cf₂, cf₃, cf₄, or cf₅. The property named CFP1 in CF_p is shared by cf₁, cf₂, cf₃, cf₄, and cf₅ (but not by CF), as are any properties found in CF_p’s implicit prototype chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CF_p.

Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object’s properties. In the above diagram, one could add a new shared property for cf₁, cf₂, cf₃, cf₄, and cf₅ by assigning a new value to the property in CF_p.

4.2.2 The Strict Variant of ECMAScript

The ECMAScript Language recognises the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript code units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is defined within a strict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode code units into a single composite program.

4.3.1 type

set of data values as defined in Clause 8 of this specification

4.3.2 primitive value

member of one of the types Undefined, Null, Boolean, Number, or String as defined in Clause 8

4.3.3 object

member of the type Object

4.3.4 constructor

function object that creates and initialises objects

4.3.5 prototype

object that provides shared properties for other objects

4.3.6 native object

object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment

4.3.7 built-in object

object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the execution of an ECMAScript program

4.3.8 host object

object supplied by the host environment to complete the execution environment of ECMAScript

4.3.9 undefined value

primitive value used when a variable has not been assigned a value

4.3.10 Undefined type

type whose sole value is the undefined value

4.3.11 null value

primitive value that represents the intentional absence of any object value

4.3.12 Null type

type whose sole value is the null value

4.3.13 Boolean value

member of the Boolean type

4.3.14 Boolean type

type consisting of the primitive values true and false

4.3.15 Boolean object

member of the Object type that is an instance of the standard built-in Boolean constructor

4.3.16 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

4.3.17 String type

set of all possible String values

4.3.18 String object

member of the Object type that is an instance of the standard built-in String constructor

4.3.19 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value

4.3.20 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity

4.3.21 Number object

member of the Object type that is an instance of the standard built-in Number constructor

4.3.22 Infinity

number value that is the positive infinite Number value

4.3.23 NaN

number value that is a IEEE 754 “Not-a-Number” value

4.3.24 function

member of the Object type that is an instance of the standard built-in Function constructor and that may be invoked as a subroutine

4.3.25 built-in function

built-in object that is a function

4.3.26 property

association between a name and a value that is a part of an object

4.3.27 method

function that is the value of a property

4.3.28 built-in method

method that is a built-in function

4.3.29 attribute

internal value that defines some characteristic of a property

4.3.30 own property

property that is directly contained by its object

4.3.31 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype

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.

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 Program, that describe how sequences of tokens can form syntactically correct ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript program, 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 program is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal Program, 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 programs. 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.

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 program exactly as written. 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 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:

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:

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:

is a convenient abbreviation for:

and that:

is a convenient abbreviation for:

which in turn is an abbreviation for:

which in turn is an abbreviation for:

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:

which is merely a convenient abbreviation for:

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

the definition

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:

indicates that the production may not be used if a LineTerminator occurs in the program 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 program.

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:

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:

7.6.1 Reserved Words

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

Syntax

7.8.1 Null Literals

Syntax

Semantics

The value of the null literal null is the sole value of the Null type, namely null.

7.8.2 Boolean Literals

Syntax

Semantics

The value of the Boolean literal true is a value of the Boolean type, namely true.

The value of the Boolean literal false is a value of the Boolean type, namely false.

7.8.3 Numeric Literals

Syntax

The source character immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

3in

Semantics

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 :: 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 is 0.

• The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 is 1.

• The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 is 2.

• The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 is 3.

• The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 is 4.

• The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 is 5.

• The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 is 6.

• The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 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 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 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.

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.

7.8.4 String Literals

A string literal is zero or more characters enclosed in single or double quotes. Each character may 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.

Syntax

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

Semantics

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of character values (CV) contributed by the various parts of the string literal. As part of this process, some 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 character sequence.

• The SV of StringLiteral :: '' is the empty character 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 character, the CV of DoubleStringCharacter.

• The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of the CV of DoubleStringCharacter followed by all the characters in the SV of DoubleStringCharacters in order.

• The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one character, the CV of SingleStringCharacter.

• The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of the CV of SingleStringCharacter followed by all the characters in the SV of SingleStringCharacters in order.

• The SV of LineContinuation :: \ LineTerminatorSequence is the empty character sequence.

• The CV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator is the SourceCharacter character itself.

• 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 SourceCharacter character itself.

• 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 a <NUL> character (Unicode value 0000).

• 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 Table 4:

• 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 SourceCharacter character itself.

• The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code unit value 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 character whose code unit value 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.

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

7.8.5 Regular Expression Literals

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 Strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions.

Syntax

Semantics

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. If the call to new RegExp would generate an error as specified in 15.10.4.1, the error must be treated as an early error (Clause 16).

7.9.1 Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

1. When, as the program 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 }.
2. When, as the program 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 Program, then a semicolon is automatically inserted at the end of the input stream.
3. When, as the program 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).

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;

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

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.

8.6.1 Property Attributes

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

A named accessor property associates a name with the attributes listed in Table 6.

If the value of an attribute is not explicitly specified by this specification for a named property, the default value defined in Table 7 is used.

8.6.2 Object Internal Properties and Methods

This specification uses various internal properties to define the semantics of object values. These internal properties are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, a TypeError exception is thrown.

The Table 8 summarises the internal properties used by this specification that are applicable to all ECMAScript objects. The Table 9 summarises the internal properties used by this specification that are only applicable to some ECMAScript objects. The descriptions in these tables indicate their behaviour for native ECMAScript objects, unless stated otherwise in this document for particular kinds of native ECMAScript objects. Host objects may support these internal properties with any implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this document.

The “Value Type Domain” columns of the following tables define the types of values associated with internal properties. The type names 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. “SpecOp” means the internal property is an internal method, an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and the type of the returned value.

Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the [[DefaultValue]] internal method may, for some objects, simply throw a TypeError exception.

All objects have an internal property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal property must eventually lead to a null value). Named 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 put access. Named accessor properties are inherited for both get access and put access.

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

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The value of the [[Class]] internal property of a host object may be any String value except one of "Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number", "Object", "RegExp", and "String". The value of a [[Class]] internal property is used internally to distinguish different kinds of objects. Note that this specification does not provide any means for a program to access that value except through Object.prototype.toString (see 15.2.4.2).

Unless otherwise specified, the common internal methods of native ECMAScript objects behave as described in 8.12. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal method (see 15.4.5.1) and String objects have a slightly different implementation of the [[GetOwnProperty]] internal method (see 15.5.5.2). Arguments objects (10.6) have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]]. Function objects (15.3) have a different implementation of [[Get]].

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

The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each property of the host 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 value of the host object’s [[Extensible]] internal property 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 a host object must not permit the addition of a new property to a host object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false.

If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false then it must not subsequently become true.

8.7.1 GetValue (V)

1. If Type(V) is not Reference, return V.
2. Let base be the result of calling GetBase(V).
3. If IsUnresolvableReference(V), throw a ReferenceError exception.
4. If IsPropertyReference(V), then
   1. If HasPrimitiveBase(V) is false, then let get be the [[Get]] internal method of base, otherwise let get be the special [[Get]] internal method defined below.
   2. Return the result of calling the get internal method using base as its this value, and passing GetReferencedName(V) for the argument.
5. 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.

The following [[Get]] internal method is used by GetValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P as its argument. The following steps are taken:

1. Let O be ToObject(base).
2. Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
3. If desc is undefined, return undefined.
4. If IsDataDescriptor(desc) is true, return desc.[[Value]].
5. Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]] (see 8.10).
6. If getter is undefined, return undefined.
7. Return the result calling the [[Call]] internal method of getter providing base as the this value and providing no arguments.

8.7.2 PutValue (V, W)

1. If Type(V) is not Reference, throw a ReferenceError exception.
2. Let base be the result of calling GetBase(V).
3. If IsUnresolvableReference(V), then
   1. If IsStrictReference(V) is true, then
      1. Throw ReferenceError exception.
   2. Call the [[Put]] internal method of the global object, passing GetReferencedName(V) for the property name, W for the value, and false for the Throw flag.
4. Else if IsPropertyReference(V), then
   1. If HasPrimitiveBase(V) is false, then let put be the [[Put]] internal method of base, otherwise let put be the special [[Put]] internal method defined below.
   2. Call the put internal method using base as its this value, and passing GetReferencedName(V) for the property name, W for the value, and IsStrictReference(V) for the Throw flag.
5. Else base must be a reference whose base is an environment record. So,
   1. Call the SetMutableBinding (10.2.1) concrete method of base, passing GetReferencedName(V), W, and IsStrictReference(V) as arguments.
6. Return.

The following [[Put]] internal method is used by PutValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P, value W, and Boolean flag Throw as arguments. The following steps are taken:

1. Let O be ToObject(base).
2. If the result of calling the [[CanPut]] internal method of O with argument P is false, then
   1. If Throw is true, then throw a TypeError exception.
   2. Else return.
3. Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
4. If IsDataDescriptor(ownDesc) is true, then
   1. If Throw is true, then throw a TypeError exception.
   2. Else return.
5. Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
6. If IsAccessorDescriptor(desc) is true, then
   1. Let setter be desc.[[Set]] (see 8.10) which cannot be undefined.
   2. Call the [[Call]] internal method of setter providing base as the this value and an argument list containing only W.
7. Else, this is a request to create an own property on the transient object O
   1. If Throw is true, then throw a TypeError exception.
8. Return.

8.10.1 IsAccessorDescriptor ( Desc )

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

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

8.10.2 IsDataDescriptor ( Desc )

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

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

8.10.3 IsGenericDescriptor ( Desc )

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

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

8.10.4 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).

1. If Desc is undefined, then return undefined.
2. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
3. If IsDataDescriptor(Desc) is true, then
   1. Call the [[DefineOwnProperty]] internal method of obj with arguments "value", Property Descriptor {[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   2. Call the [[DefineOwnProperty]] internal method of obj with arguments "writable", Property Descriptor {[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
4. Else, IsAccessorDescriptor(Desc) must be true, so
   1. Call the [[DefineOwnProperty]] internal method of obj with arguments "get", Property Descriptor {[[Value]]: Desc.[[Get]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   2. Call the [[DefineOwnProperty]] internal method of obj with arguments "set", Property Descriptor {[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
5. Call the [[DefineOwnProperty]] internal method of obj with arguments "enumerable", Property Descriptor {[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
6. Call the [[DefineOwnProperty]] internal method of obj with arguments "configurable", Property Descriptor {[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
7. Return obj.

8.10.5 ToPropertyDescriptor ( Obj )

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

1. If Type(Obj) is not Object throw a TypeError exception.
2. Let desc be the result of creating a new Property Descriptor that initially has no fields.
3. If the result of calling the [[HasProperty]] internal method of Obj with argument "enumerable" is true, then
   1. Let enum be the result of calling the [[Get]] internal method of Obj with "enumerable".
   2. Set the [[Enumerable]] field of desc to ToBoolean(enum).
4. If the result of calling the [[HasProperty]] internal method of Obj with argument "configurable" is true, then
   1. Let conf be the result of calling the [[Get]] internal method of Obj with argument "configurable".
   2. Set the [[Configurable]] field of desc to ToBoolean(conf).
5. If the result of calling the [[HasProperty]] internal method of Obj with argument "value" is true, then
   1. Let value be the result of calling the [[Get]] internal method of Obj with argument “value”.
   2. Set the [[Value]] field of desc to value.
6. If the result of calling the [[HasProperty]] internal method of Obj with argument "writable" is true, then
   1. Let writable be the result of calling the [[Get]] internal method of Obj with argument "writable".
   2. Set the [[Writable]] field of desc to ToBoolean(writable).
7. If the result of calling the [[HasProperty]] internal method of Obj with argument "get" is true, then
   1. Let getter be the result of calling the [[Get]] internal method of Obj with argument "get".
   2. If IsCallable(getter) is false and getter is not undefined, then throw a TypeError exception.
   3. Set the [[Get]] field of desc to getter.
8. If the result of calling the [[HasProperty]] internal method of Obj with argument "set" is true, then
   1. Let setter be the result of calling the [[Get]] internal method of Obj with argument "set".
   2. If IsCallable(setter) is false and setter is not undefined, then throw a TypeError exception.
   3. Set the [[Set]] field of desc to setter.
9. 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.
10. Return desc.

8.12.1 [[GetOwnProperty]] (P)

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

1. If O doesn’t have an own property with name P, return undefined.
2. Let D be a newly created Property Descriptor with no fields.
3. Let X be O’s own property named P.
4. 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
5. 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.
6. Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
7. Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
8. Return D.

However, if O is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in 15.5.5.2.

8.12.2 [[GetProperty]] (P)

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

1. Let prop be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
2. If prop is not undefined, return prop.
3. Let proto be the value of the [[Prototype]] internal property of O.
4. If proto is null, return undefined.
5. Return the result of calling the [[GetProperty]] internal method of proto with argument P.

8.12.3 [[Get]] (P)

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

1. Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
2. If desc is undefined, return undefined.
3. If IsDataDescriptor(desc) is true, return desc.[[Value]].
4. Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
5. If getter is undefined, return undefined.
6. Return the result calling the [[Call]] internal method of getter providing O as the this value and providing no arguments.

8.12.4 [[CanPut]] (P)

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

1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
2. If desc is not undefined, then
   1. If IsAccessorDescriptor(desc) is true, then
      1. If desc.[[Set]] is undefined, then return false.
      2. Else return true.
   2. Else, desc must be a DataDescriptor so return the value of desc.[[Writable]].
3. Let proto be the [[Prototype]] internal property of O.
4. If proto is null, then return the value of the [[Extensible]] internal property of O.
5. Let inherited be the result of calling the [[GetProperty]] internal method of proto with property name P.
6. If inherited is undefined, return the value of the [[Extensible]] internal property of O.
7. If IsAccessorDescriptor(inherited) is true, then
   1. If inherited.[[Set]] is undefined, then return false.
   2. Else return true.
8. Else, inherited must be a DataDescriptor
   1. If the [[Extensible]] internal property of O is false, return false.
   2. Else return the value of inherited.[[Writable]].

Host objects may define additional constraints upon [[Put]] operations. If possible, host objects should not allow [[Put]] operations in situations where this definition of [[CanPut]] returns false.

8.12.5 [[Put]] ( P, V, Throw )

When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw, the following steps are taken:

1. If the result of calling the [[CanPut]] internal method of O with argument P is false, then
   1. If Throw is true, then throw a TypeError exception.
   2. Else return.
2. Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
3. If IsDataDescriptor(ownDesc) is true, then
   1. Let valueDesc be the Property Descriptor {[[Value]]: V}.
   2. Call the [[DefineOwnProperty]] internal method of O passing P, valueDesc, and Throw as arguments.
   3. Return.
4. Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
5. If IsAccessorDescriptor(desc) is true, then
   1. Let setter be desc.[[Set]] which cannot be undefined.
   2. Call the [[Call]] internal method of setter providing O as the this value and providing V as the sole argument.
6. Else, create a named data property named P on object O as follows
   1. Let newDesc be the Property Descriptor
      {[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
   2. Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as arguments.
7. Return.

8.12.6 [[HasProperty]] (P)

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

1. Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
2. If desc is undefined, then return false.
3. Else return true.

8.12.7 [[Delete]] (P, Throw)

When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, the following steps are taken:

1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
2. If desc is undefined, then return true.
3. If desc.[[Configurable]] is true, then
   1. Remove the own property with name P from O.
   2. Return true.
4. Else if Throw, then throw a TypeError exception.
5. Return false.

8.12.8 [[DefaultValue]] (hint)

When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:

1. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
2. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.
   2. If str is a primitive value, return str.
3. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
4. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.
   2. If val is a primitive value, return val.
5. Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:

1. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
2. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.
   2. If val is a primitive value, return val.
3. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
4. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.
   2. If str is a primitive value, return str.
5. Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint were String.

The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method can return only primitive values.

8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)

In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError exception, otherwise return false”. The 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.

When the [[DefineOwnProperty]] internal method of O is called with property name P, property descriptor Desc, and Boolean flag Throw, the following steps are taken:

1. Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
2. Let extensible be the value of the [[Extensible]] internal property of O.
3. If current is undefined and extensible is false, then Reject.
4. If current is undefined and extensible is true, then
   1. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
      1. 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.
   2. Else, Desc must be an accessor Property Descriptor so,
      1. 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.
   3. Return true.
5. Return true, if every field in Desc is absent.
6. 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).
7. If the [[Configurable]] field of current is false then
   1. Reject, if the [[Configurable]] field of Desc is true.
   2. Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
8. If IsGenericDescriptor(Desc) is true, then no further validation is required.
9. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
   1. Reject, if the [[Configurable]] field of current is false.
   2. If IsDataDescriptor(current) is true, then
      1. 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. 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.
10. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
    1. If the [[Configurable]] field of current is false, then
       1. Reject, 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. Reject, 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.
11. Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so,
    1. If the [[Configurable]] field of current is false, then
       1. Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
       2. Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
12. 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.
13. Return true.

However, if O is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method defined in 15.4.5.1.

9.3.1 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

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.

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.

9.8.1 ToString Applied to the Number Type

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

1. If m is NaN, return the String "NaN".
2. If m is +0 or −0, return the String "0".
3. If m is less than zero, return the String concatenation of the String "-" and ToString(−m).
4. If m is infinity, return the String "Infinity".
5. 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.
6. 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’.
7. 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.
8. 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.
9. 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).
10. 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).

10.1.1 Strict Mode Code

An ECMAScript Program 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).

• 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 PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment 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.

10.2.1 Environment Records

There are two 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 Program and WithStatement that associate identifier bindings with the properties of some object.

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 two concrete subclasses, declarative environment record and object environment record. The abstract class includes the abstract specification methods defined in Table 17. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

10.2.2 Lexical Environment Operations

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

10.2.3 The Global Environment

The global environment is a unique Lexical Environment which is created before any ECMAScript code is executed. The global environment’s Environment Record is an object environment record whose binding object is the global object (15.1). The global environment’s outer environment reference is null.

As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

10.3.1 Identifier Resolution

Identifier resolution is the process of determining the binding of an Identifier using the LexicalEnvironment of the running execution context. During execution of ECMAScript code, the syntactic production PrimaryExpression : Identifier is evaluated using the following algorithm:

1. Let env be the running execution context’s LexicalEnvironment.
2. If the syntactic production that is being evaluated is contained in a strict mode code, then let strict be true, else let strict be false.
3. Return the result of calling GetIdentifierReference function passing env, Identifier, 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.

10.4.1 Entering Global Code

The following steps are performed when control enters the execution context for global code:

1. Initialise the execution context using the global code as described in 10.4.1.1.
2. Perform Declaration Binding Instantiation as described in 10.5 using the global code.

10.4.2 Entering Eval Code

The following steps are performed when control enters the execution context for eval code:

1. If there is no calling context or if the eval code is not being evaluated by a direct call (15.1.2.1.1) to the eval function then,
   1. Initialise the execution context as if it was a global execution context using the eval code as C as described in 10.4.1.1.
2. Else,
   1. Set the ThisBinding to the same value as the ThisBinding of the calling execution context.
   2. Set the LexicalEnvironment to the same value as the LexicalEnvironment of the calling execution context.
   3. Set the VariableEnvironment to the same value as the VariableEnvironment of the calling execution context.
3. If the eval code is strict code, then
   1. Let strictVarEnv be the result of calling NewDeclarativeEnvironment passing the LexicalEnvironment as the argument.
   2. Set the LexicalEnvironment to strictVarEnv.
   3. Set the VariableEnvironment to strictVarEnv.
4. Perform Declaration Binding Instantiation as described in 10.5 using the eval code.

10.4.3 Entering Function Code

The following steps are performed when control enters the execution context for function code contained in function object F, a caller provided thisArg, and a caller provided argumentsList:

1. If the function code is strict code, set the ThisBinding to thisArg.
2. Else if thisArg is null or undefined, set the ThisBinding to the global object.
3. Else if Type(thisArg) is not Object, set the ThisBinding to ToObject(thisArg).
4. Else set the ThisBinding to thisArg.
5. Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the [[Scope]] internal property of F as the argument.
6. Set the LexicalEnvironment to localEnv.
7. Set the VariableEnvironment to localEnv.
8. Let code be the value of F’s [[Code]] internal property.
9. Perform Declaration Binding Instantiation using the function code code and argumentsList as described in 10.5.