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© Ecma International 2009

Introduction

This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of forthcoming internationalisation facilities and future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. Although that work was not completed and not published¹ as the fourth edition of ECMAScript, it informs continuing evolution of the language. The present fifth edition of ECMAScript (published as ECMA-262 5^th edition) codifies de facto interpretations of the language specification that have become common among browser implementations and adds support for new features that have emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security.

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of December 2009.

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¹ Note: Please note that for ECMAScript Edition 4 the Ecma standard number "ECMA-262 Edition 4" was reserved but not used in the Ecma publication process. Therefore "ECMA-262 Edition 4" as an Ecma International publication does not exist.

ECMAScript Language Specification

1   Scope

This Standard defines the ECMAScript scripting language.

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2   Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of this International standard shall interpret characters in conformance with the Unicode Standard, Version 3.0 or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it (is) presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript is permitted to support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of the "future reserved words" listed in 7.6.1.2 of this specification.

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3   Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 9899:1996, Programming Languages ­-C, including amendment 1 and technical corrigenda 1 and 2

ISO/IEC 10646-1:1993, Information Technology ­- Universal Multiple-Octet Coded Character Set (UCS) plus
its amendments and corrigenda

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4   Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific host objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

A scripting language is a programming language that is used to manipulate, customise, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and nonprofessional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript can provide core scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java™, Self, and Scheme as described in:

Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing Co., 1996.

Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227–241, Orlando, FL, October 1987.

IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1   Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customised user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2   Language Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. An ECMAScript object is a collection of properties each with zero or more attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to change the value of the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, and String; an object is a member of the remaining built-in type Object; and a function is a callable object. A function that is associated with an object via a property is a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object, the Object object, the Function object, the Array object, the String object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the

JSON object, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError and URIError.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

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 initializes 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 recognizes 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   Definitions

For the purposes of this document, the following terms and definitions apply.

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 initializes 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

type whose sole value is the undefined 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   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.

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 and string grammars, and some of the terminal symbols of the syntactic grammar, 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:

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 Initializer_opt

is a convenient abbreviation for:

VariableDeclaration :

Identifier
Identifier Initializer

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.

If the phrase "[empty]" appears as the right-hand side of a production, it indicates that the production's righthand 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:

ReturnStatement :

return   [no LineTerminator here]   Expression_opt ;

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

0 1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit ::

0
1
2
3
4
5
6
7
8
9

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 ::

IdentifierNamebut notReservedWord

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 code unit

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.

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms.

When an algorithm is to produce a value as a result, the directive "return x" is used to indicate that the result of the algorithm is the value of x and that the algorithm should terminate. The notation Result(n) is used as shorthand for "the result of step n".

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:

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.

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 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.
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.

6   Source Text

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later. The text is expected to have been normalized to Unicode Normalized Form C (canonical

composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first convert to UTF-16.

SourceCharacter ::

any Unicode code unit

Throughout the rest of this document, the phrase "code unit" and the word "character" will be used to refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase "Unicode character" will be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be represented by more than one code unit). The phrase "code point" refers to such a Unicode scalar value. "Unicode character" only refers to entities represented by single Unicode scalar values: the components of a combining character sequence are still individual "Unicode characters," even though a user might think of the whole sequence as a single character.

In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a Unicode escape sequence consisting of six characters, namely \u plus four hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal, the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence contributes one character to the identifier.

NOTE Although this document sometimes refers to a "transformation" between a "character" within a "string" and the 16-bit unsigned integer that is the code unit of that character, there is actually no transformation because a "character" within a "string" is actually represented using that 16-bit unsigned value.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode character 000A is line feed) and therefore the next character is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a line feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes a character to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the string literal.

7   Lexical Conventions

The source text of an ECMAScript program 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 two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.

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

InputElementRegExp ::

WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

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 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 summarized in Table 1.

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 also occur within a StringLiteral or a RegularExpressionLiteral (where they are considered significant characters forming part of the literal value) or within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space characters are listed in Table 2.

ECMAScript implementations must recognize all of the white space characters defined in Unicode 3.0. Later editions of the Unicode Standard may define other white space characters. ECMAScript implementations may recognize 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. 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.


Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters are treated as white space but not as line terminators. The character sequence <CR><LF> is commonly used as a line terminator. It should be considered a single character for the purpose of reporting line numbers.

Syntax

LineTerminator ::

<TAB>
<VT>
<CR>
<LF>
<PS>

LineTerminatorSequence ::

<TAB>
<LF>
<CR> [lookahead ∉ <LF>]
<PS>
<CR>
<LF>

7.4   Comments

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

Because a single-line comment can contain any 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 recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 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

MultiLineComment ::

/*   MultiLineComment_opt  */

MultiLineCommentChars ::

MultiLineNotAsteriskChar   MultiLineCommentChars_opt
* PostAsteriskCommentChars_opt

PostAsteriskCommentChars ::

MultiLineNotForwardSlashOrAsteriskChar   MultiLineCommentChars_opt
* PostAsteriskCommentChars_opt

MultiLineNotAsteriskChar ::

MultiLineNotAsteriskCharbut notasterisk *

MultiLineNotForwardSlashOrAsteriskChar ::

SourceCharacterbut not forward-slash   /or asterisk *

SingleLineComment ::

/ /  SingleLineCommentChars_opt

SingleLineCommentChars ::

SingleLineCommentChar   SingleLineCommentChars_opt

SingleLineCommentChar ::

SourceCharacter but notLineTerminator

7.5   Tokens

Syntax

Token ::

WhiteSpace
IdentifierName
Punctuator
NumericLiteral
StringLiteral

7.6   Identifier Names and Identifiers

Identifier Names are tokens that are interpreted according to the grammar given in the "Identifiers" section of chapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1). The Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard. The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations.

This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.

Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single character to the IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character 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 its UnicodeEscapeSequence's CV, the result must still be a valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. All interpretations of identifiers 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). The intent is that the incoming source text has been converted to normalized form C before it reaches the compiler.

ECMAScript implementations may recognize identifier characters defined in later editions of the Unicode Standard. If portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.

Syntax

Identifier ::

IdentifierName
IdentifierNamebut notReservedWord

IdentifierName ::

IdentifierStart
IdentifierName IdentifierPart

IdentifierStart ::

UnicodeLetter>
$
_
\
UnicodeEscapeSequence

IdentifierPart ::

IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
<ZWNJ>
<ZWJ>

UnicodeLetter

any character in the Unicode categories "Uppercase letter (Lu)", "Lowercase letter
(Ll)", "Titlecase letter (Lt)", "Modifier letter (Lm)", "Other letter (Lo)", or "Letter
number (Nl)".

UnicodeCombiningMark

any character in the Unicode categories "Non-spacing mark (Mn)" or "Combining
spacing mark (Mc)"

UnicodeDigit

any character in the Unicode category "Decimal number (Nd)"

UnicodeConnectorPunctuation

any character in the Unicode category "Connector punctuation (Pc)"

UnicodeEscapeSequence

see 7.8.4.

7.6.1   Reserved Words

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

Syntax

ReservedWord ::

Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

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	do	instanceof	typeof
case	else	new	var
catch	finally	return	void
continue	for	switch	while
debugger	function	this	with
default	if	throw
delete	in	try

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

class	enum	extends	super
const	export	import

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	let	private	public
interface	package	protected	static
yield

7.7   Punctuators

Syntax

Punctuator ::one of

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

DivPunctuator ::one of

/

/=

7.8   Literals

Syntax

Literal ::

NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral

7.8.1   Null Literals

Syntax

NullLiteral ::

null

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

7.8.2   Boolean Literals

Syntax

BooleanLiteral ::

true
false

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

NumericLiteral ::

DecimalLiteral
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

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

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

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  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

StringLiteral ::

" DoubleStringCharacters_opt"
' SingleStringCharacters_opt '

DoubleStringCharacters ::

DoubleStringCharacter DoubleStringCharacters_opt

SingleStringCharacters ::

SingleStringCharacter SingleStringCharacters_opt

DoubleStringCharacter ::

SourceCharacter but not double-quote "  or backslash   \or LineTerminator
\ EscapeSequence
LineContinuation

SingleStringCharacter ::

SourceCharacter but not single-quote '  or backslash   \or LineTerminator
\ EscapeSequence
LineContinuation

LineContinuation ::

\ LineTerminatorSequence

The definitions of the nonterminal HexDigit is given in 7.6. SourceCharacter is defined in clause 6.

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.

Semantics

• 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 :: DoubleStringCharacters is a sequence of one character, the CV of DoubleStringCharacter.
• The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacter 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 :: SourceCharacterbut notdouble-quote  "orbackslash \
  orLineTerminator is the SourceCharacter character itself.
• The CV of DoubleStringCharacter :: \  EscapeSequence is the CV of the EscapeSequence.
• The CV of SingleStringCharacter :: SourceCharacterbut notsingle-quote  'orbackslash  \ orLineTerminator is the SourceCharacter character itself.
• The CV of SingleStringCharacter :: \  EscapeSequence is the CV of the EscapeSequence.
• 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 :: SourceCharacterbut notEscapeCharacterorLineTerminator 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

RegularExpressionLiteral ::

/  RegularExpressionBody  /  RegularExpressionFlags

RegularExpressionBody ::

/  RegularExpressionFirstChar  /  RegularExpressionChars

RegularExpressionChars ::

[empty]
RegularExpressionChars  RegularExpressionChar

RegularExpressionFirstChar ::

RegularExpressionNonTerminator but not * or \ or / or[
RegularExpressionBackslashSequence
RegularExpressionClass

RegularExpressionChar ::

RegularExpressionNonTerminator but not \ or / or[
RegularExpressionBackslashSequence
RegularExpressionClass

RegularExpressionBackslashSequence ::

\ NonTerminator

RegularExpressionNonTerminator ::

SourceCharacter but not LineTerminator

RegularExpressionClass ::

[RegularExpressionClassChars]

RegularExpressionClass ::

[empty]
RegularExpressionClassChars   RegularExpressionClassChar

RegularExpressionClassChar ::

RegularExpressionNonTerminator but not ] or \
RegularExpressionBackslashSequence

RegularExpressionFlags ::

[empty]
RegularExpressionFlags   IdentifierPart

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   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 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 }.
• 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.
  
  
• 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 parenthesized expression that begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

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.

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.

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, Property Identifier, Lexical Environment, and Environment Record. 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.

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   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.

8.2   The Null Type

The Null type has exactly one value, called null.

8.3   The Boolean Type

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

8.4   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 code unit value (see Clause 6). 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 position 0, the next element (if any) at position 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.

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalized form, nor do they ensure language-sensitive results.

sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalized Form C, string literals are guaranteed to be normalized (if source text is guaranteed to be normalized), as long as they do not contain any Unicode escape sequences.

8.5   The Number Type

The Number type has exactly 18437736874454810627 (that is, 2⁶⁴−2⁵³+3) values, representing the doubleprecision 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 normalized, having the form

s × m × 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 denormalized, having the form

s × m × 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.

8.6   The Object Type

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

• A named data property associates a name with an ECMAScript language value and a set of Boolean attributes.
• A named accessor property associates a name 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.
• An internal property has no name and is not directly accessible via ECMAScript language operators. Internal properties exist purely for specification purposes.

There are two kinds of access for named (non-internal) properties: get and put, corresponding to retrieval and assignment, respectively.

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

Table 5 — Attributes of a Named Data Property

Attribute Name	Value Domain	Description
[[Value]]	Any ECMAScript language type	The value retrieved by reading the property.
[[Writable]]	Boolean	If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] 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]]) will fail.

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

Table 6 — Attributes of a Named Accessor Property

Attribute Name	Default Value	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 return 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 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]]) will fail.

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.

Table 7 — Default Attribute Values

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

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.

Table 8 summarizes the internal properties used by this specification that are applicable to all ECMAScript objects. Table 9 summarizes the internal properties used by this specification that are only applicable to some ECMAScript objects. The descriptions in these tables indicates 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.

Table 8 — Internal Properties Common to All Objects

Internal Property	Value Type Domain	Description
[[Prototype]]	Object or Null	The prototype of this object.
[[Class]]	String	A String value indicating a specification defined classification of objects.
[[Extensible]]	Boolean	If true, own properties may be added to the object.
[[Get]]	SpecOp(propertyName) → any	Returns the value of the named property.
[[GetOwnProperty]]	SpecOp (propertyName) → Undefined or Property Descriptor	Returns the Property Descriptor of the named own property of this object, or undefined if absent.
[[GetProperty]]	SpecOp (propertyName) → Undefined or Property Descriptor	Returns the fully populated Property Descriptor of the named property of this object, or undefined if absent.
[[Put]]	SpecOp (propertyName, any, Boolean)	Sets the specified named property to the value of the second parameter. The flag controls failure handling.
[[CanPut]]	SpecOp (propertyName) → Boolean	Returns a Boolean value indicating whether a [[Put]] operation with propertyName can be performed.
[[HasProperty]]	SpecOp (propertyName → Boolean)	Returns a Boolean value indicating whether the object already has a property with the given name.
[[Delete]]	SpecOp (propertyName, Boolean) → Boolean	Removes the specified named own property from the object. The flag controls failure handling.
[[DefaultValue]]	SpecOp (Hint) → primitive	Hint is a String. Returns a default value for the object.
[[DefineOwnProperty]]	SpecOp (propertyName, PropertyDescriptor, Boolean) → Boolean	Creates or alters the named own property to have the state described by a Property Descriptor. The flag controls failure handling.

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.

Implementation specific extensions that modify [[Class]], [[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

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 is 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.

Table 9 — Internal Properties Only Defined for Some Objects

Internal Property	Value Type Domain	Description
[[PrimitiveValue]]	primitive	Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].
[[Construct]]	SpecOp(a List of any) → Object	Creates an object. Invoked via the new operator. The arguments to the SpecOp are the arguments passed to the new operator. Objects that implement this internal method are called constructors.
[[Call]]	SpecOp(any, a List of any) → any or Reference	Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are a this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values.
[[HasInstance]]	SpecOp(any) → Boolean	Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].
[[Scope]]	Lexical Environment	A lexical environment that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]].
[[FormalParameters]]	List of Strings	A possibly empty List containing the identifier Strings of a Function’s FormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].
[[Code]]	ECMAScript code	The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].
[[TargetFunction]]	Object	The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.
[[BoundThis]]	any	The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.
[[BoundArguments]]	List of any	The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.
[[Match]]	SpecOp(String, index) → MatchResult	Tests for a regular expression match and returns a MatchResult value (see 15.10.2.1). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]].
[[ParameterMap]]	Object	Provides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.

8.7   The Reference Specification Type

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

reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a userdefined function to return a reference. (Another reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)

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.

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.
• IsStrictReference(V). Returns the strict reference 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.

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

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]].
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]] 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.

8.8   The List 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.

8.9   The Completion Specification Type

The Completion type is used to explain the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language value or empty, and target is any ECMAScript identifier or empty.

The term "abrupt completion" refers to any completion with a type other than normal.

8.10   The Property Descriptor and Property Identifier Specification Types

The Property Descriptor type is used to explain the manipulation and reification of named 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 8.6.1. 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 8.6.1 Table 5 or Table 6.

For notational convenience within this specification, an object literal-like syntax can be used to define a property descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true} defines a data property descriptor. 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 Property Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for "the field of D named [[Value]]".

The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor value. The following abstract operations are used in this specification to operate upon Property Descriptor values:

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 Desc, 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.

8.11   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.

8.12   Algorithms for Object Internal Methods

In the following algorithm descriptions, assume O is a native ECMAScript object, P is a String, Desc is a Property Description record, and Throw is a Boolean flag.

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:

8.     Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
9.     If desc is undefined, return undefined.
10.   If IsDataDescriptor(desc) is true, return desc.[[Value]].
11.   Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
12.   If getter is undefined, return undefined.
13.   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

2. 1. 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.

3. 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.

4. 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  Type Conversion and Testing

The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are defined here to aid the specification of the semantics of the language. 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   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 10:

Table 10 — ToPrimitive Conversions

Input Type	Result
Undefined	The result equals the input argument (no conversion).
Null	The result equals the input argument (no conversion).
Boolean	The result equals the input argument (no conversion).
Number	The result equals the input argument (no conversion).
String	The result equals the input argument (no conversion).
Object	Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

9.2   ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

Table 11 — ToBoolean Conversions

Argument Type	Result
Undefined	false
Null	false
Boolean	The result equals the input argument (no conversion).
Number	The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
String	The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object	true.

9.3   ToNumber

The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:

Table 12 — To Number Conversions

Argument Type	Result
Undefined	NaN
Null	+0
Boolean	The result is 1 if the argument is true. The result is +0 if the argument is false.
Number	The result equals the input argument (no conversion).
String	See grammar and note below.
Object	Apply the following steps: 1. Let primValue be ToPrimitive(input argument, hint Number). 2. Return ToNumber(primValue).

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.

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 StringNumericLiteral StrWhiteSpace_opt is the MV of StrNumericLiteral, no matter whether white space is present or not.
• The MV of StringNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.
• The MV of StringNumericLiteral ::: 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⁻ⁿ, 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.

9.4   ToInteger

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

1. Let number be the result of calling ToNumber on the input argument.
2. If number is NaN, return +0.
3. If number is +0, −0, +∞ or –∞, return number.
4. Return the result of computing sign(number) * floor(abs(number)).

9.5   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: