# ECMAScript Language Specification ECMA-262 6th Edition – DRAFT

This is not the official ECMAScript Language Specification.

This is a draft of the next edition of the standard. See also:

- ECMAScript Language Specification, Edition 5.1 (PDF), the most recent official, final standard.
- The ES specification drafts archive for PDF and Word versions of this document, and older drafts.
- The script that produced this web page, and especially the issue tracker — please file bugs when you find them. Patches are welcome too.

For copyright information, see Ecma International’s legal disclaimer in the document itself.

# Draft

# ECMA-262

# 6th Edition / Draft March 17, 2015

# Ecma/TC39/2015/0XX

# ECMAScript 2015

# Language Specification

Draft
Release Candidate #3

Report Errors and Issues at: https://bugs.ecmascript.org

Product: Draft for 6th Edition

Component: choose an appropriate one

Version: Rev 36, March 17, 2015 Draft

# Contents

# Introduction

This is the sixth edition of ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world’s most widely used general purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications. The sixth edition is the most extensive update to ECMAScript since the publication of the first edition in 1997.

Goals for the sixth edition include providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements include modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins has been expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplemental characters in strings and regular expressions. The built-ins are now extensible via subclassing.

Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. Dozens of individuals representing many organizations have made very significant contributions within TC39 to the development of this edition and to the prior editions. In addition, a vibrant informal community has emerged supporting TC39’s ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.

New uses and requirements for ECMAScript continue to emerge. The sixth edition provides the foundation for regular, incremental language and library enhancements.

Allen Wirfs-Brock
ECMA-262, 6th Edition Project Editor

This Ecma Standard has been adopted by the General Assembly of <month> <year>.

ECMA-262 Edition History

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 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.

After publication of the third edition, ECMAScript 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. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.

The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had 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. The Fifth Edition was adopted by the Ecma General Assembly of December 2009.

The Fifth Edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.

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ECMAScript 2015 Language Specification

# 1 Scope

This Standard defines the ECMAScript 2015 general purpose programming language.

# 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 ECMAScript must interpret source text input in conformance with the Unicode Standard, Version 5.1.0 or later and ISO/IEC 10646. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the Unicode set, collection 10646.

A conforming implementation of ECMAScript that provides an application programming interface that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.

A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may 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 may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of the “future reserved words” listed in subclause 11.6.2.2 of this specification.

A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause 16.1.

# 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.

IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2008)

ISO/IEC 10646:2003: Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor

The Unicode Standard, Version 5.0, as amended by Unicode 5.1.0, or successor.
http://www.unicode.org/versions/latest

Unicode Standard Annex #15, Unicode Normalization Forms, version Unicode 5.1.0, or successor.
http://www.unicode.org/reports/tr15/

Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax, version Unicode 5.1.0, or successor. http://www.unicode.org/reports/tr31/

ECMA-402, ECMAScript 2015 Internationalization API Specification.
http://www.ecma-international.org/publications/standards/Ecma-402.htm

ECMA-404, The JSON Data Interchange Format.
http://www.ecma-international.org/publications/standards/Ecma-404.htm

# 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 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.

ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general purpose programming language. A scripting language is a programming language that is used to manipulate, customize, 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 non-professional 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 is now used to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.

ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so has the features and facilities it provides. ECMAScript is now a fully featured general propose programming language.

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

ISO/IEC 9899:1996, Programming Languages – C.

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 customized 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 ECMAScript 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. In ECMAScript, an object is a collection of zero or more properties each with 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 assign a different value to 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, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called 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; objects that are fundamental to the runtime semantics of the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and, reflection objects including Proxy and Reflect.

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.

Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.

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

Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based 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.

Figure 1 — Object/Prototype Relationships

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, while 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 CFp. The constructor, CF, has two properties itself, named `P1` and `P2`, which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named `CFP1` in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp’s implicit prototype chain that are not named `q1`, `q2`, or `CFP1`. Notice that there is no implicit prototype link between CF and CFp.

Unlike most 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 CFp.

Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with the sixth edition, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.

# 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 source text units. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.

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

# 4.3 Terms and 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 6 of this specification

# 4.3.2 primitive value

member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause 6

NOTE A primitive value is a datum that is represented directly at the lowest level of the language implementation.

# 4.3.3 object

member of the type Object

NOTE An object is a collection of properties and has a single prototype object. The prototype may be the null value.

# 4.3.4 constructor

function object that creates and initializes objects

NOTE The value of a constructor’s `prototype` property is a prototype object that is used to implement inheritance and shared properties.

# 4.3.5 prototype

object that provides shared properties for other objects

NOTE When a constructor creates an object, that object implicitly references the constructor’s `prototype` property for the purpose of resolving property references. The constructor’s `prototype` property can be referenced by the program expression `constructor.prototype`, and properties added to an object’s prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the `Object.create` built-in function.

# 4.3.6 ordinary object

object that has the default behaviour for the essential internal methods that must be supported by all objects.

# 4.3.7 exotic object

object that does not have the default behaviour for one or more of the essential internal methods that must be supported by all objects.

NOTE Any object that is not an ordinary object is an exotic object.

# 4.3.8 standard object

object whose semantics are defined by this specification

# 4.3.9 built-in object

object specified and supplied by an ECMAScript implementation

NOTE Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-in constructor is a built-in object that is also a constructor.

# 4.3.10 undefined value

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

# 4.3.11 Undefined type

type whose sole value is the undefined value

# 4.3.12 null value

primitive value that represents the intentional absence of any object value

# 4.3.13 Null type

type whose sole value is the null value

# 4.3.14 Boolean value

member of the Boolean type

NOTE There are only two Boolean values, true and false

# 4.3.15 Boolean type

type consisting of the primitive values true and false

# 4.3.16 Boolean object

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

NOTE A Boolean object is created by using the `Boolean` constructor in a `new` expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

# 4.3.17 String value

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

NOTE A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

# 4.3.18 String type

set of all possible String values

# 4.3.19 String object

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

NOTE A String object is created by using the `String` constructor in a `new` expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the `String` constructor as a function (21.1.1.1).

# 4.3.20 Number value

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

NOTE A Number value is a member of the Number type and is a direct representation of a number.

# 4.3.21 Number type

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

# 4.3.22 Number object

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

NOTE A Number object is created by using the `Number` constructor in a `new` expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the `Number` constructor as a function (20.1.1.1).

# 4.3.23 Infinity

number value that is the positive infinite Number value

# 4.3.24 NaN

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

# 4.3.25 Symbol value

primitive value that represents a unique, non-String Object property key

# 4.3.26 Symbol type

set of all possible Symbol values

# 4.3.27 Symbol object

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

# 4.3.28 function

member of the Object type that may be invoked as a subroutine

NOTE In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript.

# 4.3.29 built-in function

built-in object that is a function

NOTE Examples of built-in functions include `parseInt` and `Math.exp`. An implementation may provide implementation-dependent built-in functions that are not described in this specification.

# 4.3.30 property

part of an object that associates a key (either a String value or a Symbol value) and a value.

NOTE Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

# 4.3.31 method

function that is the value of a property

NOTE When a function is called as a method of an object, the object is passed to the function as its this value.

# 4.3.32 built-in method

method that is a built-in function

NOTE Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify and provide other additional built-in methods.

# 4.3.33 attribute

internal value that defines some characteristic of a property

# 4.3.34 own property

property that is directly contained by its object

# 4.3.35 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

# 4.4 Organization of This Specification

The remainder of this specification is organized as follows:

Clause 5 defines the notational conventions used throughout the specification.

Clauses 6−9 define the execution environment within which ECMAScript programs operate.

Clauses 10−16 define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.

Clauses 17−26 define the ECMAScript standard library. It includes the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.

# 5 Notational Conventions

# 5.1 Syntactic and Lexical Grammars

# 5.1.1 Context-Free Grammars

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

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

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

# 5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 11. This grammar has as its terminal symbols Unicode code points that conform to the rules for SourceCharacter defined in 10.1. It defines a set of productions, starting from the goal symbol InputElementDiv, InputElementTemplateTail, or InputElementRegExp, or InputElementRegExpOrTemplateTail, that describe how sequences of such code points 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 (11.9). 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 21.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of code points 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 7.1.3.1.

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, 14, and 15. 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 two alternative goal symbols Script and Module, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.

When a stream of code points is to be parsed as an ECMAScript Script or Module, 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 input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), 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 12, 13, 14 and 15 is not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. 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 line terminator character appears in certain “awkward” places.

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

# 5.1.5 Grammar Notation

Terminal symbols of the lexical, RegExp, and numeric string grammars are shown in `fixed width` font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges.

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

WhileStatement :
`while` `(` Expression `)` Statement

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

ArgumentList :
AssignmentExpression
ArgumentList `,` AssignmentExpression

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

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

VariableDeclaration :
BindingIdentifier Initializeropt

is a convenient abbreviation for:

VariableDeclaration :
BindingIdentifier
BindingIdentifier Initializer

and that:

IterationStatement :
`for` `(` LexicalDeclaration Expressionopt `;` Expressionopt `)` Statement

is a convenient abbreviation for:

IterationStatement :
`for` `(` LexicalDeclaration `;` Expressionopt `)` Statement
`for` `(` LexicalDeclaration Expression `;` Expressionopt `)` Statement

which in turn is an abbreviation for:

IterationStatement :
`for` `(` LexicalDeclaration `;` `)` Statement
`for` `(` LexicalDeclaration `;` Expression `)` Statement
`for` `(` LexicalDeclaration Expression `;` `)` Statement
`for` `(` LexicalDeclaration Expression `;` Expression `)` Statement

so, in this example, the nonterminal IterationStatement actually has four alternative right-hand sides.

A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:

StatementList[Return] :
ReturnStatement
ExpressionStatement

is a convenient abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement

and that:

StatementList[Return, In] :
ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement
StatementList_In :
ReturnStatement
ExpressionStatement
StatementList_Return_In :
ReturnStatement
ExpressionStatement

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right-hand side of a production can also be parameterized. For example:

StatementList :
ReturnStatement
ExpressionStatement[In]

is equivalent to saying:

StatementList :
ReturnStatement
ExpressionStatement_In

A nonterminal reference may have both a parameter list and an “opt” suffix. For example:

VariableDeclaration :
BindingIdentifier Initializer[In]opt

is an abbreviation for:

VariableDeclaration :
BindingIdentifier
BindingIdentifier Initializer_In

Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production’s left-hand side symbol. For example:

VariableDeclaration[In] :
BindingIdentifier Initializer[?In]

is an abbreviation for:

VariableDeclaration :
BindingIdentifier Initializer
VariableDeclaration_In :
BindingIdentifier Initializer_In

If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production’s nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production’s nonterminal symbol. This means that:

StatementList[Return] :
[+Return] ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement

and that

StatementList[Return] :
[~Return] ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ExpressionStatement

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

NonZeroDigit :: one of
`1` `2` `3` `4` `5` `6` `7` `8` `9`

which is merely a convenient abbreviation for:

NonZeroDigit ::
`1`
`2`
`3`
`4`
`5`
`6`
`7`
`8`
`9`

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

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token is a member of the given set. The set can be written as a list of terminals enclosed in curly brackets. 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. If the set consists of a single terminal the phrase “[lookahead ≠ terminal]” may be used.

For example, given the definitions

DecimalDigit :: one of
`0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit

the definition

LookaheadExample ::
`n` [lookahead ∉ {`1`, `3`, `5`, `7`, `9`}] DecimalDigits
DecimalDigit [lookahead ∉ DecimalDigit]

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

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

ThrowStatement :
`throw` [no LineTerminator here] Expression `;`

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

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

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

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

Identifier ::
IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points 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 point

# 5.2 Algorithm Conventions

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

Algorithms may be explicitly parameterized, in which case the names and usage of the parameters must be provided as part of the algorithm’s definition. In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as operationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.operationName(arg1, arg2).

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

When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.

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

Block :
`{` StatementList `}`

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

Runtime Semantics: Evaluation

Block : `{` StatementList `}`

1. Return the result of evaluating StatementList.

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

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

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

A step may specify the iterative application of its substeps.

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

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which unless otherwise noted 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) produces 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) produces 1 if x is positive and −1 if x is negative. The sign function is not used in this standard for cases when x is zero.

The mathematical function min(x1, x2, ..., xn) produces the mathematically smallest of x1 through xn. The mathematical function max(x1, x2, ..., xn) produces the mathematically largest of x1 through xn. The domain and range of these mathematical functions include +∞ and −∞.

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) produces the largest integer (closest to positive infinity) that is not larger than x.

NOTE floor(x) = x−(x modulo 1).

# 5.3 Static Semantic Rules

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

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

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

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

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

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

# 6 ECMAScript Data Types and Values

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

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.

# 6.1 ECMAScript Language Types

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

# 6.1.1 The Undefined Type

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

# 6.1.2 The Null Type

The Null type has exactly one value, called null.

# 6.1.3 The Boolean Type

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

# 6.1.4 The String Type

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

Where ECMAScript operations interpret String values, each element is interpreted as a single UTF-16 code unit. However, ECMAScript does not place any restrictions or requirements on the sequence of code units in a String value, so they may be ill-formed when interpreted as UTF-16 code unit sequences. Operations that do not interpret String contents treat them as sequences of undifferentiated 16-bit unsigned integers. The function `String.prototype.normalize` (see 21.1.3.12) can be used to explicitly normalize a string value. `String.prototype.localeCompare` (see 21.1.3.10) internally normalizes strings values, but no other operations implicitly normalize the strings upon which they operate. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results.

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

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

- A code unit in the range 0 to 0xD7FF or in the range 0xE000 to 0xFFFF is interpreted as a code point with the same value.

- A sequence of two code units, where the first code unit c1 is in the range 0xD800 to 0xDBFF and the second code unit c2 is in the range 0xDC00 to 0xDFFF, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 – 0xDC00) + 0x10000. (See 10.1.2)

- A code unit that is in the range 0xD800 to 0xDFFF, but is not part of a surrogate pair, is interpreted as a code point with the same value.

# 6.1.5 The Symbol Type

The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.

# 6.1.5.1 Well-Known Symbols

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all Code Realms (8.2).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.

Table 1— Well-known Symbols

Specification Name | [[Description]] | Value and Purpose
--- | --- | ---
@@hasInstance | `"Symbol.hasInstance"` | A method that determines if a constructor object recognizes an object as one of the constructor’s instances. Called by the semantics of the `instanceof` operator.
@@isConcatSpreadable | `"Symbol.isConcatSpreadable"` | A Boolean valued property that if true indicates that an object should be flattened to its array elements by `Array.prototype.concat`.
@@iterator | `"Symbol.iterator"` | A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match | `"Symbol.match"` | A regular expression method that matches the regular expression against a string. Called by the `String.prototype.match` method.
@@replace | `"Symbol.replace"` | A regular expression method that replaces matched substrings of a string. Called by the `String.prototype.replace` method.
@@search | `"Symbol.search"` | A regular expression method that returns the index within a string that matches the regular expression. Called by the `String.prototype.search` method.
@@species | `"Symbol.species"` | A function valued property that is the constructor function that is used to create derived objects.
@@split | `"Symbol.split"` | A regular expression method that splits a string at the indices that match the regular expression. Called by the `String.prototype.split` method.
@@toPrimitive | `"Symbol.toPrimitive"` | A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag | `"Symbol.toStringTag"` | A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method `Object.prototype.toString`.
@@unscopables | `"Symbol.unscopables"` | An object valued property whose own property names are property names that are excluded from the `with` environment bindings of the associated object.

# 6.1.6 The Number Type

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

NOTE The bit pattern that might be observed in an ArrayBuffer (see 24.1) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

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 × 2e

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 × 2e

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, ties to even” mode.)

Some ECMAScript operators deal only with integers in specific ranges such as −231 through 231−1, inclusive, or in the range 0 through 216−1, inclusive. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.

# 6.1.7 The Object Type

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

- A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.

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

Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty string, are valid as property keys. A property name is a property key that is a String value.

An integer index is a String-valued property key that is a canonical numeric String (see 7.1.16) and whose numeric value is either +0 or a positive integer ≤ 253−1. An array index is an integer index whose numeric value i is in the range +0 ≤ i < 232−1.

Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.

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

# 6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 2.

Table 2 — Attributes of a Data Property

Attribute Name | Value Domain | Description
--- | --- | ---
[[Value]] | Any ECMAScript language type | The value retrieved by a get access of the property.
[[Writable]] | Boolean | If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]] | Boolean | If true, the property will be enumerated by a for-in enumeration (see 13.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]] | Boolean | If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

An accessor property associates a key value with the attributes listed in Table 3.

Table 3 — Attributes of an Accessor Property

Attribute Name | Value Domain | Description
--- | --- | ---
[[Get]] | Object or Undefined | If the value is an Object it must be a function Object. The function’s [[Call]] internal method (Table 6) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.

[[Set]]

Object or Undefined

If the value is an Object it must be a function Object. The function’s [[Call]] internal method (Table 6) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.

[[Enumerable]] | Boolean | If true, the property is to be enumerated by a for-in enumeration (see 13.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]] | Boolean | If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the initial values of a property’s attributes are not explicitly specified by this specification, the default value defined in Table 4 is used.

Table 4 — Default Attribute Values

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

# 6.1.7.2 Object Internal Methods and Internal Slots

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

Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.

Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 5 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

The “Signature” column of Table 5 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type. An internal method implicitly returns a Completion Record as described in 6.2.2. In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

Table 5 — Essential Internal Methods

Internal Method | Signature | Description
--- | --- | ---
[[GetPrototypeOf]] | () → Object or Null | Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]] | (Object or Null) → Boolean | Associate with this object another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[IsExtensible]] | ( ) → Boolean | Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]] | ( ) → Boolean | Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
[[GetOwnProperty]] | (propertyKey) → Undefined or Property Descriptor | Return a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[HasProperty]] | (propertyKey) → Boolean | Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]] | (propertyKey, Receiver) → any | Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as the this value when evaluating the code.
[[Set]] | (propertyKey,value, Receiver) → Boolean | Set the value of this object property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the this value when evaluating the code. Returns true if that the property value was set or false if that it could not be set.
[[Delete]] | (propertyKey) → Boolean | Remove the own property whose key is propertyKey from this object . Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
[[DefineOwnProperty]] | (propertyKey,PropertyDescriptor) → Boolean | Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Return true if that the property was successfully created/updated or false if the property could not be created or updated.
[[Enumerate]] | ()→Object | Return an iterator object that produces the keys of the string-keyed enumerable properties of the object.
[[OwnPropertyKeys]] | ()→ List of propertyKey | Return a List whose elements are all of the own property keys for the object.

Table 6 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal methods. A constructor (also referred to as a constructor function) is a function object that supports the [[Construct]] internal method.

Table 6 — Additional Essential Internal Methods of Function Objects

Internal Method | Signature | Description
--- | --- | ---
[[Call]] | (any, a List of any) → any | Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable.

[[Construct]]

(a List of any, Object)
→ Object

Creates an object. Invoked via the `new` or `super` operators. The first arguments to the internal method is a list containing the arguments of the operator. The second argument is the object to which the `new` operator was initially applied. Objects that implement this internal method are called constructors. A Function object is not necessarily a constructor and such non-constructor Function objects do not have a [[Construct]] internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 9. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.

# 6.1.7.3 Invariants of the Essential Internal Methods

The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.

Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.

An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.

Definitions:

● The target of an internal method is the object upon which the internal method is called.

● A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.

● A non-existent property is a property that does not exist as an own property on a non-extensible target.

● All references to SameValue are according to the definition of SameValue algorithm specified in 7.2.9.

[[GetPrototypeOf]] ( )

● The Type of the return value must be either Object or Null.

● If target is non-extensible, and [[GetPrototypeOf]] returns a value v, then any future calls to [[GetPrototypeOf]] should return the SameValue as v.

NOTE An object’s prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

[[SetPrototypeOf]] (V)

● The Type of the return value must be Boolean.

● If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target’s observed [[GetPrototypeOf]] value.

[[PreventExtensions]] ( )

● The Type of the return value must be Boolean.

● If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.

[[GetOwnProperty]] (P)

● The Type of the return value must be either Property Descriptor or Undefined.

● If the Type of the return value is Property Descriptor, the return value must be a complete property descriptor (see 6.2.4.6).

● If a property P is described as a data property with Desc.[[Value]] equal to v and Desc.[[Writable]] and Desc.[[Configurable]] are both false, then the SameValue must be returned for the Desc.[[Value]] attribute of the property on all future calls to [[GetOwnProperty]] ( P ).

● If P’s attributes other than [[Writable]] may change over time or if the property might disappear, then P’s [[Configurable]] attribute must be true.

● If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.

● If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).

NOTE As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the Desc.[[Writable]] and Desc.[[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other internal methods.

[[DefineOwnProperty]] (P, Desc)

● The Type of the return value must be Boolean.

● [[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:

1. P is a non-configurable writable own data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.

2. All attributes in Desc are the SameValue as P’s attributes.

● [[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

[[HasProperty]] ( P )

● The Type of the return value must be Boolean.

● If P was previously observed as a non-configurable data or accessor own property of the target, [[HasProperty]] must return true.

[[Get]] (P, Receiver)

● If P was previously observed as a non-configurable, non-writable own data property of the target with value v, then [[Get]] must return the SameValue.

● If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

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

● The Type of the return value must be Boolean.

● If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P’s [[Value]] attribute.

● If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

[[Delete]] ( P )

● The Type of the return value must be Boolean.

● If P was previously observed to be a non-configurable own data or accessor property of the target, [[Delete]] must return false.

[[Enumerate]] ( )

● The Type of the return value must be Object.

[[OwnPropertyKeys]] ( )

● The return value must be a List.

●  The Type of each element of the returned List is either String or Symbol.

● The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.

● If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].

[[Construct]] ( )

● The Type of the return value must be Object.

# 6.1.7.4 Well-Known Intrinsic Objects

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

Within this specification a reference such as %name% means the intrinsic object, associated with the current Realm, corresponding to the name. Determination of the current Realm and its intrinsics is described in 8.1.2.5. The well-known intrinsics are listed in Table 7.

Table 7 — Well-known Intrinsic Objects

Intrinsic Name | Global Name | ECMAScript Language Association
--- | --- | ---
%Array% | `Array` | The `Array` constructor (22.1.1)
%ArrayBuffer% | `ArrayBuffer` | The `ArrayBuffer` constructor (24.1.2)
%ArrayBufferPrototype% | `ArrayBuffer.prototype` | The initial value of the `prototype` data property of %ArrayBuffer%.
%ArrayIteratorPrototype% |  | The prototype of Array iterator objects (22.1.5)
%ArrayPrototype% | `Array.prototype` | The initial value of the `prototype` data property of %Array% (22.1.3)
%ArrayProto_values% | `Array.prototype.values` | The initial value of the `values` data property of %ArrayPrototype% (22.1.3.29)
%Boolean% | `Boolean` | The `Boolean` constructor (19.3.1)
%BooleanPrototype% | `Boolean.prototype` | The initial value of the `prototype` data property of %Boolean% (19.3.3)
%DataView% | `DataView` | The `DataView` constructor (24.2.2)
%DataViewPrototype% | `DataView.prototype` | The initial value of the `prototype` data property of %DataView%
%Date% | `Date` | The `Date` constructor (20.3.2)
%DatePrototype% | `Date.prototype` | The initial value of the `prototype` data property of %Date%.
%decodeURI% | `decodeURI` | The `decodeURI` function (18.2.6.2)
%decodeURIComponent% | `decodeURIComponent` | The `decodeURIComponent` function (18.2.6.3)
%encodeURI% | `encodeURI` | The `encodeURI` function (18.2.6.4)
%encodeURIComponent% | `encodeURIComponent` | The `encodeURIComponent` function (18.2.6.5)
%Error% | `Error` | The `Error` constructor (19.5.1)
%ErrorPrototype% | `Error.prototype` | The initial value of the `prototype` data property of %Error%
%eval% | `eval` | The `eval` function (18.2.1)
%EvalError% | `EvalError` | The `EvalError` constructor (19.5.5.1)
%EvalErrorPrototype% | `EvalError.prototype` | The initial value of the `prototype` property of %EvalError%
%Float32Array% | `Float32Array` | The `Float32Array` constructor (22.2)
%Float32ArrayPrototype% | `Float32Array.prototype` | The initial value of the `prototype` data property of %Float32Array%.
%Float64Array% | `Float64Array` | The `Float64Array` constructor (22.2)
%Float64ArrayPrototype% | `Float64Array.prototype` | The initial value of the `prototype` data property of %Float64Array%
%Function% | `Function` | The `Function` constructor (19.2.1)
%FunctionPrototype% | `Function.prototype` | The initial value of the `prototype` data property of %Function%
%Generator% |  | The initial value of the `prototype` property of %GeneratorFunction%
%GeneratorFunction% |  | The constructor of generator objects (25.2.1)
%GeneratorPrototype% |  | The initial value of the `prototype` property of %Generator%
%Int8Array% | `Int8Array` | The `Int8Array` constructor (22.2)
%Int8ArrayPrototype% | `Int8Array.prototype` | The initial value of the `prototype` data property of %Int8Array%
%Int16Array% | `Int16Array` | The `Int16Array` constructor (22.2)
%Int16ArrayPrototype% | `Int16Array.prototype` | The initial value of the `prototype` data property of %Int16Array%
%Int32Array% | `Int32Array` | The `Int32Array` constructor (22.2)
%Int32ArrayPrototype% | `Int32Array.prototype` | The initial value of the `prototype` data property of %Int32Array%
%isFinite% | `isFinite` | The `isFinite` function (18.2.2)
%isNaN% | `isNaN` | The `isNaN` function (18.2.3)
%IteratorPrototype% |  | An object that all standard built-in iterator objects indirectly inherit from
%JSON% | `JSON` | The `JSON` object (24.3)
%Map% | `Map` | The `Map` constructor (23.1.1)
%MapIteratorPrototype% |  | The prototype of Map iterator objects (23.1.5)
%MapPrototype% | `Map.prototype` | The initial value of the `prototype` data property of %Map%
%Math% | `Math` | The `Math` object (20.2)
%Number% | `Number` | The `Number` constructor (20.1.1)
%NumberPrototype% | `Number.prototype` | The initial value of the `prototype` property of %Number%
%Object% | `Object` | The `Object` constructor (19.1.1)
%ObjectPrototype% | `Object.prototype` | The initial value of the `prototype` data property of %Object%. (19.1.3)
%ObjProto_toString% | `Object.prototype. toString` | The initial value of the `toString` data property of %ObjectPrototype% (19.1.3.6)
%parseFloat% | `parseFloat` | The `parseFloat` function (18.2.4)
%parseInt% | `parseInt` | The `parseInt` function (18.2.5)
%Promise% | `Promise` | The `Promise` constructor (25.4.3)
%PromisePrototype% | `Promise.prototype` | The initial value of the `prototype` data property of %Promise%
%Proxy% | `Proxy` | The `Proxy` constructor (26.2.1)
%RangeError% | `RangeError` | The `RangeError` constructor (19.5.5.2)
%RangeErrorPrototype% | `RangeError.prototype` | The initial value of the `prototype` property of %RangeError%
%ReferenceError% | `ReferenceError` | The `ReferenceError` constructor (19.5.5.3)
%ReferenceErrorPrototype% | `ReferenceError. prototype` | The initial value of the `prototype` property of %ReferenceError%
%Reflect% | `Reflect` | The `Reflect` object (26.1)
%RegExp% | `RegExp` | The `RegExp` constructor (21.2.3)
%RegExpPrototype% | `RegExp.prototype` | The initial value of the `prototype` data property of %RegExp%
%Set% | `Set` | The `Set` constructor (23.2.1)
%SetIteratorPrototype% |  | The prototype of Set iterator objects (23.2.5)
%SetPrototype% | `Set.prototype` | The initial value of the `prototype` data property of %Set%
%String% | `String` | The `String` constructor (21.1.1)
%StringIteratorPrototype% |  | The prototype of String iterator objects (21.1.5)
%StringPrototype% | `String.prototype` | The initial value of the `prototype` data property of %String%
%Symbol% | `Symbol` | The `Symbol` constructor (19.4.1)
%SymbolPrototype% | `Symbol.prototype` | The initial value of the `prototype` data property of %Symbol%. (19.4.3)
%SyntaxError% | `SyntaxError` | The `SyntaxError` constructor (19.5.5.4)
%SyntaxErrorPrototype% | `SyntaxError.prototype` | The initial value of the `prototype` property of %SyntaxError%
%ThrowTypeError% |  | A function object that unconditionally throws a new instance of %TypeError%
%TypedArray% |  | The super class of all typed Array constructors (22.2.1)
%TypedArrayPrototype% |  | The initial value of the `prototype` property of %TypedArray%
%TypeError% | `TypeError` | The `TypeError` constructor (19.5.5.5)
%TypeErrorPrototype% | `TypeError.prototype` | The initial value of the `prototype` property of %TypeError%
%Uint8Array% | `Uint8Array` | The `Uint8Array` constructor (22.2)
%Uint8ArrayPrototype% | `Uint8Array.prototype` | The initial value of the `prototype` data property of %Uint8Array%
%Uint8ClampedArray% | `Uint8ClampedArray` | The `Uint8ClampedArray` constructor (22.2)
%Uint8ClampedArrayPrototype% | `Uint8ClampedArray. prototype` | The initial value of the `prototype` data property of %Uint8ClampedArray%
%Uint16Array% | `Uint16Array` | The `Uint16Array` constructor (22.2)
%Uint16ArrayPrototype% | `Uint16Array.prototype` | The initial value of the `prototype` data property of %Uint16Array%
%Uint32Array% | `Uint32Array` | The `Uint32Array` constructor (22.2)
%Uint32ArrayPrototype% | `Uint32Array.prototype` | The initial value of the `prototype` data property of %Uint32Array%
%URIError% | `URIError` | The `URIError` constructor (19.5.5.6)
%URIErrorPrototype% | `URIError.prototype` | The initial value of the `prototype` property of %URIError%
%WeakMap% | `WeakMap` | The `WeakMap` constructor (23.3.1)
%WeakMapPrototype% | `WeakMap.prototype` | The initial value of the `prototype` data property of %WeakMap%
%WeakSet% | `WeakSet` | The `WeakSet` constructor (23.4.1)
%WeakSetPrototype% | `WeakSet.prototype` | The initial value of the `prototype` data property of %WeakSet%

# 6.2 ECMAScript Specification Types

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

# 6.2.1 The List and Record Specification Type

The List type is used to explain the evaluation of argument lists (see 12.3.6) in `new` expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.

For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, «1, 2» defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as «».

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

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

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

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

# 6.2.2 The Completion Record Specification Type

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

Values of the Completion type are Record values whose fields are defined as by Table 8.

Table 8 — Completion Record Fields

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

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

# 6.2.2.1 NormalCompletion

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

1. Return NormalCompletion(argument).

Is a shorthand that is defined as follows:

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

# 6.2.2.2 Implicit Completion Values

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

1. Return `"Infinity"`.

means the same thing as:

1. Return NormalCompletion(`"Infinity"`).

However, if the value expression of a “return” statement is a Completion Record construction literal, the resulting Completion Record is returned. If the value expression is a call to an abstract operation, the “return” statement simply returns the Completion Record produced by the abstract operation.

The abstract operation Completion(completionRecord) is used to emphasize that a previously computed Completion Record is being returned. The Completion abstract operation takes a single argument, completionRecord, and performs the following steps: such as

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

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

1. Return NormalCompletion(undefined).

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

# 6.2.2.3 Throw an Exception

Algorithms steps that say to throw an exception, such as

1. Throw a TypeError exception.

mean the same things as:

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

# 6.2.2.4 ReturnIfAbrupt

Algorithms steps that say

1. ReturnIfAbrupt(argument).

mean the same thing as:

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

# 6.2.3 The Reference Specification Type

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

A Reference is a resolved name or property 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 Symbol, a Number, or an Environment Record (8.1.1). A base value of undefined indicates that the Reference could not be resolved to a binding. The referenced name is a String or Symbol value.

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

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

- GetBase(V). Returns the base value component of the reference V.

- GetReferencedName(V). Returns the referenced name component of the reference V.

- IsStrictReference(V). Returns the strict reference flag component of the reference V.

- HasPrimitiveBase(V). Returns true if Type(base) is Boolean, String, Symbol, or Number.

- IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true; otherwise returns false.

- IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.

- IsSuperReference(V). Returns true if this reference has a thisValue component.

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

# 6.2.3.1 GetValue (V)

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

NOTE The object that may be created in step 5.a.ii is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

# 6.2.3.2 PutValue (V, W)

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

NOTE The object that may be created in step 6.a.ii is not accessible outside of the above algorithm and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

# 6.2.3.3 GetThisValue (V)

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

# 6.2.3.4 InitializeReferencedBinding (V, W)

1. ReturnIfAbrupt(V).
2. ReturnIfAbrupt(W).
3. Assert: Type(V) is Reference.
4. Assert: IsUnresolvableReference(V) is false.
5. Let base be GetBase(V).
6. Assert: base is an Environment Record.
7. Return base.InitializeBinding(GetReferencedName(V), W).

# 6.2.4 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field’s name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. In addition, any field may be present or absent. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

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 Table 2 or Table 3.

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

# 6.2.4.1 IsAccessorDescriptor ( Desc )

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

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

# 6.2.4.2 IsDataDescriptor ( Desc )

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

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

# 6.2.4.3 IsGenericDescriptor ( Desc )

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

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

# 6.2.4.4 FromPropertyDescriptor ( Desc )

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

1. If Desc is undefined, return undefined.
2. Let obj be ObjectCreate(%ObjectPrototype%).
3. Assert: obj is an extensible ordinary object with no own properties.
4. If Desc has a [[Value]] field, then
   1. Call CreateDataProperty(obj, `"value"`, Desc.[[Value]]).
5. If Desc has a [[Writable]] field, then
   1. Call CreateDataProperty(obj, `"writable"`, Desc.[[Writable]]).
6. If Desc has a [[Get]] field, then
   1. Call CreateDataProperty(obj, `"get",` Desc.[[Get]]).
7. If Desc has a [[Set]] field, then
   1. Call CreateDataProperty(obj, `"set"`, Desc.[[Set]])
8. If Desc has an [[Enumerable]] field, then
   1. Call CreateDataProperty(obj, `"enumerable"`, Desc.[[Enumerable]]).
9. If Desc has a [[Configurable]] field, then
   1. Call CreateDataProperty(obj , `"configurable"`, Desc.[[Configurable]]).
10. Assert: all of the above CreateDataProperty operations return true.
11. Return obj.

# 6.2.4.5 ToPropertyDescriptor ( Obj )

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

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

# 6.2.4.6 CompletePropertyDescriptor ( Desc )

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

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

# 6.2.5 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 8.1.

# 6.2.6 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

For notational convenience within this specification, an array-like syntax can be used to express to the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to express access to its 3rd byte.

The following abstract operations are used in this specification to operate upon Data Block values:

# 6.2.6.1 CreateByteDataBlock(size)

When the abstract operation CreateByteDataBlock is called with integer argument size, the following steps are taken:

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

# 6.2.6.2 CopyDataBlockBytes(toBlock, toIndex, fromBlock, fromIndex, count)

When the abstract operation CopyDataBlockBytes is called the following steps are taken:

1. Assert: fromBlock and toBlock are distinct Data Block values.
2. Assert: fromIndex, toIndex, and count are positive integer values.
3. Let fromSize be the number of bytes in fromBlock.
4. Assert: fromIndex+count ≤ fromSize.
5. Let toSize be the number of bytes in toBlock.
6. Assert: toIndex+count ≤ toSize.
7. Repeat, while count>0
   1. Set toBlock[toIndex] to the value of fromBlock[fromIndex].
   2. Increment toIndex and fromIndex each by 1.
   3. Decrement count by 1.
8. Return NormalCompletion(empty)

# 7 Abstract Operations

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

# 7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type or of a Completion Record value. But no other specification types are used with these operations.

# 7.1.1 ToPrimitive ( input [, PreferredType] )

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 9:

Table 9 — ToPrimitive Conversions

Input Type | Result
--- | ---
Completion Record | If input is an abrupt completion, return input. Otherwise return ToPrimitive(input.[[value]]) also passing the optional hint PreferredType.
Undefined | Return input.
Null | Return input.
Boolean | Return input.
Number | Return input.
String | Return input.
Symbol | Return input.
Object | Perform the steps following this table.

When Type(input) is Object, the following steps are taken:

1. If PreferredType was not passed, let hint be `"default"`.
2. Else if PreferredType is hint String, let hint be `"string"`.
3. Else PreferredType is hint Number, let hint be `"number"`.
4. Let exoticToPrim be GetMethod(input, @@toPrimitive).
5. ReturnIfAbrupt(exoticToPrim).
6. If exoticToPrim is not undefined, then
   1. Let result be Call(exoticToPrim, input, «hint»).
   2. ReturnIfAbrupt(result).
   3. If Type(result) is not Object, return result.
   4. Throw a TypeError exception.
7. If hint is `"default"`, let hint be `"number"`.
8. Return OrdinaryToPrimitive(input,hint).

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

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

NOTE When ToPrimitive is called with no hint, then it generally behaves as if the hint were Number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Date objects (see 20.3.4.45) and Symbol objects (see 19.4.3.4) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.

# 7.1.2 ToBoolean ( argument )

The abstract operation ToBoolean converts argument to a value of type Boolean according to Table 10:

Table 10 — ToBoolean Conversions

Argument Type | Result
--- | ---
Completion Record | If argument is an abrupt completion, return argument. Otherwise return ToBoolean(argument.[[value]]).
Undefined | Return false.
Null | Return false.
Boolean | Return argument.
Number | Return false if argument is +0, −0, or NaN; otherwise return true.
String | Return false if argument is the empty String (its length is zero); otherwise return true.
Symbol | Return true.
Object | Return true.

# 7.1.3 ToNumber ( argument )

The abstract operation ToNumber converts argument to a value of type Number according to Table 11:

Table 11 — ToNumber Conversions

Argument Type | Result
--- | ---
Completion Record | If argument is an abrupt completion, return argument. Otherwise return ToNumber(argument.[[value]]).
Undefined | Return NaN.
Null | Return +0.
Boolean | Return 1 if argument is true. Return +0 if argument is false.
Number | Return argument (no conversion).
String | See grammar and conversion algorithm below.
Symbol | Throw a TypeError exception.

Object

Apply the following steps:

1. Let primValue be ToPrimitive(argument, hint Number).
2. Return ToNumber(primValue).

# 7.1.3.1 ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String interpreted as a sequence of UTF-16 encoded code points (6.1.4). If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

NOTE The terminal symbols of this grammar are all composed of Unicode BMP code points so the result will be NaN if the string contains the UTF-16 encoding of any supplementary code points or any unpaired surrogate code points

## Syntax

StringNumericLiteral :::
StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::
WhiteSpace
LineTerminator
StrNumericLiteral :::
StrDecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
StrDecimalLiteral :::
StrUnsignedDecimalLiteral
`+` StrUnsignedDecimalLiteral
`-` StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::
Infinity
DecimalDigits `.` DecimalDigitsopt ExponentPartopt
`.` DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one of
`0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
ExponentPart :::
ExponentIndicator SignedInteger
ExponentIndicator ::: one of
`e` `E`
SignedInteger :::
DecimalDigits
`+` DecimalDigits
`-` DecimalDigits

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (11.8.3)

NOTE Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 11.8.3):

- A StringNumericLiteral may include leading and/or trailing 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 include a `+` or `-` to indicate its sign.

- A StringNumericLiteral that is empty or contains only white space is converted to +0.

- `Infinity` `and –Infinity` are recognized as a StringNumericLiteral but not as a NumericLiteral.

# 7.1.3.1.1 Runtime Semantics: MV’s

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 11.8.3), 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. 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 on any grammar symbol, not provided below, is the MV for that symbol defined in 11.8.3.1.

- The MV of StringNumericLiteral ::: [empty] is 0.

- The MV of StringNumericLiteral ::: StrWhiteSpace is 0.

- The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV of StrNumericLiteral, no matter whether white space is present or not.

- The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.

- The MV of StrNumericLiteral ::: BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.

- The MV of StrNumericLiteral ::: OctalIntegerLiteral is the MV of OctalIntegerLiteral.

- The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.

- The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

- The MV of StrDecimalLiteral ::: `+` StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

- The MV of StrDecimalLiteral ::: `-` StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point +0 or −0 as appropriate.)

- The MV of StrUnsignedDecimalLiteral ::: Infinity is 1010000 (a value so large that it will round to +∞).

- The MV of StrUnsignedDecimalLiteral ::: DecimalDigits `.` is the MV of DecimalDigits.

- The MV of StrUnsignedDecimalLiteral ::: DecimalDigits `.` DecimalDigits is the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10−n), where n is the number of code points in the second DecimalDigits.

- The MV of StrUnsignedDecimalLiteral ::: DecimalDigits `.` ExponentPart is the MV of DecimalDigits times 10e, 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 10e, where n is the number of code points 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 code points in DecimalDigits.

- The MV of StrUnsignedDecimalLiteral ::: `.` DecimalDigits ExponentPart is the MV of DecimalDigits times 10e−n, where n is the number of code points 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 10e, where e is the MV of ExponentPart.

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 code point 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 6.1.6), 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.

# 7.1.4 ToInteger ( argument )

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

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, return +0.
4. If number is +0, −0, +∞, or −∞, return number.
5. Return the number value that is the same sign as number and whose magnitude is floor(abs(number)).

# 7.1.5 ToInt32 ( argument )

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

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +∞, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int32bit be int modulo 232.
6. If int32bit ≥ 231, return int32bit − 232, otherwise return int32bit.

NOTE Given the above definition of ToInt32:

- The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

- ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)

- ToInt32 maps −0 to +0.

# 7.1.6 ToUint32 ( argument )

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

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +∞, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int32bit be int modulo 232.
6. Return int32bit.

NOTE Given the above definition of ToUint32:

- Step 6 is the only difference between ToUint32 and ToInt32.

- The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

- ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)

- ToUint32 maps −0 to +0.

# 7.1.7 ToInt16 ( argument )

The abstract operation ToInt16 converts argument to one of 216 integer values in the range −32768 through 32767, inclusive. This abstract operation functions as follows:

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +∞, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int16bit be int modulo 216.
6. If int16bit ≥ 215, return int16bit − 216, otherwise return int16bit.

# 7.1.8 ToUint16 ( argument )

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

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +∞, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int16bit be int modulo 216.
6. Return int16bit.

NOTE Given the above definition of ToUint16:

- The substitution of 216 for 232 in step 5 is the only difference between ToUint32 and ToUint16.
- ToUint16 maps −0 to +0.

# 7.1.9 ToInt8 ( argument )

The abstract operation ToInt8 converts argument to one of 28 integer values in the range −128 through 127, inclusive. This abstract operation functions as follows:

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +∞, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int8bit be int modulo 28.
6. If int8bit ≥ 27, return int8bit − 28, otherwise return int8bit.

# 7.1.10 ToUint8 ( argument )

The abstract operation ToUint8 converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, +0, −0, +&ininfin;, or −∞, return +0.
4. Let int be the mathematical value that is the same sign as number and whose magnitude is floor(abs(number)).
5. Let int8bit be int modulo 28.
6. Return int8bit.

# 7.1.11 ToUint8Clamp ( argument )

The abstract operation ToUint8Clamp converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:

1. Let number be ToNumber(argument).
2. ReturnIfAbrupt(number).
3. If number is NaN, return +0.
4. If number ≤ 0, return +0.
5. If number ≥ 255, return 255.
6. Let f be floor(number).
7. If f + 0.5 < number, return f + 1.
8. If number < f + 0.5, return f.
9. If f is odd, return f + 1.
10. Return f.

NOTE Note that unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp rounds rather than truncates non-integer values and does not convert +∞ to 0. ToUint8Clamp does “round half to even” tie-breaking. This differs from `Math.round` which does “round half up” tie-breaking.

# 7.1.12 ToString ( argument )

The abstract operation ToString converts argument to a value of type String according to Table 12:

Table 12 — ToString Conversions

Argument Type | Result
--- | ---
Completion Record | If argument is an abrupt completion, return argument. Otherwise return ToString(argument.[[value]]).
Undefined | Return `"undefined"`.
Null | Return `"null"`.
Boolean | If argument is true, return `"true"`. If argument is false, return `"false"`.
Number | See 7.1.12.1.
String | Return argument.
Symbol | Throw a TypeError exception.
Object | Apply the following steps: 1. Let primValue be ToPrimitive(argument, hint String). 2. Return ToString(primValue).

# 7.1.12.1 ToString Applied to the Number Type

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

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

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

- If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number value as x.

- The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

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

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

NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various `netlib` mirror sites.

# 7.1.13 ToObject ( argument )

The abstract operation ToObject converts argument to a value of type Object according to Table 13:

Table 13 — ToObject Conversions

Argument Type | Result
--- | ---
Completion Record | If argument is an abrupt completion, return argument. Otherwise return ToObject(argument.[[value]]).
Undefined | Throw a TypeError exception.
Null | Throw a TypeError exception.
Boolean | Return a new Boolean object whose [[BooleanData]] internal slot is set to the value of argument. See 19.3 for a description of Boolean objects.
Number | Return a new Number object whose [[NumberData]] internal slot is set to the value of argument. See 20.1 for a description of Number objects.
String | Return a new String object whose [[StringData]] internal slot is set to the value of argument. See 21.1 for a description of String objects.
Symbol | Return a new Symbol object whose [[SymbolData]] internal slot is set to the value of argument. See 19.4 for a description of Symbol objects.
Object | Return argument.

# 7.1.14 ToPropertyKey ( argument )

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

1. Let key be ToPrimitive(argument, hint String).
2. ReturnIfAbrupt(key).
3. If Type(key) is Symbol, then
   1. Return key.
4. Return ToString(key).

# 7.1.15 ToLength ( argument )

The abstract operation ToLength converts argument to an integer suitable for use as the length of an array-like object. It performs the following steps:

1. ReturnIfAbrupt(argument).
2. Let len be ToInteger(argument).
3. ReturnIfAbrupt(len).
4. If len ≤ +0, return +0.
5. If len is +∞, return 253-1.
6. Return min(len, 253-1).

# 7.1.16 CanonicalNumericIndexString ( argument )

The abstract operation CanonicalNumericIndexString returns argument converted to a numeric value if it is a String representation of a Number that would be produced by ToString, or the string `"-0"`. Otherwise, it returns undefined. This abstract operation functions as follows:

1. Assert: Type(argument) is String.
2. If argument is `"-0"`, return −0.
3. Let n be ToNumber(argument).
4. If SameValue(ToString(n), argument) is false, return undefined.
5. Return n.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return undefined.

# 7.2 Testing and Comparison Operations

# 7.2.1 RequireObjectCoercible ( argument )

The abstract operation RequireObjectCoercible throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:

Table 14 — RequireObjectCoercible Results

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

# 7.2.2 IsArray ( argument )

The abstract operation IsArray takes one argument argument, and performs the following steps:

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

# 7.2.3 IsCallable ( argument )

The abstract operation IsCallable determines if argument, which must be an ECMAScript language value or a Completion Record, is a callable function with a [[Call]] internal method.

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

# 7.2.4 IsConstructor ( argument )

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

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

# 7.2.5 IsExtensible (O)

The abstract operation IsExtensible is used to determine whether additional properties can be added to the object that is O. A Boolean value is returned. This abstract operation performs the following steps:

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

# 7.2.6 IsInteger ( argument )

The abstract operation IsInteger determines if argument is a finite integer numeric value.

1. ReturnIfAbrupt(argument).
2. If Type(argument) is not Number, return false.
3. If argument is NaN, +∞, or −∞, return false.
4. If floor(abs(argument)) ≠ abs(argument), return false.
5. Return true.

# 7.2.7 IsPropertyKey ( argument )

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

1. ReturnIfAbrupt(argument).
2. If Type(argument) is String, return true.
3. If Type(argument) is Symbol, return true.
4. Return false.

# 7.2.8 IsRegExp ( argument )

The abstract operation IsRegExp with argument argument performs the following steps:

1. If Type(argument) is not Object, return false.
2. Let isRegExp be Get(argument, @@match).
3. ReturnIfAbrupt(isRegExp).
4. If isRegExp is not undefined, return ToBoolean(isRegExp).
5. If argument has a [[RegExpMatcher]] internal slot, return true.
6. Return false.

# 7.2.9 SameValue(x, y)

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

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).
3. If Type(x) is different from Type(y), return false.
4. If Type(x) is Undefined, return true.
5. If Type(x) is Null, return true.
6. If Type(x) is Number, then
   1. If x is NaN and y is NaN, return true.
   2. If x is +0 and y is -0, return false.
   3. If x is -0 and y is +0, return false.
   4. If x is the same Number value as y, return true.
   5. Return false.
7. If Type(x) is String, then
   1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices) return true; otherwise, return false.
8. If Type(x) is Boolean, then
   1. If x and y are both true or both false, return true; otherwise, return false.
9. If Type(x) is Symbol, then
   1. If x and y are both the same Symbol value, return true; otherwise, return false.
10. Return true if x and y are the same Object value. Otherwise, return false.

# 7.2.10 SameValueZero(x, y)

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

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).
3. If Type(x) is different from Type(y), return false.
4. If Type(x) is Undefined, return true.
5. If Type(x) is Null, return true.
6. If Type(x) is Number, then
   1. If x is NaN and y is NaN, return true.
   2. If x is +0 and y is -0, return true.
   3. If x is -0 and y is +0, return true.
   4. If x is the same Number value as y, return true.
   5. Return false.
7. If Type(x) is String, then
   1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices) return true; otherwise, return false.
8. If Type(x) is Boolean, then
   1. If x and y are both true or both false, return true; otherwise, return false.
9. If Type(x) is Symbol, then
   1. If x and y are both the same Symbol value, return true; otherwise, return false.
10. Return true if x and y are the same Object value. Otherwise, return false.

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

# 7.2.11 Abstract Relational Comparison

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

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).
3. If the LeftFirst flag is true, then
   1. Let px be ToPrimitive(x, hint Number).
   2. ReturnIfAbrupt(px).
   3. Let py be ToPrimitive(y, hint Number).
   4. ReturnIfAbrupt(py).
4. Else the order of evaluation needs to be reversed to preserve left to right evaluation
   1. Let py be ToPrimitive(y, hint Number).
   2. ReturnIfAbrupt(py).
   3. Let px be ToPrimitive(x, hint Number).
   4. ReturnIfAbrupt(px).
5. If both px and py are Strings, then
   1. If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
   2. If px is a prefix of py, return true.
   3. Let k be the smallest nonnegative integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
   4. Let m be the integer that is the code unit value at index k within px.
   5. Let n be the integer that is the code unit value at index k within py.
   6. If m < n, return true. Otherwise, return false.
6. Else,
   1. Let nx be ToNumber(px). Because px and py are primitive values evaluation order is not important.
   2. ReturnIfAbrupt(nx).
   3. Let ny be ToNumber(py).
   4. ReturnIfAbrupt(ny).
   5. If nx is NaN, return undefined.
   6. If ny is NaN, return undefined.
   7. If nx and ny are the same Number value, return false.
   8. If nx is +0 and ny is −0, return false.
   9. If nx is −0 and ny is +0, return false.
   10. If nx is +∞, return false.
   11. If ny is +∞, return true.
   12. If ny is −∞, return false.
   13. If nx is −∞, return true.
   14. If the mathematical value of nx is less than the mathematical value of ny —note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.

NOTE 1 Step 5 differs from step 11 in the algorithm for the addition operator `+` (12.7.3) in using “and” instead of “or”.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

# 7.2.12 Abstract Equality Comparison

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

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).
3. If Type(x) is the same as Type(y), then
   1. Return the result of performing Strict Equality Comparison x === y.
4. If x is null and y is undefined, return true.
5. If x is undefined and y is null, return true.
6. If Type(x) is Number and Type(y) is String,
   return the result of the comparison x == ToNumber(y).
7. If Type(x) is String and Type(y) is Number,
   return the result of the comparison ToNumber(x) == y.
8. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
9. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
10. If Type(x) is either String, Number, or Symbol and Type(y) is Object, then
    return the result of the comparison x == ToPrimitive(y).
11. If Type(x) is Object and Type(y) is either String, Number, or Symbol, then
    return the result of the comparison ToPrimitive(x) == y.
12. Return false.

# 7.2.13 Strict Equality Comparison

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

1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is Number, then
   1. If x is NaN, return false.
   2. If y is NaN, return false.
   3. If x is the same Number value as y, return true.
   4. If x is +0 and y is −0, return true.
   5. If x is −0 and y is +0, return true.
   6. Return false.
5. If Type(x) is String, then
   1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true.
   2. Else, return false.
6. If Type(x) is Boolean, then
   1. If x and y are both true or both false, return true.
   2. Else, return false.
7. If x and y are the same Symbol value, return true.
8. If x and y are the same Object value, return true.
9. Return false.

NOTE This algorithm differs from the SameValue Algorithm (7.2.9) in its treatment of signed zeroes and NaNs.

# 7.3 Operations on Objects

# 7.3.1 Get (O, P)

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

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

# 7.3.2 GetV (V, P)

The abstract operation GetV is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. The operation is called with arguments V and P where V is the value and P is the property key. This abstract operation performs the following steps:

1. Assert: IsPropertyKey(P) is true.
2. Let O be ToObject(V).
3. ReturnIfAbrupt(O).
4. Return O.[[Get]](P, V).

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

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

1. Assert: Type(O) is Object.
2. Assert: IsPropertyKey(P) is true.
3. Assert: Type(Throw) is Boolean.
4. Let success be O.[[Set]](P, V, O).
5. ReturnIfAbrupt(success).
6. If success is false and Throw is true, throw a TypeError exception.
7. Return success.

# 7.3.4 CreateDataProperty (O, P, V)

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

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

NOTE This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

# 7.3.5 CreateMethodProperty (O, P, V)

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

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

NOTE This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

# 7.3.6 CreateDataPropertyOrThrow (O, P, V)

The abstract operation CreateDataPropertyOrThrow is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:

1. Assert: Type(O) is Object.
2. Assert: IsPropertyKey(P) is true.
3. Let success be CreateDataProperty(O, P, V).
4. ReturnIfAbrupt(success).
5. If success is false, throw a TypeError exception.
6. Return success.

NOTE This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.

# 7.3.7 DefinePropertyOrThrow (O, P, desc)

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

1. Assert: Type(O) is Object.
2. Assert: IsPropertyKey(P) is true.
3. Let success be O.[[DefineOwnProperty]](P, desc).
4. ReturnIfAbrupt(success).
5. If success is false, throw a TypeError exception.
6. Return success.

# 7.3.8 DeletePropertyOrThrow (O, P)

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

1. Assert: Type(O) is Object.
2. Assert: IsPropertyKey(P) is true.
3. Let success be O.[[Delete]](P).
4. ReturnIfAbrupt(success).
5. If success is false, throw a TypeError exception.
6. Return success.

# 7.3.9 GetMethod (O, P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let func be GetV(O, P).
3. ReturnIfAbrupt(func).
4. If func is either undefined or null, return undefined.
5. If IsCallable(func) is false, throw a TypeError exception.
6. Return func.

# 7.3.10 HasProperty (O, P)

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

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

# 7.3.11 HasOwnProperty (O, P)

The abstract operation HasOwnProperty is used to determine whether an object has an own property with the specified property key. A Boolean value is returned. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

1. Assert: Type(O) is Object.
2. Assert: IsPropertyKey(P) is true.
3. Let desc be O.[[GetOwnProperty]](P).
4. ReturnIfAbrupt(desc).
5. If desc is undefined, return false.
6. Return true.

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

The abstract operation Call is used to call the [[Call]] internal method of a function object. The operation is called with arguments F, V , and optionally argumentsList where F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, an empty List is used as its value. This abstract operation performs the following steps:

1. ReturnIfAbrupt(F).
2. If argumentsList was not passed, let argumentsList be a new empty List.
3. If IsCallable(F) is false, throw a TypeError exception.
4. Return F.[[Call]](V, argumentsList).

# 7.3.13 Invoke(O,P, [argumentsList])

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

1. Assert: P is a valid property key.
2. If argumentsList was not passed, let argumentsList be a new empty List.
3. Let func be GetV(O, P).
4. Return Call(func, O, argumentsList).

# 7.3.14 Construct (F, [argumentsList], [newTarget])

The abstract operation Construct is used to call the [[Construct]] internal method of a function object. The operation is called with arguments F, and optionally argumentsList, and newTarget where F is the function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, an empty List is used as its value. If newTarget is not present, F is used as its value. This abstract operation performs the following steps:

1. If newTarget was not passed, let newTarget be F.
2. If argumentsList was not passed, let argumentsList be a new empty List.
3. Assert: IsConstructor (F) is true.
4. Assert: IsConstructor (newTarget) is true.
5. Return F.[[Construct]](argumentsList, newTarget).

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

# 7.3.15 SetIntegrityLevel (O, level)

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

1. Assert: Type(O) is Object.
2. Assert: level is either `"sealed"` or `"frozen"`.
3. Let status be O.[[PreventExtensions]]().
4. ReturnIfAbrupt(status).
5. If status is false, return false.
6. Let keys be O.[[OwnPropertyKeys]]().
7. ReturnIfAbrupt(keys).
8. If level is `"sealed"`, then
   1. Repeat for each element k of keys,
      1. Let status be DefinePropertyOrThrow(O, k, PropertyDescriptor{ [[Configurable]]: false}).
      2. ReturnIfAbrupt(status).
9. Else level is `"frozen"`,
   1. Repeat for each element k of keys,
      1. Let currentDesc be O.[[GetOwnProperty]](k).
      2. ReturnIfAbrupt(currentDesc).
      3. If currentDesc is not undefined, then
         1. If IsAccessorDescriptor(currentDesc) is true, then
            1. Let desc be the PropertyDescriptor{[[Configurable]]: false}.
         2. Else,
            1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
         3. Let status be DefinePropertyOrThrow(O, k, desc).
         4. ReturnIfAbrupt(status).
10. Return true.

# 7.3.16 TestIntegrityLevel (O, level)

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

1. Assert: Type(O) is Object.
2. Assert: level is either `"sealed"` or `"frozen"`.
3. Let status be IsExtensible(O).
4. ReturnIfAbrupt(status).
5. If status is true, return false
6. NOTE If the object is extensible, none of its properties are examined.
7. Let keys be O.[[OwnPropertyKeys]]().
8. ReturnIfAbrupt(keys).
9. Repeat for each element k of keys,
   1. Let currentDesc be O.[[GetOwnProperty]](k).
   2. ReturnIfAbrupt(currentDesc).
   3. If currentDesc is not undefined, then
      1. If currentDesc.[[Configurable]] is true, return false.
      2. If level is `"frozen"` and IsDataDescriptor(currentDesc) is true, then
         1. If currentDesc.[[Writable]] is true, return false.
10. Return true.

# 7.3.17 CreateArrayFromList (elements)

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

1. Assert: elements is a List whose elements are all ECMAScript language values.
2. Let array be ArrayCreate(0) (see 9.4.2.2).
3. Let n be 0.
4. For each element e of elements
   1. Let status be CreateDataProperty(array, ToString(n), e).
   2. Assert: status is true.
   3. Increment n by 1.
5. Return array.

# 7.3.18 CreateListFromArrayLike (obj [, elementTypes] )

The abstract operation CreateListFromArrayLike is used to create a List value whose elements are provided by the indexed properties of an array-like object, obj. The optional argument elementTypes is a List containing the names of ECMAScript Language Types that are allowed for element values of the List that is created. This abstract operation performs the following steps:

1. ReturnIfAbrupt(obj).
2. If elementTypes was not passed, let elementTypes be (Undefined, Null, Boolean, String, Symbol, Number, Object).
3. If Type(obj) is not Object, throw a TypeError exception.
4. Let len be ToLength(Get(obj, `"length"`)).
5. ReturnIfAbrupt(len).
6. Let list be an empty List.
7. Let index be 0.
8. Repeat while index < len
   1. Let indexName be ToString(index).
   2. Let next be Get(obj, indexName).
   3. ReturnIfAbrupt(next).
   4. If Type(next) is not an element of elementTypes, throw a TypeError exception.
   5. Append next as the last element of list.
   6. Set index to index + 1.
9. Return list.

# 7.3.19 OrdinaryHasInstance (C, O)

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

1. If IsCallable(C) is false, return false.
2. If C has a [[BoundTargetFunction]] internal slot, then
   1. Let BC be the value of C’s [[BoundTargetFunction]] internal slot.
   2. Return InstanceofOperator(O,BC) (see 12.9.4).
3. If Type(O) is not Object, return false.
4. Let P be Get(C, `"prototype"`).
5. ReturnIfAbrupt(P).
6. If Type(P) is not Object, throw a TypeError exception.
7. Repeat
   1. Let O be O.[[GetPrototypeOf]]().
   2. ReturnIfAbrupt(O).
   3. If O is `null`, return false.
   4. If SameValue(P, O) is true, return true.

# 7.3.20 SpeciesConstructor ( O, defaultConstructor )

The abstract operation SpeciesConstructor is used to retrieve the constructor that should be used to create new objects that are derived from the argument object O. The defaultConstructor argument is the constructor to use if a constructor @@species property cannot be found starting from O. This abstract operation performs the following steps:

1. Assert: Type(O) is Object.
2. Let C be Get(O, `"constructor"`).
3. ReturnIfAbrupt(C).
4. If C is undefined, return defaultConstructor.
5. If Type(C) is not Object, throw a TypeError exception.
6. Let S be Get(C, @@species).
7. ReturnIfAbrupt(S).
8. If S is either undefined or null, return defaultConstructor.
9. If IsConstructor(S) is true, return S.
10. Throw a TypeError exception.

# 7.3.21 EnumerableOwnNames (O)

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

1. Assert: Type(O) is Object.
2. Let ownKeys be O.[[OwnPropertyKeys]]().
3. ReturnIfAbrupt(ownKeys).
4. Let names be a new empty List.
5. Repeat, for each element key of ownKeys in List order
   1. If Type(key) is String, then
      1. Let desc be O.[[GetOwnProperty]](key).
      2. ReturnIfAbrupt(desc).
      3. If desc is not undefined, then
         1. If desc.[[Enumerable]] is true, append key to names.
6. Order the elements of names so they are in the same relative order as would be produced by the Iterator that would be returned if the [[Enumerate]] internal method was invoked on O.
7. Return names.

NOTE The order of elements is returned list is the same as the enumeration order that used by a for-in statement.

# 7.3.22 GetFunctionRealm ( obj )

The abstract operation GetFunctionRealm with argument obj performs the following steps:

1. Assert: obj is a callable object.
2. If obj has a [[Realm]] internal slot, then
   1. Return obj’s [[Realm]] internal slot.
3. If obj is a Bound Function exotic object, then
   1. Let target be obj’s [[BoundTargetFunction]] internal slot.
   2. Return GetFunctionRealm(target).
4. If obj is a Proxy exotic object, then
   1. If the value of the [[ProxyHandler]] internal slot of obj is null, throw a TypeError exception.
   2. Let proxyTarget be the value of obj’s [[ProxyTarget]] internal slot.
   3. Return GetFunctionRealm(proxyTarget).
5. Return the running execution context’s Realm.

NOTE Step 5 will only be reached if target is a non-standard exotic function object that does not have a [[Realm]] internal slot.

# 7.4 Operations on Iterator Objects

See Common Iteration Interfaces (25.1).

# 7.4.1 GetIterator ( obj, method )

The abstract operation GetIterator with argument obj and optional argument method performs the following steps:

1. ReturnIfAbrupt(obj).
2. If method was not passed, then
   1. Let method be GetMethod(obj, @@iterator).
   2. ReturnIfAbrupt(method).
3. Let iterator be Call(method,obj).
4. ReturnIfAbrupt(iterator).
5. If Type(iterator) is not Object, throw a TypeError exception.
6. Return iterator.

# 7.4.2 IteratorNext ( iterator, value )

The abstract operation IteratorNext with argument iterator and optional argument value performs the following steps:

1. If value was not passed, then
   1. Let result be Invoke(iterator, `"next"`, «‍ »).
2. Else,
   1. Let result be Invoke(iterator, `"next"`, «‍value»).
3. ReturnIfAbrupt(result).
4. If Type(result) is not Object, throw a TypeError exception.
5. Return result.

# 7.4.3 IteratorComplete ( iterResult )

The abstract operation IteratorComplete with argument iterResult performs the following steps:

1. Assert: Type(iterResult) is Object.
2. Return ToBoolean(Get(iterResult, `"done"`)).

# 7.4.4 IteratorValue ( iterResult )

The abstract operation IteratorValue with argument iterResult performs the following steps:

1. Assert: Type(iterResult) is Object.
2. Return Get(iterResult, `"value"`).

# 7.4.5 IteratorStep ( iterator )

The abstract operation IteratorStep with argument iterator requests the next value from iterator and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. IteratorStep performs the following steps:

1. Let result be IteratorNext(iterator).
2. ReturnIfAbrupt(result).
3. Let done be IteratorComplete(result).
4. ReturnIfAbrupt(done).
5. If done is true, return false.
6. Return result.

# 7.4.6 IteratorClose( iterator, completion )

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

1. Assert: Type(iterator) is Object.
2. Assert: completion is a Completion Record.
3. Let return be GetMethod(iterator, `"return"`).
4. ReturnIfAbrupt(return).
5. If return is undefined, return Completion(completion).
6. Let innerResult be Call(return, iterator, «‍ »).
7. If completion.[[type]] is throw, return Completion(completion).
8. If innerResult.[[type]] is throw, return Completion(innerResult).
9. If Type(innerResult.[[value]]) is not Object, throw a TypeError exception.
10. Return Completion(completion).

# 7.4.7 CreateIterResultObject ( value, done )

The abstract operation CreateIterResultObject with arguments value and done creates an object that supports the IteratorResult interface by performing the following steps:

1. Assert: Type(done) is Boolean.
2. Let obj be ObjectCreate(%ObjectPrototype%).
3. Perform CreateDataProperty(obj, `"value"`, value).
4. Perform CreateDataProperty(obj, `"done"`, done).
5. Return obj.

# 7.4.8 CreateListIterator ( list )

The abstract operation CreateListIterator with argument list creates an Iterator (25.1.1.2) object whose next method returns the successive elements of list. It performs the following steps:

1. Let iterator be ObjectCreate(%IteratorPrototype%, «[[IteratorNext]], [[IteratedList]], [[ListIteratorNextIndex]]»).
2. Set iterator’s [[IteratedList]] internal slot to list.
3. Set iterator’s [[ListIteratorNextIndex]] internal slot to 0.
4. Let next be a new built-in function object as defined in ListIterator `next` (7.4.8.1).
5. Set iterator’s [[IteratorNext]] internal slot to next.
6. Perform CreateMethodProperty(iterator, `"next"`, next).
7. Return iterator.

# 7.4.8.1 ListIterator next( )

The ListIterator `next` method is a standard built-in function object (clause 17) that performs the following steps:

1. Let O be the this value.
2. Let f be the active function object.
3. If O does not have a [[IteratorNext]] internal slot, throw a TypeError exception.
4. Let next be the value of the [[IteratorNext]] internal slot of O.
5. If SameValue(f, next) is false, throw a TypeError exception.
6. If O does not have a [[IteratedList]] internal slot, throw a TypeError exception.
7. Let list be the value of the [[IteratedList]] internal slot of O.
8. Let index be the value of the [[ListIteratorNextIndex]] internal slot of O.
9. Let len be the number of elements of list.
10. If index ≥ len, then
    1. Return CreateIterResultObject(undefined, true).
11. Set the value of the [[ListIteratorNextIndex]] internal slot of O to index+1.
12. Return CreateIterResultObject(list[index], false).

NOTE A ListIterator `next` method will throw an exception if applied to any object other than the one with which it was originally associated.

# 8 Executable Code and Execution Contexts

# 8.1 Lexical Environments

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

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

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

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

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

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

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

# 8.1.1 Environment Records

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

For specification purposes Environment Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses, declarative Environment Record, object Environment Record, and global Environment Record. Function Environment Records and module Environment Records are subclasses of declarative Environment Record. The abstract class includes the abstract specification methods defined in Table 15. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 15 — Abstract Methods of Environment Records

Method | Purpose
--- | ---
HasBinding(N) | Determine if an Environment Record has a binding for the String value N. Return true if it does and false if it does not
CreateMutableBinding(N, D) | Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted.

CreateImmutableBinding(N, S)

Create a new but uninitialized immutable binding in an Environment Record. The String value N is the text of the bound name. If S is true then attempts to access the value of the binding before it is initialized or set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding. S is an optional parameter that defaults to false.

InitializeBinding(N,V) | Set the value of an already existing but uninitialized binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N,V, S) | Set the value of an already existing mutable binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception.

GetBindingValue(N,S)

Returns the value of an already existing binding from an Environment Record. The String value N is the text of the bound name. S is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.

DeleteBinding(N) | Delete a binding from an Environment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding() | Determine if an Environment Record establishes a `this` binding. Return true if it does and false if it does not.
HasSuperBinding() | Determine if an Environment Record establishes a `super` method binding. Return true if it does and false if it does not.
WithBaseObject () | If this Environment Record is associated with a `with` statement, return the with object. Otherwise, return undefined.

# 8.1.1.1 Declarative Environment Records

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

The behaviour of the concrete specification methods for declarative environment records is defined by the following algorithms.

# 8.1.1.1.1 HasBinding(N)

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

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

# 8.1.1.1.2 CreateMutableBinding (N, D)

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

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

# 8.1.1.1.3 CreateImmutableBinding (N, S)

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

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

# 8.1.1.1.4 InitializeBinding (N,V)

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

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

# 8.1.1.1.5 SetMutableBinding (N,V,S)

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

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

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

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

# 8.1.1.1.6 GetBindingValue(N,S)

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

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

# 8.1.1.1.7 DeleteBinding (N)

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

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

# 8.1.1.1.8 HasThisBinding ()

Regular declarative Environment Records do not provide a `this` binding.

1. Return false.

# 8.1.1.1.9 HasSuperBinding ()

Regular declarative Environment Records do not provide a `super` binding.

1. Return false.

# 8.1.1.1.10 WithBaseObject()

Declarative Environment Records always return undefined as their WithBaseObject.

1. Return undefined.

# 8.1.1.2 Object Environment Records

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

Object Environment Records created for `with` statements (13.10) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a withEnvironment Boolean value that is associated with each object Environment Record. By default, the value of withEnvironment is false for any object Environment Record.

The behaviour of the concrete specification methods for object environment records is defined by the following algorithms.

# 8.1.1.2.1 HasBinding(N)

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

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

# 8.1.1.2.2 CreateMutableBinding (N, D)

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

1. Let envRec be the object Environment Record for which the method was invoked.
2. Let bindings be the binding object for envRec.
3. If D is true then let configValue be true otherwise let configValue be false.
4. Return DefinePropertyOrThrow(bindings, N, PropertyDescriptor{[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}).

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

# 8.1.1.2.3 CreateImmutableBinding (N, S)

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

# 8.1.1.2.4 InitializeBinding (N,V)

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

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

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

# 8.1.1.2.5 SetMutableBinding (N,V,S)

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

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

# 8.1.1.2.6 GetBindingValue(N,S)

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

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

# 8.1.1.2.7 DeleteBinding (N)

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

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

# 8.1.1.2.8 HasThisBinding ()

Regular object environment records do not provide a `this` binding.

1. Return false.

# 8.1.1.2.9 HasSuperBinding ()

Regular object environment records do not provide a `super` binding.

1. Return false.

# 8.1.1.2.10 WithBaseObject()

Object environment records return undefined as their WithBaseObject unless their withEnvironment flag is true.

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

# 8.1.1.3 Function Environment Records

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

Function Environment Records have the additional state fields listed in Table 16.

Table 16 — Additional Fields of Function Environment Records

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

Function Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed in Table 17:

Table 17 — Additional Methods of Function Environment Records

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

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

# 8.1.1.3.1 BindThisValue(V)

1. Let envRec be the function Environment Record for which the method was invoked.
2. Assert: envRec.[[thisBindingStatus]] is not `"lexical"`.
3. If envRec.[[thisBindingStatus]] is `"initialized"`, throw a ReferenceError exception.
4. Set envRec.[[thisValue]] to V.
5. Set envRec.[[thisBindingStatus]] to `"initialized"`.
6. Return V.

# 8.1.1.3.2 HasThisBinding ()

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

# 8.1.1.3.3 HasSuperBinding ()

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

# 8.1.1.3.4 GetThisBinding ()

1. Let envRec be the function Environment Record for which the method was invoked.
2. Assert: envRec.[[thisBindingStatus]] is not `"lexical"`.
3. If envRec.[[thisBindingStatus]] is `"uninitialized"`, throw a ReferenceError exception.
4. Return envRec.[[thisValue]].

# 8.1.1.3.5 GetSuperBase ()

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

# 8.1.1.4 Global Environment Records

A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common Realm (8.2). A global Environment Record provides the bindings for built-in globals (clause 18), properties of the global object, and for all top-level declarations (13.1.8, 13.1.10) that occur within a Script.

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

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

Global Environment Records have the additional fields listed in Table 18 and the additional methods listed in Table 19.

Table 18 — Additional Fields of Global Environment Records

Field Name | Value | Meaning
--- | --- | ---
[[ObjectRecord]] | Object Environment Record | Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings in global code for the associated Realm.
[[DeclarativeRecord]] | Declarative Environment Record | Contains bindings for all declarations in global code for the associated Realm code except for FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings.
[[VarNames]] | List of String | The string names bound by FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration declarations in global code for the associated Realm.

Table 19 — Additional Methods of Global Environment Records

Method | Purpose
--- | ---
GetThisBinding() | Return the value of this Environment Record’s `this` binding.
HasVarDeclaration (N) | Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, or GeneratorDeclaration.
HasLexicalDeclaration (N) | Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N) | Determines if the argument is the name of a global object property that may not be shadowed by a global lexically binding.
CanDeclareGlobalVar (N) | Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N) | Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.

CreateGlobalVarBinding(N, D)

Used to create and initialize to undefined a global `var` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `var`. The String value N is the bound name. If D is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.

CreateGlobalFunctionBinding(N, V, D)

Create and initialize a global `function` binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `function`. The String value N is the bound name. V is the initialization value. If the optional Boolean argument D is true the binding is may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

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

# 8.1.1.4.1 HasBinding(N)

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

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

# 8.1.1.4.2 CreateMutableBinding (N, D)

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

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

# 8.1.1.4.3 CreateImmutableBinding (N, S)

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

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

# 8.1.1.4.4 InitializeBinding (N,V)

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

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

# 8.1.1.4.5 SetMutableBinding (N,V,S)

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

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

# 8.1.1.4.6 GetBindingValue(N,S)

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

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

# 8.1.1.4.7 DeleteBinding (N)

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

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

# 8.1.1.4.8 HasThisBinding ()

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

1. Return true.

# 8.1.1.4.9 HasSuperBinding ()

1. Return false.

# 8.1.1.4.10 WithBaseObject()

Global Environment Records always return undefined as their WithBaseObject.

1. Return undefined.

# 8.1.1.4.11 GetThisBinding ()

1. Let envRec be the global Environment Record for which the method was invoked.
2. Let ObjRec be envRec.[[ObjectRecord]].
3. Let bindings be the binding object for ObjRec.
4. Return bindings.

# 8.1.1.4.12 HasVarDeclaration (N)

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

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

# 8.1.1.4.13 HasLexicalDeclaration (N)

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

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

# 8.1.1.4.14 HasRestrictedGlobalProperty (N)

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

1. Let envRec be the global Environment Record for which the method was invoked.
2. Let ObjRec be envRec.[[ObjectRecord]].
3. Let globalObject be the binding object for ObjRec.
4. Let existingProp be globalObject.[[GetOwnProperty]](N).
5. ReturnIfAbrupt(existingProp).
6. If existingProp is undefined, return false.
7. If existingProp.[[Configurable]] is true, return false.
8. Return true.

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

# 8.1.1.4.15 CanDeclareGlobalVar (N)

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

1. Let envRec be the global Environment Record for which the method was invoked.
2. Let ObjRec be envRec.[[ObjectRecord]].
3. Let globalObject be the binding object for ObjRec.
4. Let hasProperty be HasOwnProperty(globalObject, N).
5. ReturnIfAbrupt(hasProperty).
6. If hasProperty is true, return true.
7. Return IsExtensible(globalObject).

# 8.1.1.4.16 CanDeclareGlobalFunction (N)

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

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

# 8.1.1.4.17 CreateGlobalVarBinding (N, D)

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

1. Let envRec be the global Environment Record for which the method was invoked.
2. Let ObjRec be envRec.[[ObjectRecord]].
3. Let globalObject be the binding object for ObjRec.
4. Let hasProperty be HasOwnProperty(globalObject, N).
5. ReturnIfAbrupt(hasProperty).
6. Let extensible be IsExtensible(globalObject).
7. ReturnIfAbrupt(extensible).
8. If hasProperty is false and extensible is true, then
   1. Let status be ObjRec.CreateMutableBinding(N, D).
   2. ReturnIfAbrupt(status).
   3. Let status be ObjRec.InitializeBinding(N, undefined).
   4. ReturnIfAbrupt(status).
9. Let varDeclaredNames be envRec.[[VarNames]].
10. If varDeclaredNames does not contain the value of N, then
    1. Append N to varDeclaredNames.
11. Return NormalCompletion(empty).

# 8.1.1.4.18 CreateGlobalFunctionBinding (N, V, D)

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

1. Let envRec be the global Environment Record for which the method was invoked.
2. Let ObjRec be envRec.[[ObjectRecord]].
3. Let globalObject be the binding object for ObjRec.
4. Let existingProp be globalObject.[[GetOwnProperty]](N).
5. ReturnIfAbrupt(existingProp).
6. If existingProp is undefined or existingProp.[[Configurable]] is true, then
   1. Let desc be the PropertyDescriptor{[[Value]]:V, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: D}.
7. Else,
   1. Let desc be the PropertyDescriptor{[[Value]]:V }.
8. Let status be DefinePropertyOrThrow(globalObject, N, desc).
9. ReturnIfAbrupt(status).
10. Let status be Set(globalObject, N, V, false).
11. Record that the binding for N in ObjRec has been initialized.
12. ReturnIfAbrupt(status).
13. Let varDeclaredNames be envRec.[[VarNames]].
14. If varDeclaredNames does not contain the value of N, then
    1. Append N to varDeclaredNames.
15. Return NormalCompletion(empty).

NOTE Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Steps 10-12 are equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

# 8.1.1.5 Module Environment Records

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

Module Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed in Table 20:

Table 20 — Additional Methods of Module Environment Records

Method | Purpose
--- | ---
CreateImportBinding(N, M, N2 ) | Create an immutable indirect binding in a module Environment Record. The String value N is the text of the bound name. M is a Module Record (see 15.2.1.14), and N2 is a binding that exists in M’s module Environment Record.
GetThisBinding() | Return the value of this Environment Record’s `this` binding.

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

# 8.1.1.5.1 GetBindingValue(N,S)

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

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

NOTE Because a Module is always strict mode code, calls to GetBindingValue should always pass true as the value of S.

# 8.1.1.5.2 DeleteBinding (N)

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

1. Let envRec be the module Environment Record for which the method was invoked.
2. If envRec does not have a binding for the name that is the value of N, return true.
3. Return false.

NOTE Because the bindings of a module Environment Record are not deletable.

# 8.1.1.5.3 HasThisBinding ()

Module Environment Records provide a `this` binding.

1. Return true.

# 8.1.1.5.4 GetThisBinding ()

1. Return undefined.

# 8.1.1.5.5 CreateImportBinding (N, M, N2)

The concrete Environment Record method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. M is a Module Record (see 15.2.1.14), and N2 is the name of a binding that exists in M’s module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of value of the target binding.

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

# 8.1.2 Lexical Environment Operations

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

# 8.1.2.1 GetIdentifierReference (lex, name, strict)

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

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

# 8.1.2.2 NewDeclarativeEnvironment (E)

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

1. Let env be a new Lexical Environment.
2. Let envRec be a new declarative Environment Record containing no bindings.
3. Set env’s EnvironmentRecord to be envRec.
4. Set the outer lexical environment reference of env to E.
5. Return env.

# 8.1.2.3 NewObjectEnvironment (O, E)

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

1. Let env be a new Lexical Environment.
2. Let envRec be a new object Environment Record containing O as the binding object.
3. Set env’s EnvironmentRecord to envRec.
4. Set the outer lexical environment reference of env to E.
5. Return env.

# 8.1.2.4 NewFunctionEnvironment ( F, newTarget )

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

1. Assert: F is an ECMAScript function.
2. Assert: Type(newTarget) is Undefined or Object.
3. Let env be a new Lexical Environment.
4. Let envRec be a new function Environment Record containing no bindings.
5. Set envRec.[[FunctionObject]] to F.
6. If F’s [[ThisMode]] internal slot is lexical, set envRec.[[thisBindingStatus]] to `"lexical"`.
7. Else, Set envRec.[[thisBindingStatus]] to `"uninitialized"`.
8. Let home be the value of F’s [[HomeObject]] internal slot.
9. Set envRec.[[HomeObject]] to home.
10. Set envRec.[[NewTarget]] to newTarget.
11. Set env’s EnvironmentRecord to be envRec.
12. Set the outer lexical environment reference of env to the value of F’s [[Environment]] internal slot.
13. Return env.

# 8.1.2.5 NewGlobalEnvironment ( G )

When the abstract operation NewGlobalEnvironment is called with an ECMAScript Object G as its argument, the following steps are performed:

1. Let env be a new Lexical Environment.
2. Let objRec be a new object Environment Record containing G as the binding object.
3. Let dclRec be a new declarative Environment Record containing no bindings.
4. Let globalRec be a new global Environment Record.
5. Set globalRec.[[ObjectRecord]] to objRec.
6. Set globalRec.[[DeclarativeRecord]] to dclRec.
7. Set globalRec.[[VarNames]] to a new empty List.
8. Set env’s EnvironmentRecord to globalRec.
9. Set the outer lexical environment reference of env to null
10. Return env.

# 8.1.2.6 NewModuleEnvironment (E)

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

1. Let env be a new Lexical Environment.
2. Let envRec be a new module Environment Record containing no bindings.
3. Set env’s EnvironmentRecord to be envRec.
4. Set the outer lexical environment reference of env to E.
5. Return env.

# 8.2 Code Realms

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

A Realm is specified as a Record with the fields specified in Table 21:

Table 21 — Realm Record Fields

Field Name | Value | Meaning
--- | --- | ---
[[intrinsics]] | Record whose field names are intrinsic keys and whose values are objects | These are the intrinsic values used by code associated with this Realm
[[globalThis]] | Object | The global object for this Realm
[[globalEnv]] | Lexical Environment | The global environment for this Realm

[[templateMap]]

A List of Record { [[strings]]: List, [[array]]: Object}.

Template objects are canonicalized separately for each Realm using its [[templateMap]]. Each [[strings]] value is a List containing, in source text order, the raw string values of a TemplateLiteral that has been evaluated. The associated [[array]] value is the corresponding template object that is passed to a tag function.

An implementation may define other, implementation specific fields.

# 8.2.1 CreateRealm ( )

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

1. Let realmRec be a new Record.
2. Perform CreateIntrinsics(realmRec).
3. Set realmRec.[[globalThis]] to undefined.
4. Set realmRec.[[globalEnv]] to undefined.
5. Set realmRec.[[templateMap]] to a new empty List.
6. Return realmRec.

# 8.2.2 CreateIntrinsics ( realmRec )

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

1. Let intrinsics be a new Record.
2. Set realmRec.[[intrinsics]] to intrinsics.
3. Let objProto be ObjectCreate(null).
4. Set intrinsics.[[%ObjectPrototype%]] to objProto.
5. Let throwerSteps be the algorithm steps specified in 9.2.7.1 for the %ThrowTypeError% function.
6. Let thrower be CreateBuiltinFunction(realmRec, throwerSteps, null).
7. Set intrinsics.[[%ThrowTypeError%]] to thrower.
8. Let noSteps be an empty sequence of algorithm steps.
9. Let funcProto be CreateBuiltinFunction(realmRec, noSteps, objProto).
10. Set intrinsics.[[%FunctionPrototype%]] to funcProto.
11. Call thrower.[[SetPrototypeOf]](funcProto).
12. Perform AddRestrictedFunctionProperties(funcProto, realmRec).
13. Set fields of intrinsics with the values listed in Table 7 that have not already been handled above. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 18-26. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(realmRec, <steps>, <prototype>, <slots>) where <steps> is the definition of that function provided by this specification, <prototype> is the specified value of the function’s [[Prototype]] internal slot and <slots> is a list of the names, if any, of the functions specified internal slots. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
14. Return intrinsics.

# 8.2.3 SetRealmGlobalObject ( realmRec, globalObj )

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

1. If globalObj is undefined, then
   1. Let intrinsics be realmRec.[[intrinsics]].
   2. Let globalObj be ObjectCreate(intrinsics.[[%ObjectPrototype%]]).
2. Assert: Type(globalObj) is Object.
3. Set realmRec.[[globalThis]] to globalObj.
4. Let newGlobalEnv be NewGlobalEnvironment(globalObj).
5. Set realmRec.[[globalEnv]] to newGlobalEnv.
6. Return realmRec.

# 8.2.4 SetDefaultGlobalBindings ( realmRec )

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

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

# 8.3 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context that is actually executing code. This is known as the running execution context. A stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 22.

Table 22 —State Components for All Execution Contexts

Component | Purpose
--- | ---
code evaluation state | Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function | If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
Realm | The Realm from which associated code accesses ECMAScript resources.

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

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

Execution contexts for ECMAScript code have the additional state components listed in Table 23.

Table 23 — Additional State Components for ECMAScript Code Execution Contexts

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

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value.

Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 24.

Table 24 — Additional State Components for Generator Execution Contexts

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

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

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

# 8.3.1 ResolveBinding ( name, [env] )

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

1. If env was not passed or if env is undefined, then
   1. Let env be the running execution context’s LexicalEnvironment.
2. Assert: env is a Lexical Environment.
3. If the code matching the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let strict be false.
4. Return GetIdentifierReference(env, name, strict ).

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

# 8.3.2 GetThisEnvironment ( )

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

1. Let lex be the running execution context’s LexicalEnvironment.
2. Repeat
   1. Let envRec be lex’s EnvironmentRecord.
   2. Let exists be envRec.HasThisBinding().
   3. If exists is true, return envRec.
   4. Let outer be the value of lex’s outer environment reference.
   5. Let lex be outer.

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

# 8.3.3 ResolveThisBinding ( )

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

1. Let envRec be GetThisEnvironment( ).
2. Return envRec.GetThisBinding().

# 8.3.4 GetNewTarget ( )

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

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

# 8.3.5 GetGlobalObject ( )

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

1. Let ctx be the running execution context.
2. Let currentRealm be ctx’s Realm.
3. Return currentRealm.[[globalThis]].

# 8.4 Jobs and Job Queues

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

Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 25. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.

Table 25 — PendingJob Record Fields

Field Name | Value | Meaning
--- | --- | ---
[[Job]] | The name of a Job abstract operation | This is the abstract operation that is performed when execution of this PendingJob is initiated. Jobs are abstract operations that use NextJob rather than Return to indicate that they have completed.
[[Arguments]] | A List | The List of argument values that are to be passed to [[Job]] when it is activated.
[[Realm]] | A Realm Record | The Realm for the initial execution context when this Pending Job is initiated.
[[HostDefined]] | Any, default value is undefined. | Field reserved for use by host environments that need to associate additional information with a pending Job.

A Job Queue is a FIFO queue of PendingJob records. Each Job Queue has a name and the full set of available Job Queues are defined by an ECMAScript implementation. Every ECMAScript implementation has at least the Job Queues defined in Table 26.

Table 26 — Required Job Queues

Name | Purpose
--- | ---
ScriptJobs | Jobs that validate and evaluate ECMAScript Script and Module source text. See clauses 10 and 15.
PromiseJobs | Jobs that are responses to the settlement of a Promise (see 25.4).

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

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

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

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

# 8.4.1 EnqueueJob ( queueName, job, arguments)

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

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

# 8.4.2 NextJob result

An algorithm step such as:

1. NextJob result.

is used in Job abstract operations in place of:

1. Return result.

Job abstract operations must not contain a Return step or a ReturnIfAbrupt step. The NextJob result operation is equivalent to the following steps:

1. If result is an abrupt completion, perform implementation defined unhandled exception processing.
2. Suspend the running execution context and remove it from the execution context stack.
3. Assert: The execution context stack is now empty.
4. Let nextQueue be a non-empty Job Queue chosen in an implementation defined manner. If all Job Queues are empty, the result is implementation defined.
5. Let nextPending be the PendingJob record at the front of nextQueue. Remove that record from nextQueue.
6. Let newContext be a new execution context.
7. Set newContext’s Realm to nextPending.[[Realm]].
8. Push newContext onto the execution context stack; newContext is now the running execution context.
9. Perform any implementation or host environment defined job initialization using nextPending.
10. Perform the abstract operation named by nextPending.[[Job]] using the elements of nextPending.[[Arguments]] as its arguments.

# 8.5 ECMAScript Initialization()

An ECMAScript implementation performs the following steps prior to the execution of any Jobs or the evaluation of any ECMAScript code:

1. Let realm be CreateRealm().
2. Let newContext be a new execution context.
3. Set the Function of newContext to null.
4. Set the Realm of newContext to realm.
5. Push newContext onto the execution context stack; newContext is now the running execution context.
6. Let status be InitializeHostDefinedRealm(realm).
7. If status is an abrupt completion, then
   1. Assert: The first realm could not be created.
   2. Terminate ECMAScript execution.
8. In an implementation dependent manner, obtain the ECMAScript source texts (see clause 10) for zero or more ECMAScript scripts and/or ECMAScript modules. For each such sourceText do,
   1. If sourceText is the source code of a script, then
      1. Perform EnqueueJob(`"ScriptJobs"`, ScriptEvaluationJob, « sourceText »).
   2. Else sourceText is the source code of a module,
      1. Perform EnqueueJob(`"ScriptJobs"`, TopLevelModuleEvaluationJob, « sourceText »).
9. NextJob NormalCompletion(undefined).

# 8.5.1 InitializeHostDefinedRealm ( realm )

The abstract operation InitializeHostDefinedRealm with parameter realm performs the following steps:

1. If this implementation requires use of an exotic object to serve as realm’s global object, let global be such an object created in an implementation defined manner. Otherwise, let global be undefined indicating that an ordinary object should be created as the global object.
2. Perform SetRealmGlobalObject(realm, global).
3. Let globalObj be SetDefaultGlobalBindings(realm).
4. ReturnIfAbrupt(globalObj).
5. Create any implementation defined global object properties on globalObj.
6. Return NormalCompletion(undefined).

# 9 Ordinary and Exotic Objects Behaviours

# 9.1 Ordinary Object Internal Methods and Internal Slots

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

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

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

# 9.1.1 [[GetPrototypeOf]] ( )

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

1. Return the value of the [[Prototype]] internal slot of O.

# 9.1.2 [[SetPrototypeOf]] (V)

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

1. Assert: Either Type(V) is Object or Type(V) is Null.
2. Let extensible be the value of the [[Extensible]] internal slot of O.
3. Let current be the value of the [[Prototype]] internal slot of O.
4. If SameValue(V, current), return true.
5. If extensible is false, return false.
6. Let p be V.
7. Let done be false.
8. Repeat while done is false,
   1. If p is null, let done be true.
   2. Else, if SameValue(p, O) is true, return false.
   3. Else,
      1. If the [[GetPrototypeOf]] internal method of p is not the ordinary object internal method defined in 9.1.1, let done be true.
      2. Else, let p be the value of p’s [[Prototype]] internal slot.
9. Set the value of the [[Prototype]] internal slot of O to V.
10. Return true.

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

# 9.1.3 [[IsExtensible]] ( )

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

1. Return the value of the [[Extensible]] internal slot of O.

# 9.1.4 [[PreventExtensions]] ( )

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

1. Set the value of the [[Extensible]] internal slot of O to false.
2. Return true.

# 9.1.5 [[GetOwnProperty]] (P)

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

1. Return OrdinaryGetOwnProperty(O, P).

# 9.1.5.1 OrdinaryGetOwnProperty (O, P)

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

1. Assert: IsPropertyKey(P) is true.
2. If O does not have an own property with key P, return undefined.
3. Let D be a newly created Property Descriptor with no fields.
4. Let X be O’s own property whose key is P.
5. 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
6. 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.
7. Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
8. Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
9. Return D.

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

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

1. Return OrdinaryDefineOwnProperty(O, P, Desc).

# 9.1.6.1 OrdinaryDefineOwnProperty (O, P, Desc)

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

1. Let current be O.[[GetOwnProperty]](P).
2. ReturnIfAbrupt(current).
3. Let extensible be the value of the [[Extensible]] internal slot of O.
4. Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

# 9.1.6.2 IsCompatiblePropertyDescriptor (Extensible, Desc, Current)

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

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

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

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

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

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

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

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

# 9.1.7 [[HasProperty]](P)

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

1. Return OrdinaryHasProperty(O, P).

# 9.1.7.1 OrdinaryHasProperty (O, P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let hasOwn be OrdinaryGetOwnProperty(O, P).
3. If hasOwn is not undefined, return true.
4. Let parent be O.[[GetPrototypeOf]]().
5. ReturnIfAbrupt(parent).
6. If parent is not null, then
   1. Return parent.[[HasProperty]](P).
7. Return false.

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

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

1. Assert: IsPropertyKey(P) is true.
2. Let desc be O.[[GetOwnProperty]](P).
3. ReturnIfAbrupt(desc).
4. If desc is undefined, then
   1. Let parent be O.[[GetPrototypeOf]]().
   2. ReturnIfAbrupt(parent).
   3. If parent is null, return undefined.
   4. Return parent.[[Get]](P, Receiver).
5. If IsDataDescriptor(desc) is true, return desc.[[Value]].
6. Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
7. If getter is undefined, return undefined.
8. Return Call(getter, Receiver).

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

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

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

# 9.1.10 [[Delete]] (P)

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

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

# 9.1.11 [[Enumerate]] ()

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

1. Return an Iterator object (25.1.1.2) whose `next` method iterates over all the String-valued keys of enumerable properties of O. The Iterator object must inherit from %IteratorPrototype% (25.1.2). The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator’s `next` method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator’s `next` method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator’s `next` method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator’s `next` method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained as if by invoking the prototype object’s [[Enumerate]] internal method. [[Enumerate]] must obtain the own property keys of the target object as if by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained as if by calling its [[GetOwnProperty]] internal method.

NOTE The following is an informative definition of an ECMAScript generator function that conforms to these rules:

    function* enumerate(obj) {

    let visited=new Set;

    for (let key of Reflect.ownKeys(obj)) {

    if (typeof key === "string") {

    let desc = Reflect.getOwnPropertyDescriptor(obj,key);

    if (desc) {

    visited.add(key);

    if (desc.enumerable) yield key;

    }

    }

    }

    let proto = Reflect.getPrototypeOf(obj)

    if (proto === null) return;

    for (let protoName of Reflect.enumerate(proto)) {

    if (!visited.has(protoName)) yield protoName;

    }

    }

# 9.1.12 [[OwnPropertyKeys]] ( )

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

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

# 9.1.13 ObjectCreate(proto, internalSlotsList)

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

1. If internalSlotsList was not provided, let internalSlotsList be an empty List.
2. Let obj be a newly created object with an internal slot for each name in internalSlotsList.
3. Set obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
4. Set the [[Prototype]] internal slot of obj to proto.
5. Set the [[Extensible]] internal slot of obj to true.
6. Return obj.

# 9.1.14 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto, internalSlotsList )

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

1. Assert: intrinsicDefaultProto is a string value that is this specification’s name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
2. Let proto be GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
3. ReturnIfAbrupt(proto).
4. Return ObjectCreate(proto, internalSlotsList).

# 9.1.15 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )

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

1. Assert: intrinsicDefaultProto is a string value that is this specification’s name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
2. Assert: IsConstructor (constructor) is true.
3. Let proto be Get(constructor, `"prototype"`).
4. ReturnIfAbrupt(proto).
5. If Type(proto) is not Object, then
   1. Let realm be GetFunctionRealm(constructor).
   2. ReturnIfAbrupt(realm).
   3. Let proto be realm’s intrinsic object named intrinsicDefaultProto.
6. Return proto.

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

# 9.2 ECMAScript Function Objects

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

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

Table 27 — Internal Slots of ECMAScript Function Objects

Internal Slot | Type | Description
--- | --- | ---
[[Environment]] | Lexical Environment | The Lexical Environment that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[FormalParameters]] | Parse Node | The root parse node of the source text that defines the function’s formal parameter list.
[[FunctionKind]] | String | Either `"normal"`, `"classConstructor"` or `"generator"`.
[[ECMAScriptCode]] | Parse Node | The root parse node of the source text that defines the function’s body.
[[ConstructorKind]] | String | Either `"base"` or `"derived"`.
[[Realm]] | Realm Record | The Code Realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.

[[ThisMode]]

(lexical, strict, global)

Defines how `this` references are interpreted within the formal parameters and code body of the function. lexical means that `this` refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.

[[Strict]] | Boolean | true if this is a strict mode function, false if this is not a strict mode function.
[[HomeObject]] | Object | If the function uses `super`, this is the object whose [[GetPrototypeOf]] provides the object where `super` property lookups begin.

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method. ECMAScript function objects whose code is not strict mode code have the [[GetOwnProperty]] internal method defined here.

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

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

1. Assert: F is an ECMAScript function object.
2. If F’s [[FunctionKind]] internal slot is `"classConstructor"`, throw a TypeError exception.
3. Let callerContext be the running execution context.
4. Let calleeContext be PrepareForOrdinaryCall(F, undefined).
5. Assert: calleeContext is now the running execution context.
6. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
7. Let result be OrdinaryCallEvaluateBody(F, argumentsList).
8. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
9. If result.[[type]] is return, return NormalCompletion(result.[[value]]).
10. ReturnIfAbrupt(result).
11. Return NormalCompletion(undefined).

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

# 9.2.1.1 PrepareForOrdinaryCall( F, newTarget )

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

1. Assert: Type(newTarget) is Undefined or Object.
2. Let callerContext be the running execution context.
3. Let calleeContext be a new ECMAScript code execution context.
4. Set the Function of calleeContext to F.
5. Let calleeRealm be the value of F’s [[Realm]] internal slot.
6. Set the Realm of calleeContext to calleeRealm.
7. Let localEnv be NewFunctionEnvironment(F, newTarget).
8. Set the LexicalEnvironment of calleeContext to localEnv.
9. Set the VariableEnvironment of calleeContext to localEnv.
10. If callerContext is not already suspended, Suspend callerContext.
11. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
12. NOTE Any exception objects produced after this point are associated with calleeRealm.
13. Return calleeContext.

# 9.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )

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

1. Let thisMode be the value of F’s [[ThisMode]] internal slot.
2. If thisMode is lexical, return NormalCompletion(undefined).
3. Let calleeRealm be the value of F’s [[Realm]] internal slot.
4. Let localEnv be the LexicalEnvironment of calleeContext.
5. If thisMode is strict, let thisValue be thisArgument.
6. Else
   1. if thisArgument is null or undefined, then
      1. Let thisValue be calleeRealm.[[globalThis]].
   2. Else
      1. Let thisValue be ToObject(thisArgument).
      2. Assert: thisValue is not an abrupt completion.
      3. NOTE ToObject produces wrapper objects using calleeRealm.
7. Let envRec be localEnv’s EnvironmentRecord.
8. Assert: The next step never returns an abrupt completion because envRec.[[thisBindingStatus]] is `"uninitialized"`.
9. Return envRec.BindThisValue(thisValue).

# 9.2.1.3 OrdinaryCallEvaluateBody ( F, argumentsList )

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

1. Let status be FunctionDeclarationInstantiation(F, argumentsList).
2. ReturnIfAbrupt(status)
3. Return the result of EvaluateBody of the parsed code that is the value of F's [[ECMAScriptCode]] internal slot passing F as the argument.

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

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

1. Assert: F is an ECMAScript function object.
2. Assert: Type(newTarget) is Object.
3. Let callerContext be the running execution context.
4. Let kind be F’s [[ConstructorKind]] internal slot.
5. If kind is `"base"`, then
   1. Let thisArgument be OrdinaryCreateFromConstructor(newTarget, `"%ObjectPrototype%"`).
   2. ReturnIfAbrupt(thisArgument).
6. Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
7. Assert: calleeContext is now the running execution context.
8. If kind is `"base"`, perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
9. Let constructorEnv be the LexicalEnvironment of calleeContext.
10. Let envRec be constructorEnv’s EnvironmentRecord.
11. Let result be OrdinaryCallEvaluateBody(F, argumentsList).
12. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
13. If result.[[type]] is return, then
    1. If Type(result.[[value]]) is Object, return NormalCompletion(result.[[value]]).
    2. If kind is `"base"`, return NormalCompletion(thisArgument).
    3. If result.[[value]] is not undefined, throw a TypeError exception.
14. Else, ReturnIfAbrupt(result).
15. Return envRec.GetThisBinding().

# 9.2.3 FunctionAllocate (functionPrototype, strict [,functionKind] )

The abstract operation FunctionAllocate requires the two arguments functionPrototype and strict. It also accepts one optional argument, functionKind. FunctionAllocate performs the following steps:

1. Assert: Type(functionPrototype) is Object.
2. Assert: If functionKind is present, its value is either `"normal"`, `"non-constructor"` or `"generator"`.
3. If functionKind is not present, let functionKind be `"normal"`.
4. If functionKind is `"non-constructor"`, then
   1. Let functionKind be `"normal"`.
   2. Let needsConstruct be false.
5. Else let needsConstruct be true.
6. Let F be a newly created ECMAScript function object with the internal slots listed in Table 27. All of those internal slots are initialized to undefined.
7. Set F’s essential internal methods to the default ordinary object definitions specified in 9.1.
8. Set F’s [[Call]] internal method to the definition specified in 9.2.1.
9. If needsConstruct is true, then
   1. Set F’s [[Construct]] internal method to the definition specified in 9.2.2.
   2. If functionKind is `"generator"`, set the [[ConstructorKind]] internal slot of F to `"derived"`.
   3. Else, set the [[ConstructorKind]] internal slot of F to `"base"`.
   4. NOTE Generator functions are tagged as `"derived"` constructors to prevent [[Construct]] from preallocating a generator instance. Generator instance objects are allocated when EvaluateBody is applied to the GeneratorBody of a generator function.
10. Set the [[Strict]] internal slot of F to strict.
11. Set the [[FunctionKind]] internal slot of F to functionKind.
12. Set the [[Prototype]] internal slot of F to functionPrototype.
13. Set the [[Extensible]] internal slot of F to true.
14. Set the [[Realm]] internal slot of F to the running execution context’s Realm.
15. Return F.

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

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

1. Assert: F is an extensible object that does not have a `length` own property.
2. Let len be the ExpectedArgumentCount of ParameterList.
3. Let status be DefinePropertyOrThrow(F, `"length"`, PropertyDescriptor{[[Value]]: len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true}).
4. Assert: status is not an abrupt completion.
5. Let Strict be the value of the [[Strict]] internal slot of F.
6. Set the [[Environment]] internal slot of F to the value of Scope.
7. Set the [[FormalParameters]] internal slot of F to ParameterList .
8. Set the [[ECMAScriptCode]] internal slot of F to Body.
9. If kind is Arrow, set the [[ThisMode]] internal slot of F to lexical.
10. Else if Strict is true, set the [[ThisMode]] internal slot of F to strict.
11. Else set the [[ThisMode]] internal slot of F to global.
12. Return F.

# 9.2.5 FunctionCreate (kind, ParameterList, Body, Scope, Strict, prototype)

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

1. If the prototype argument was not passed, then
   1. Let prototype be the intrinsic object %FunctionPrototype%.
2. If kind is not Normal, let allocKind be `"non-constructor"`.
3. Else let allocKind be `"normal"`.
4. Let F be FunctionAllocate(prototype, Strict, allocKind).
5. Return FunctionInitialize(F, kind, ParameterList, Body, Scope).

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

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

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

# 9.2.7 AddRestrictedFunctionProperties ( F, realm )

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

1. Assert: realm.[[intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
2. Let thrower be realm.[[intrinsics]].[[%ThrowTypeError%]].
3. Let status be DefinePropertyOrThrow(F, `"caller"`, PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true}).
4. Assert: status is not an abrupt completion.
5. Return DefinePropertyOrThrow(F , `"arguments"`, PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true}).
6. Assert: The above returned value is not an abrupt completion.

# 9.2.7.1 %ThrowTypeError% ( )

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

1. Throw a TypeError exception.

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

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

# 9.2.8 MakeConstructor (F, writablePrototype, prototype)

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

1. Assert: F is an ECMAScript function object.
2. Assert: F has a [[Construct]] internal method.
3. Assert: F is an extensible object that does not have a `prototype` own property.
4. Let installNeeded be false.
5. If the prototype argument was not provided, then
   1. Let installNeeded be true.
   2. Let prototype be ObjectCreate(%ObjectPrototype%).
6. If the writablePrototype argument was not provided, then
   1. Let writablePrototype be true.
7. If installNeeded, then
   1. Let status be DefinePropertyOrThrow(prototype, `"constructor"`, PropertyDescriptor{[[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: writablePrototype }).
   2. Assert: status is not an abrupt completion.
8. Let status be DefinePropertyOrThrow(F, `"prototype"`, PropertyDescriptor{[[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false}).
9. Assert: status is not an abrupt completion.
10. Return NormalCompletion(undefined).

# 9.2.9 MakeClassConstructor ( F)

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

1. Assert: F is an ECMAScript function object.
2. Assert: F’s [[FunctionKind]] internal slot is `"normal"`.
3. Set F’s [[FunctionKind]] internal slot to `"classConstructor"`.
4. Return NormalCompletion(undefined).

# 9.2.10 MakeMethod ( F, homeObject)

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

1. Assert: F is an ECMAScript function object.
2. Assert: Type(homeObject ) is Object.
3. Set the [[HomeObject]] internal slot of F to homeObject.
4. Return NormalCompletion(undefined).

# 9.2.11 SetFunctionName (F, name, prefix)

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

1. Assert: F is an extensible object that does not have a `name` own property.
2. Assert: Type(name) is either Symbol or String.
3. Assert: If prefix was passed then Type(prefix) is String.
4. If Type(name) is Symbol, then
   1. Let description be name’s [[Description]] value.
   2. If description is undefined, let name be the empty String.
   3. Else, let name be the concatenation of `"["`, description, and `"]"`.
5. If prefix was passed, then
   1. Let name be the concatenation of prefix, code unit 0x0020 (SPACE), and name.
6. Return DefinePropertyOrThrow(F, `"name"`, PropertyDescriptor{[[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true}).
7. Assert: the result is never an abrupt completion.

# 9.2.12 FunctionDeclarationInstantiation(func, argumentsList)

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

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

1. Let calleeContext be the running execution context.
2. Let env be the LexicalEnvironment of calleeContext.
3. Let envRec be env’s Environment Record.
4. Let code be the value of the [[ECMAScriptCode]] internal slot of func.
5. Let strict be the value of the [[Strict]] internal slot of func.
6. Let formals be the value of the [[FormalParameters]] internal slot of func.
7. Let parameterNames be the BoundNames of formals.
8. If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
9. Let simpleParameterList be IsSimpleParameterList of formals.
10. Let hasParameterExpressions be ContainsExpression of formals.
11. Let varNames be the VarDeclaredNames of code.
12. Let varDeclarations be the VarScopedDeclarations of code.
13. Let lexicalNames be the LexicallyDeclaredNames of code.
14. Let functionNames be an empty List.
15. Let functionsToInitialize be an empty List.
16. For each d in varDeclarations, in reverse list order do
    1. If d is neither a VariableDeclaration or a ForBinding, then
       1. Assert: d is either a FunctionDeclaration or a GeneratorDeclaration.
       2. Let fn be the sole element of the BoundNames of d.
       3. If fn is not an element of functionNames, then
          1. Insert fn as the first element of functionNames.
          2. NOTE If there are multiple FunctionDeclarations or GeneratorDeclarations for the same name, the last declaration is used.
          3. Insert d as the first element of functionsToInitialize.
17. Let argumentsObjectNeeded be true.
18. If the value of the [[ThisMode]] internal slot of func is lexical, then
    1. NOTE Arrow functions never have an arguments objects.
    2. Let argumentsObjectNeeded be false.
19. Else if `"arguments"` is an element of parameterNames, then
    1. Let argumentsObjectNeeded be false.
20. Else if hasParameterExpressions is false, then
    1. If `"arguments"` is an element of functionNames or if `"arguments"` is an element of lexicalNames, then
       1. Let argumentsObjectNeeded be false.
21. For each String paramName in parameterNames, do
    1. Let alreadyDeclared be envRec.HasBinding(paramName).
    2. NOTE Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
    3. If alreadyDeclared is false, then
       1. Let status be envRec.CreateMutableBinding(paramName).
       2. If hasDuplicates is true, then
          1. Let status be envRec.InitializeBinding(paramName, undefined).
       3. Assert: status is never an abrupt completion for either of the above operations.
22. If argumentsObjectNeeded is true, then
    1. If strict is true or if simpleParameterList is false, then
       1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
    2. Else,
       1. NOTE mapped argument object is only provided for non-strict functions that don’t have a rest parameter, any parameter default value initializers, or any destructured parameters .
       2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
    3. ReturnIfAbrupt(ao).
    4. If strict is true, then
       1. Let status be envRec.CreateImmutableBinding(`"arguments"`).
    5. Else,
       1. Let status be envRec.CreateMutableBinding(`"arguments"`).
    6. Assert: status is never an abrupt completion.
    7. Call envRec.InitializeBinding(`"arguments"`, ao).
    8. Append `"arguments"` to parameterNames.
23. Let iteratorRecord be Record {[[iterator]]: CreateListIterator(argumentsList), [[done]]: false}.
24. If hasDuplicates is true, then
    1. Let formalStatus be IteratorBindingInitialization for formals with iteratorRecord and undefined as arguments.
25. Else,
    1. Let formalStatus be IteratorBindingInitialization for formals with iteratorRecord and envRec as arguments.
26. ReturnIfAbrupt(formalStatus).
27. If hasParameterExpressions is false, then
    1. NOTE Only a single lexical environment is needed for the parameters and top-level vars.
    2. Let instantiatedVarNames be a copy of the List parameterNames.
    3. For each n in varNames, do
       1. If n is not an element of instantiatedVarNames, then
          1. Append n to instantiatedVarNames.
          2. Let status be envRec.CreateMutableBinding(n).
          3. Assert: status is never an abrupt completion.
          4. Call envRec.InitializeBinding(n, undefined).
    4. Let varEnv be env.
    5. Let varEnvRec be envRec.
28. Else,
    1. NOTE A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
    2. Let varEnv be NewDeclarativeEnvironment(env).
    3. Let varEnvRec be varEnv’s EnvironmentRecord.
    4. Set the VariableEnvironment of calleeContext to varEnv.
    5. Let instantiatedVarNames be a new empty List.
    6. For each n in varNames, do
       1. If n is not an element of instantiatedVarNames, then
          1. Append n to instantiatedVarNames.
          2. Let status be varEnvRec.CreateMutableBinding(n).
          3. Assert: status is never an abrupt completion.
          4. If n is not an element of parameterNames or if n is an element of functionNames, let initialValue be undefined.
          5. else,
             1. Let initialValue be envRec.GetBindingValue(n, false).
             2. ReturnIfAbrupt(initialValue).
          6. Call varEnvRec.InitializeBinding(n, initialValue).
          7. NOTE vars whose names are the same as a formal parameter, initially have the same value as the corresponding initialized parameter.
29. NOTE: Annex B.3.3 adds additional steps at this point.
30. If strict is false, then
    1. Let lexEnv be NewDeclarativeEnvironment(varEnv).
    2. NOTE: Non-strict functions use a separate lexical Environment Record for top-level lexical declarations so that a direct `eval` (see 12.3.4.1) can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct `eval` always places all declarations into a new Environment Record.
31. Else, let lexEnv be varEnv.
32. Let lexEnvRec be lexEnv’s EnvironmentRecord.
33. Set the LexicalEnvironment of calleeContext to lexEnv.
34. Let lexDeclarations be the LexicallyScopedDeclarations of code.
35. For each element d in lexDeclarations do
    1. NOTE A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d do
       1. If IsConstantDeclaration of d is true, then
          1. Let status be lexEnvRec.CreateImmutableBinding(dn, true).
       2. Else,
          1. Let status be lexEnvRec.CreateMutableBinding(dn, false).
    3. Assert: status is never an abrupt completion.
36. For each parsed grammar phrase f in functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be the result of performing InstantiateFunctionObject for f with argument lexEnv.
    3. Let status be varEnvRec.SetMutableBinding(fn, fo, false).
    4. Assert: status is never an abrupt completion.
37. Return NormalCompletion(empty).

NOTE 1 B.3.3 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate the sixth edition of ECMA-262.

NOTE 2 Parameter Initializers may contain direct eval expressions (12.3.4.1). Any top level declarations of such evals are only visible to the eval code (10.2). The creation of the environment for such declarations is described in 14.1.18.

# 9.3 Built-in Function Objects

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

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

Unless otherwise specified every built-in function object has the %FunctionPrototype% object (19.2.3) as the initial value of its [[Prototype]] internal slot.

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

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

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

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

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

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

1. Let callerContext be the running execution context.
2. If callerContext is not already suspended, Suspend callerContext.
3. Let calleeContext be a new ECMAScript code execution context.
4. Set the Function of calleeContext to F.
5. Let calleeRealm be the value of F’s [[Realm]] internal slot.
6. Set the Realm of calleeContext to calleeRealm.
7. Perform any necessary implementation defined initialization of calleeContext.
8. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
9. Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
10. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
11. Return result.

NOTE 1 When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.

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

The [[Construct]] internal method for built-in function object F is called with parameters argumentsList and newTarget. The steps performed are the same as [[Call]] (see 9.3.1) except that step 9 is replaced by:

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

# 9.3.3 CreateBuiltinFunction(realm, steps, prototype, internalSlotsList)

The abstract operation CreateBuiltinFunction takes arguments realm, prototype, and steps. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, an empty List is used. CreateBuiltinFunction returns a built-in function object created by the following steps:

1. Assert: realm is a Realm Record.
2. Assert: steps is either a set of algorithm steps or other definition of a functions behaviour provided in this specification.
3. Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList. The initial value of each of those internal slots is undefined.
4. Set the [[Realm]] internal slot of func to realm.
5. Set the [[Prototype]] internal slot of func to prototype.
6. Return func.

Each built-in function defined in this specification is created as if by calling the CreateBuiltinFunction abstract operation, unless otherwise specified.

# 9.4 Built-in Exotic Object Internal Methods and Slots

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

# 9.4.1 Bound Function Exotic Objects

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

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

Table 28 — Internal Slots of Exotic Bound Function Objects

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

Unlike ECMAScript function objects, bound function objects do not use an alternative definition of the [[GetOwnProperty]] internal methods. Bound function objects provide all of the essential internal methods as specified in 9.1. However, they use the following definitions for the essential internal methods of function objects.

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

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

1. Let target be the value of F’s [[BoundTargetFunction]] internal slot.
2. Let boundThis be the value of F’s [[BoundThis]] internal slot.
3. Let boundArgs be the value of F’s [[BoundArguments]] internal slot.
4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
5. Return Call(target, boundThis, args).

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

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

1. Let target be the value of F’s [[BoundTargetFunction]] internal slot.
2. Assert: target has a [[Construct]] internal method.
3. Let boundArgs be the value of F’s [[BoundArguments]] internal slot.
4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
5. If SameValue(F, newTarget) is true, let newTarget be target.
6. Return Construct(target, args, newTarget).

# 9.4.1.3 BoundFunctionCreate (targetFunction, boundThis, boundArgs)

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

1. Assert: Type(targetFunction) is Object.
2. Let proto be targetFunction.[[GetPrototypeOf]]().
3. ReturnIfAbrupt(proto).
4. Let obj be a newly created object.
5. Set obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
6. Set the [[Call]] internal method of obj as described in 9.4.1.1.
7. If targetFunction has a [[Construct]] internal method, then
   1. Set the [[Construct]] internal method of obj as described in 9.4.1.2.
8. Set the [[Prototype]] internal slot of obj to proto.
9. Set the [[Extensible]] internal slot of obj to true.
10. Set the [[BoundTargetFunction]] internal slot of obj to targetFunction.
11. Set the [[BoundThis]] internal slot of obj to the value of boundThis.
12. Set the [[BoundArguments]] internal slot of obj to boundArgs.
13. Return obj.

# 9.4.2 Array Exotic Objects

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

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

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

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

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

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

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

# 9.4.2.2 ArrayCreate(length, proto)

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

1. Assert: length is an integer Number ≥ 0.
2. If length is −0, let length be +0.
3. If length>232-1, throw a RangeError exception.
4. If the proto argument was not passed, let proto be the intrinsic object %ArrayPrototype%.
5. Let A be a newly created Array exotic object.
6. Set A’s essential internal methods except for [[DefineOwnProperty]] to the default ordinary object definitions specified in 9.1.
7. Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.2.1.
8. Set the [[Prototype]] internal slot of A to proto.
9. Set the [[Extensible]] internal slot of A to true.
10. Perform OrdinaryDefineOwnProperty(A, `"length"`, PropertyDescriptor{[[Value]]: length, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}).
11. Assert: the preceding step never produces an abrupt completion.
12. Return A.

# 9.4.2.3 ArraySpeciesCreate(originalArray, length)

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

1. Assert: length is an integer Number ≥ 0.
2. If length is −0, let length be +0.
3. Let C be undefined.
4. Let isArray be IsArray(originalArray).
5. ReturnIfAbrupt(isArray).
6. If isArray is true, then
   1. Let C be Get(originalArray, `"constructor"`).
   2. ReturnIfAbrupt(C).
   3. If IsConstructor(C) is true, then
      1. Let thisRealm be the running execution context’s Realm.
      2. Let realmC be GetFunctionRealm(C).
      3. ReturnIfAbrupt(realmC).
      4. If thisRealm and realmC are not the same Realm Record, then
         1. If SameValue(C, realmC.[[intrinsics]].[[%Array%]]) is true, let C be undefined.
   4. If Type(C) is Object, then
      1. Let C be Get(C, @@species).
      2. ReturnIfAbrupt(C).
      3. If C is null, let C be undefined.
7. If C is undefined, return ArrayCreate(length).
8. If IsConstructor(C) is false, throw a TypeError exception.
9. Return Construct(C, «length»).

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

# 9.4.2.4 ArraySetLength(A, Desc)

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

1. If the [[Value]] field of Desc is absent, then
   1. Return OrdinaryDefineOwnProperty(A, `"length"`, Desc).
2. Let newLenDesc be a copy of Desc.
3. Let newLen be ToUint32(Desc.[[Value]]).
4. ReturnIfAbrupt(newLen).
5. Let numberLen be ToNumber(Desc.[[Value]]).
6. ReturnIfAbrupt(newLen).
7. If newLen ≠ numberLen, throw a RangeError exception.
8. Set newLenDesc.[[Value]] to newLen.
9. Let oldLenDesc be OrdinaryGetOwnProperty(A, `"length"`).
10. Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
11. Let oldLen be oldLenDesc.[[Value]].
12. If newLen ≥oldLen, then
    1. Return OrdinaryDefineOwnProperty(A, `"length"`, newLenDesc).
13. If oldLenDesc.[[Writable]] is false, return false.
14. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
15. Else,
    1. Need to defer setting the [[Writable]] attribute to false in case any elements cannot be deleted.
    2. Let newWritable be false.
    3. Set newLenDesc.[[Writable]] to true.
16. Let succeeded be OrdinaryDefineOwnProperty(A, `"length"`, newLenDesc).
17. Assert: succeeded is not an abrupt completion.
18. If succeeded is false, return false.
19. While newLen < oldLen repeat,
    1. Set oldLen to oldLen – 1.
    2. Let deleteSucceeded be A.[[Delete]](ToString(oldLen)).
    3. Assert: deleteSucceeded is not an abrupt completion.
    4. If deleteSucceeded is false, then
       1. Set newLenDesc.[[Value]] to oldLen + 1.
       2. If newWritable is false, set newLenDesc.[[Writable]] to false.
       3. Let succeeded be OrdinaryDefineOwnProperty(A, `"length"`, newLenDesc).
       4. Assert: succeeded is not an abrupt completion.
       5. Return false.
20. If newWritable is false, then
    1. Return OrdinaryDefineOwnProperty(A, `"length"`, PropertyDescriptor{[[Writable]]: false}). This call will always return true.
21. Return true.

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

# 9.4.3 String Exotic Objects

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

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

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

# 9.4.3.1 [[GetOwnProperty]] ( P )

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

1. Assert: IsPropertyKey(P) is true.
2. Let desc be OrdinaryGetOwnProperty(S, P).
3. If desc is not undefined return desc.
4. Return StringGetIndexProperty(S, P).

# 9.4.3.1.1 StringGetIndexProperty (S, P)

When the abstract operation StringGetIndexProperty is called with an exotic String object S and with property key P, the following steps are taken:

1. If Type(P) is not String, return undefined.
2. Let index be CanonicalNumericIndexString (P).
3. Assert: index is not an abrupt completion.
4. If index is undefined, return undefined.
5. If IsInteger(index) is false, return undefined.
6. If index = −0, return undefined.
7. Let str be the String value of the [[StringData]] internal slot of S.
8. Let len be the number of elements in str.
9. If index < 0 or len ≤ index, return undefined.
10. Let resultStr be a String value of length 1, containing one code unit from str, specifically the code unit at index index.
11. Return a PropertyDescriptor{ [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false }.

# 9.4.3.2 [[HasProperty]](P)

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

1. Let elementDesc be StringGetIndexProperty(S, P).
2. If elementDesc is not undefined, return true.
3. Return OrdinaryHasProperty(S, P)..

# 9.4.3.3 [[OwnPropertyKeys]] ( )

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

1. Let keys be a new empty List.
2. Let str be the String value of the [[StringData]] internal slot of O.
3. Let len be the number of elements in str.
4. For each integer i starting with 0 such that i < len, in ascending order,
   1. Add ToString(i) as the last element of keys
5. For each own property key P of O such that P is an integer index and ToInteger(P) ≥ len, in ascending numeric index order,
   1. Add P as the last element of keys.
6. For each own property key P of O such that Type(P) is String and P is not an integer index, in property creation order,
   1. Add P as the last element of keys.
7. For each own property key P of O such that Type(P) is Symbol, in property creation order,
   1. Add P as the last element of keys.
8. Return keys.

# 9.4.3.4 StringCreate( value, prototype)

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

1. ReturnIfAbrupt(prototype).
2. Assert: Type(value) is String.
3. Let S be a newly created String exotic object.
4. Set the [[StringData]] internal slot of S to value.
5. Set S’s essential internal methods to the default ordinary object definitions specified in 9.1.
6. Set the [[GetOwnProperty]] internal method of S as specified in 9.4.3.1.
7. Set the [[HasProperty]] internal method of S as specified in 9.4.3.2.
8. Set the [[OwnPropertyKeys]] internal method of S as specified in 9.4.3.3.
9. Set the [[Prototype]] internal slot of S to prototype.
10. Set the [[Extensible]] internal slot of S to true.
11. Let length be the number of code unit elements in value.
12. Let status be DefinePropertyOrThrow(S, `"length"`, PropertyDescriptor{[[Value]]: length, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
13. Assert: status is not an abrupt completion.
14. Return S.

# 9.4.4 Arguments Exotic Objects

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

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

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

NOTE 1 For non-strict functions the integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

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

# 9.4.4.1 [[GetOwnProperty]] (P)

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

1. Let args be the arguments object.
2. Let desc be OrdinaryGetOwnProperty(args, P).
3. If desc is undefined, return desc.
4. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
5. Let isMapped be HasOwnProperty(map, P).
6. Assert: isMapped is never an abrupt completion.
7. If the value of isMapped is true, then
   1. Set desc.[[Value]] to Get(map, P).
8. If IsDataDescriptor(desc) is true and P is `"caller"` and desc.[[Value]] is a strict mode Function object, throw a TypeError exception.
9. Return desc.

If an implementation does not provide a built-in `caller` property for argument exotic objects then step 8 of this algorithm is must be skipped.

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

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

1. Let args be the arguments object.
2. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
3. Let isMapped be HasOwnProperty(map, P).
4. Let allowed be OrdinaryDefineOwnProperty(args, P, Desc).
5. ReturnIfAbrupt(allowed).
6. If allowed is false, return false.
7. If the value of isMapped is true, then
   1. If IsAccessorDescriptor(Desc) is true, then
      1. Call map.[[Delete]](P).
   2. Else
      1. If Desc.[[Value]] is present, then
         1. Let setStatus be Set(map, P, Desc.[[Value]], false).
         2. Assert: setStatus is true because formal parameters mapped by argument objects are always writable.
      2. If Desc.[[Writable]] is present and its value is false, then
         1. Call map.[[Delete]](P).
8. Return true.

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

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

1. Let args be the arguments object.
2. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
3. Let isMapped be HasOwnProperty(map, P).
4. Assert: isMapped is not an abrupt completion.
5. If the value of isMapped is false, then
   1. Let v be the result of calling the default ordinary object [[Get]] internal method (9.1.8) on args passing P and Receiver as the arguments.
6. Else map contains a formal parameter mapping for P,
   1. Let v be Get(map, P).
7. ReturnIfAbrupt(v).
8. Return v.

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

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

1. Let args be the arguments object.
2. If SameValue(args, Receiver) is false, then
   1. Let isMapped be false.
3. Else,
   1. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
   2. Let isMapped be HasOwnProperty(map, P).
   3. Assert: isMapped is not an abrupt completion.
4. If the value of isMapped is false, then
   1. Return the result of calling the default ordinary object [[Set]] internal method (9.1.9) on args passing P, V and Receiver as the arguments.
5. Else map contains a formal parameter mapping for P,
   1. Return Set(map, P, V, false).

# 9.4.4.5 [[Delete]] (P)

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

1. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
2. Let isMapped be HasOwnProperty(map, P).
3. Assert: isMapped is not an abrupt completion.
4. Let result be the result of calling the default [[Delete]] internal method for ordinary objects (9.1.10) on the arguments object passing P as the argument.
5. ReturnIfAbrupt(result).
6. If result is true and the value of isMapped is true, then
   1. Call map.[[Delete]](P).
7. Return result.

# 9.4.4.6 CreateUnmappedArgumentsObject(argumentsList)

The abstract operation CreateUnmappedArgumentsObject called with an argument argumentsList performs the following steps:

1. Let len be the number of elements in argumentsList.
2. Let obj be ObjectCreate(%ObjectPrototype%, «‍[[ParameterMap]]»).
3. Set obj’s [[ParameterMap]] internal slot to undefined.
4. Perform DefinePropertyOrThrow(obj, `"length"`, PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
5. Let index be 0.
6. Repeat while index < len,
   1. Let val be argumentsList[index].
   2. Perform CreateDataProperty(obj, ToString(index), val).
   3. Let index be index + 1
7. Perform DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {[[Value]]:%ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
8. Perform DefinePropertyOrThrow(obj, `"caller"`, PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false}).
9. Perform DefinePropertyOrThrow(obj, `"callee"`, PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false}).
10. Assert: the above property definitions will not produce an abrupt completion.
11. Return obj

# 9.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env )

The abstract operation CreateMappedArgumentsObject is called with object func, parsed grammar phrase formals, List argumentsList, and Environment Record env. The following steps are performed:

1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
2. Let len be the number of elements in argumentsList.
3. Let obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
4. Set the [[GetOwnProperty]] internal method of obj as specified in 9.4.4.1.
5. Set the [[DefineOwnProperty]] internal method of obj as specified in 9.4.4.2.
6. Set the [[Get]] internal method of obj as specified in 9.4.4.3.
7. Set the [[Set]] internal method of obj as specified in 9.4.4.4.
8. Set the [[Delete]] internal method of obj as specified in 9.4.4.5.
9. Set the remainder of obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
10. Set the [[Prototype]] internal slot of obj to %ObjectPrototype%.
11. Set the [[Extensible]] internal slot of obj to true.
12. Let parameterNames be the BoundNames of formals.
13. Let numberOfParameters be the number of elements in parameterNames
14. Let index be 0.
15. Repeat while index < len ,
    1. Let val be argumentsList[index].
    2. Perform CreateDataProperty(obj, ToString(index), val).
    3. Let index be index + 1
16. Perform DefinePropertyOrThrow(obj, `"length"`, PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
17. Let map be ObjectCreate(null).
18. Let mappedNames be an empty List.
19. Let index be numberOfParameters − 1.
20. Repeat while index ≥ 0 ,
    1. Let name be parameterNames[index].
    2. If name is not an element of mappedNames, then
       1. Add name as an element of the list mappedNames.
       2. If index < len, then
          1. Let g be MakeArgGetter(name, env).
          2. Let p be MakeArgSetter(name, env).
          3. Call map.[[DefineOwnProperty]](ToString(index), PropertyDescriptor{[[Set]]: p, [[Get]]: g, [[Enumerable]]: false, [[Configurable]]: true}).
    3. Let index be index − 1
21. Set the [[ParameterMap]] internal slot of obj to map.
22. Perform DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {[[Value]]:%ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
23. Perform DefinePropertyOrThrow(obj, `"callee"`, PropertyDescriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
24. Assert: the above property definitions will not produce an abrupt completion.
25. Return obj

# 9.4.4.7.1 MakeArgGetter ( name, env)

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

1. Let realm be the current Realm.
2. Let steps be the steps of an ArgGetter function as specified below.
3. Let getter be CreateBuiltinFunction(realm, steps, %FunctionPrototype%, «‍[[name]], [[env]]» ).
4. Set getter’s [[name]] internal slot to name.
5. Set getter’s [[env]] internal slot to env.
6. Return getter.

An ArgGetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:

1. Let name be the value of f’s [[name]] internal slot.
2. Let env be the value of f’s [[env]] internal slot
3. Return env.GetBindingValue(name, false).

NOTE ArgGetter functions are never directly accessible to ECMAScript code.

# 9.4.4.7.2 MakeArgSetter ( name, env)

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

1. Let realm be the current Realm.
2. Let steps be the steps of an ArgSetter function as specified below.
3. Let setter be CreateBuiltinFunction(realm, steps, %FunctionPrototype%, «‍[[name]], [[env]]» ).
4. Set setter’s [[name]] internal slot to name.
5. Set setter’s [[env]] internal slot to env.
6. Return setter.

An ArgSetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:

1. Let name be the value of f’s [[name]] internal slot.
2. Let env be the value of f’s [[env]] internal slot
3. Return env.SetMutableBinding(name, value, false).

NOTE ArgSetter functions are never directly accessible to ECMAScript code.

# 9.4.5 Integer Indexed Exotic Objects

An Integer Indexed object is an exotic object that performs special handling of integer index property keys.

Integer Indexed exotic objects have the same internal slots as ordinary objects additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.

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

# 9.4.5.1 [[GetOwnProperty]] ( P )

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

1. Assert: IsPropertyKey(P) is true.
2. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
3. If Type(P) is String, then
   1. Let numericIndex be CanonicalNumericIndexString(P).
   2. Assert: numericIndex is not an abrupt completion.
   3. If numericIndex is not undefined, then
      1. Let value be IntegerIndexedElementGet (O, numericIndex).
      2. ReturnIfAbrupt(value).
      3. If value is undefined, return undefined.
      4. Return a PropertyDescriptor{ [[Value]]: value, [[Enumerable]]: true, [[Writable]]: true, [[Configurable]]: false }.
4. Return OrdinaryGetOwnProperty(O, P).

# 9.4.5.2 [[HasProperty]](P)

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

1. Assert: IsPropertyKey(P) is true.
2. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
3. If Type(P) is String, then
   1. Let numericIndex be CanonicalNumericIndexString(P).
   2. Assert: numericIndex is not an abrupt completion.
   3. If numericIndex is not undefined, then
      1. Let buffer be the value of O’s [[ViewedArrayBuffer]] internal slot.
      2. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
      3. If IsInteger(numericIndex) is false, return false
      4. If numericIndex = −0, return false.
      5. If numericIndex < 0, return false.
      6. If numericIndex ≥ the value of O’s [[ArrayLength]] internal slot, return false.
      7. Return true.
4. Return OrdinaryHasProperty(O, P).

# 9.4.5.3 [[DefineOwnProperty]] ( P, Desc)

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

1. Assert: IsPropertyKey(P) is true.
2. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
3. If Type(P) is String, then
   1. Let numericIndex be CanonicalNumericIndexString (P).
   2. Assert: numericIndex is not an abrupt completion.
   3. If numericIndex is not undefined, then
      1. If IsInteger(numericIndex) is false, return false
      2. Let intIndex be numericIndex.
      3. If intIndex = −0, return false.
      4. If intIndex < 0, return false.
      5. Let length be the value of O’s [[ArrayLength]] internal slot.
      6. If intIndex ≥ length, return false.
      7. If IsAccessorDescriptor(Desc) is true, return false.
      8. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is true, return false.
      9. If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] is false, return false.
      10. If Desc has a [[Writable]] field and if Desc.[[Writable]] is false, return false.
      11. If Desc has a [[Value]] field, then
          1. Let value be Desc.[[Value]].
          2. Return IntegerIndexedElementSet (O, intIndex, value).
      12. Return true.
4. Return OrdinaryDefineOwnProperty(O, P, Desc).

# 9.4.5.4 [[Get]] (P, Receiver)

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

1. Assert: IsPropertyKey(P) is true.
2. If Type(P) is String and if SameValue(O, Receiver) is true, then
   1. Let numericIndex be CanonicalNumericIndexString (P).
   2. Assert: numericIndex is not an abrupt completion.
   3. If numericIndex is not undefined, then
      1. Return IntegerIndexedElementGet (O, numericIndex).
3. Return the result of calling the default ordinary object [[Get]] internal method (9.1.8) on O passing P and Receiver as arguments.

# 9.4.5.5 [[Set]] ( P, V, Receiver)

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

1. Assert: IsPropertyKey(P) is true.
2. If Type(P) is String and if SameValue(O, Receiver) is true, then
   1. Let numericIndex be CanonicalNumericIndexString (P).
   2. Assert: numericIndex is not an abrupt completion.
   3. If numericIndex is not undefined, then
      1. Return IntegerIndexedElementSet (O, numericIndex, V).
3. Return the result of calling the default ordinary object [[Set]] internal method (9.1.8) on O passing P, V, and Receiver as arguments.

# 9.4.5.6 [[OwnPropertyKeys]] ()

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

1. Let keys be a new empty List.
2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
3. Let len be the value of O’s [[ArrayLength]] internal slot.
4. For each integer i starting with 0 such that i < len, in ascending order,
   1. Add ToString(i) as the last element of keys.
5. For each own property key P of O such that Type(P) is String and P is not an integer index, in property creation order
   1. Add P as the last element of keys.
6. For each own property key P of O such that Type(P) is Symbol, in property creation order
   1. Add P as the last element of keys.
7. Return keys.

# 9.4.5.7 IntegerIndexedObjectCreate (prototype, internalSlotsList)

The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:

1. Let A be a newly created object with an internal slot for each name in internalSlotsList.
2. Set A’s essential internal methods to the default ordinary object definitions specified in 9.1.
3. Set the [[GetOwnProperty]] internal method of A as specified in 9.4.5.1.
4. Set the [[HasProperty]] internal method of A as specified in 9.4.5.2.
5. Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.5.3.
6. Set the [[Get]] internal method of A as specified in 9.4.5.4.
7. Set the [[Set]] internal method of A as specified in 9.4.5.5.
8. Set the [[OwnPropertyKeys]] internal method of A as specified in 9.4.5.6.
9. Set the [[Prototype]] internal slot of A to prototype.
10. Set the [[Extensible]] internal slot of A to true.
11. Return A.

# 9.4.5.8 IntegerIndexedElementGet ( O, index )

The abstract operation IntegerIndexedElementGet with arguments O and index performs the following steps:

1. Assert: Type(index) is Number.
2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
3. Let buffer be the value of O’s [[ViewedArrayBuffer]] internal slot.
4. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
5. If IsInteger(index) is false, return undefined
6. If index = −0, return undefined.
7. Let length be the value of O’s [[ArrayLength]] internal slot.
8. If index < 0 or index ≥ length, return undefined.
9. Let offset be the value of O’s [[ByteOffset]] internal slot.
10. Let arrayTypeName be the string value of O’s [[TypedArrayName]] internal slot.
11. Let elementSize be the Number value of the Element Size value specified in Table 49 for arrayTypeName.
12. Let indexedPosition = (index × elementSize) + offset.
13. Let elementType be the string value of the Element Type value in Table 49 for arrayTypeName.
14. Return GetValueFromBuffer(buffer, indexedPosition, elementType).

# 9.4.5.9 IntegerIndexedElementSet ( O, index, value )

The abstract operation IntegerIndexedElementSet with arguments O, index, and value performs the following steps:

1. Assert: Type(index) is Number.
2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
3. Let numValue be ToNumber(value).
4. ReturnIfAbrupt(numValue).
5. Let buffer be the value of O’s [[ViewedArrayBuffer]] internal slot.
6. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
7. If IsInteger(index) is false, return false
8. If index = −0, return false.
9. Let length be the value of O’s [[ArrayLength]] internal slot.
10. If index < 0 or index ≥ length, return false.
11. Let offset be the value of O’s [[ByteOffset]] internal slot.
12. Let arrayTypeName be the string value of O’s [[TypedArrayName]] internal slot.
13. Let elementSize be the Number value of the Element Size value specified in Table 49 for arrayTypeName.
14. Let indexedPosition = (index × elementSize) + offset.
15. Let elementType be the string value of the Element Type value in Table 49 for arrayTypeName.
16. Let status be SetValueInBuffer(buffer, indexedPosition, elementType, numValue).
17. ReturnIfAbrupt(status).
18. Return true.

# 9.4.6 Module Namespace Exotic Objects

A module namespace object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using `export *` export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes {[[Configurable]]: false, [[Enumerable]]: true}. Module namespace objects are not extensible.

Module namespace objects have the internal slots defined in Table 29.

Table 29 — Internal Slots of Module Namespace Exotic Objects

Internal Slot | Type | Description
--- | --- | ---
[[Module]] | Module Record | The Module Record whose exports this namespace exposes.
[[Exports]] | List of String | A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those string values had been sorted using `Array.prototype.sort` using SortCompare as comparefn.

Module namespace exotic objects provide alternative definitions for all of the internal methods.

# 9.4.6.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of a module namespace exotic object O is called the following steps are taken:

1. Return null.

# 9.4.6.2 [[SetPrototypeOf]] (V)

When the [[SetPrototypeOf]] internal method of a module namespace exotic object O is called with argument V the following steps are taken:

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

# 9.4.6.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a module namespace exotic object O is called the following steps are taken:

1. Return false.

# 9.4.6.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a module namespace exotic object O is called the following steps are taken:

1. Return true.

# 9.4.6.5 [[GetOwnProperty]] (P)

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

1. If Type(P) is Symbol, return OrdinaryGetOwnProperty(O, P).
2. Throw a TypeError exception.

# 9.4.6.6 [[DefineOwnProperty]] (P, Desc)

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

1. Return false.

# 9.4.6.7 [[HasProperty]] (P)

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

1. If Type(P) is Symbol, return OrdinaryHasProperty(O, P).
2. Let exports be the value of O’s [[Exports]] internal slot.
3. If P is an element of exports, return true.
4. Return false.

# 9.4.6.8 [[Get]] (P, Receiver)

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

1. Assert: IsPropertyKey(P) is true.
2. If Type(P) is Symbol, then
   1. Return the result of calling the default ordinary object [[Get]] internal method (9.1.8) on O passing P and Receiver as arguments.
3. Let exports be the value of O’s [[Exports]] internal slot.
4. If P is not an element of exports, return undefined.
5. Let m be the value of O’s [[Module]] internal slot.
6. Let binding be m.ResolveExport(P, «», «»).
7. ReturnIfAbrupt(binding).
8. Assert: binding is neither null nor `"ambiguous"`.
9. Let targetModule be binding.[[module]],
10. Assert: targetModule is not undefined.
11. Let targetEnv be targetModule.[[Environment]].
12. If targetEnv is undefined, throw a ReferenceError exception.
13. Let targetEnvRec be targetEnv’s EnvironmentRecord.
14. Return targetEnvRec.GetBindingValue(binding.[[bindingName]], true).

NOTE ResolveExport is idempotent and side-effect free. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.

# 9.4.6.9 [[Set]] ( P, V, Receiver)

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

1. Return false.

# 9.4.6.10 [[Delete]] (P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let exports be the value of O’s [[Exports]] internal slot.
3. If P is an element of exports, return false.
4. Return true.

# 9.4.6.11 [[Enumerate]] ()

When the [[Enumerate]] internal method of a module namespace exotic object O is called the following steps are taken:

1. Let exports be the value of O’s [[Exports]] internal slot.
2. Return CreateListIterator(exports).

# 9.4.6.12 [[OwnPropertyKeys]] ( )

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

1. Let exports be a copy of the value of O’s [[Exports]] internal slot.
2. Let symbolKeys be the result of calling the default ordinary object [[OwnPropertyKeys]] internal method (9.1.12) on O passing no arguments.
3. Append all the entries of symbolKeys to the end of exports.
4. Return exports.

# 9.4.6.13 ModuleNamespaceCreate (module, exports)

The abstract operation ModuleNamespaceCreate with arguments module, and exports is used to specify the creation of new module namespace exotic objects. It performs the following steps:

1. Assert: module is a Module Record (see 15.2.1.14).
2. Assert: module.[[Namespace]] is undefined.
3. Assert: exports is a List of string values.
4. Let M be a newly created object.
5. Set M’s essential internal methods to the definitions specified in 9.4.6.
6. Set M’s [[Module]] internal slot to module.
7. Set M’s [[Exports]] internal slot to exports.
8. Create own properties of M corresponding to the definitions in 26.3.
9. Set module.[[Namespace]] to M.
10. Return M.

# 9.5 Proxy Object Internal Methods and Internal Slots

A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy objects has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy’s handler object, or null. Methods (see Table 30) of a handler object may be used to augment the implementation for one or more of the proxy object’s internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy’s target object.

Table 30 — Proxy Handler Methods

Internal Method | Handler Method
--- | ---
[[GetPrototypeOf]] | `getPrototypeOf`
[[SetPrototypeOf]] | `setPrototypeOf`
[[IsExtensible]] | `isExtensible`
[[PreventExtensions]] | `preventExtensions`
[[GetOwnProperty]] | `getOwnPropertyDescriptor`
[[HasProperty]] | `has`
[[Get]] | `get`
[[Set]] | `set`
[[Delete]] | `deleteProperty`
[[DefineOwnProperty]] | `defineProperty`
[[Enumerate]] | `enumerate`
[[OwnPropertyKeys]] | `ownKeys`
[[Call]] | `apply`
[[Construct]] | `construct`

When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy’s target object as a parameter. A proxy’s handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy’s target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHander]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy object to throw a TypeError exception.

Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

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

# 9.5.1 [[GetPrototypeOf]] ( )

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"getPrototypeOf"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return target.[[GetPrototypeOf]]().
8. Let handlerProto be Call(trap, handler, «target»).
9. ReturnIfAbrupt(handlerProto).
10. If Type(handlerProto) is neither Object nor Null, throw a TypeError exception.
11. Let extensibleTarget be IsExtensible(target).
12. ReturnIfAbrupt(extensibleTarget).
13. If extensibleTarget is true, return handlerProto.
14. Let targetProto be target.[[GetPrototypeOf]]().
15. ReturnIfAbrupt(targetProto).
16. If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
17. Return handlerProto.

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

- The result of [[GetPrototypeOf]] must be either an Object or null.

- If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object’s target object.

# 9.5.2 [[SetPrototypeOf]] (V)

When the [[SetPrototypeOf]] internal method of a Proxy exotic object O is called with argument V the following steps are taken:

1. Assert: Either Type(V) is Object or Type(V) is Null.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"setPrototypeOf"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[SetPrototypeOf]](V).
9. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target, V»)).
10. ReturnIfAbrupt(booleanTrapResult).
11. Let extensibleTarget be IsExtensible(target).
12. ReturnIfAbrupt(extensibleTarget).
13. If extensibleTarget is true, return booleanTrapResult.
14. Let targetProto be target.[[GetPrototypeOf]]().
15. ReturnIfAbrupt(targetProto).
16. If booleanTrapResult is true and SameValue(V, targetProto) is false, throw a TypeError exception.
17. Return booleanTrapResult.

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

- The result of [[SetPrototypeOf]] is a Boolean value.

- If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

# 9.5.3 [[IsExtensible]] ( )

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"isExtensible"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return target.[[IsExtensible]]().
8. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target»)).
9. ReturnIfAbrupt(booleanTrapResult).
10. Let targetResult be target.[[IsExtensible]]().
11. ReturnIfAbrupt(targetResult).
12. If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
13. Return booleanTrapResult.

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

- The result of [[IsExtensible]] is a Boolean value.

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

# 9.5.4 [[PreventExtensions]] ( )

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"preventExtensions"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return target.[[PreventExtensions]]().
8. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target»)).
9. ReturnIfAbrupt(booleanTrapResult).
10. If booleanTrapResult is true, then
    1. Let targetIsExtensible be target.[[IsExtensible]]().
    2. ReturnIfAbrupt(targetIsExtensible).
    3. If targetIsExtensible is true, throw a TypeError exception.
11. Return booleanTrapResult.

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

- The result of [[PreventExtensions]] is a Boolean value.

- [[PreventExtensions]] applied to the proxy object only returns true if [[IsExtensible]] applied to the proxy object’s target object is false.

# 9.5.5 [[GetOwnProperty]] (P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"getOwnPropertyDescriptor"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[GetOwnProperty]](P).
9. Let trapResultObj be Call(trap, handler, «target, P»).
10. ReturnIfAbrupt(trapResultObj).
11. If Type(trapResultObj) is neither Object nor Undefined, throw a TypeError exception.
12. Let targetDesc be target.[[GetOwnProperty]](P).
13. ReturnIfAbrupt(targetDesc).
14. If trapResultObj is undefined, then
    1. If targetDesc is undefined, return undefined.
    2. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
    3. Let extensibleTarget be IsExtensible(target).
    4. ReturnIfAbrupt(extensibleTarget).
    5. If ToBoolean(extensibleTarget) is false, throw a TypeError exception.
    6. Return undefined.
15. Let extensibleTarget be IsExtensible(target).
16. ReturnIfAbrupt(extensibleTarget).
17. Let resultDesc be ToPropertyDescriptor(trapResultObj).
18. ReturnIfAbrupt(resultDesc).
19. Call CompletePropertyDescriptor(resultDesc).
20. Let valid be IsCompatiblePropertyDescriptor (extensibleTarget, resultDesc, targetDesc).
21. If valid is false, throw a TypeError exception.
22. If resultDesc.[[Configurable]] is false, then
    1. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
       1. Throw a TypeError exception.
23. Return resultDesc.

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

- The result of [[GetOwnProperty]] must be either an Object or undefined.

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

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

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

- A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.

# 9.5.6 [[DefineOwnProperty]] (P, Desc)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"defineProperty"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[DefineOwnProperty]](P, Desc).
9. Let descObj be FromPropertyDescriptor(Desc).
10. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target, P, descObj»)).
11. ReturnIfAbrupt(booleanTrapResult).
12. If booleanTrapResult is false, return false.
13. Let targetDesc be target.[[GetOwnProperty]](P).
14. ReturnIfAbrupt(targetDesc).
15. Let extensibleTarget be IsExtensible(target).
16. ReturnIfAbrupt(extensibleTarget).
17. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is false, then
    1. Let settingConfigFalse be true.
18. Else let settingConfigFalse be false.
19. If targetDesc is undefined, then
    1. If extensibleTarget is false, throw a TypeError exception.
    2. If settingConfigFalse is true, throw a TypeError exception.
20. Else targetDesc is not undefined,
    1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc , targetDesc) is false, throw a TypeError exception.
    2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
21. Return true.

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

- The result of [[DefineOwnProperty]] is a Boolean value.

- A property cannot be added, if the target object is not extensible.

- A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.

- If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

# 9.5.7 [[HasProperty]] (P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"has"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[HasProperty]](P).
9. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target, P»)).
10. ReturnIfAbrupt(booleanTrapResult).
11. If booleanTrapResult is false, then
    1. Let targetDesc be target.[[GetOwnProperty]](P).
    2. ReturnIfAbrupt(targetDesc).
    3. If targetDesc is not undefined, then
       1. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
       2. Let extensibleTarget be IsExtensible(target).
       3. ReturnIfAbrupt(extensibleTarget).
       4. If extensibleTarget is false, throw a TypeError exception.
12. Return booleanTrapResult.

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

- The result of [[HasProperty]] is a Boolean value.

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

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

# 9.5.8 [[Get]] (P, Receiver)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"get"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[Get]](P, Receiver).
9. Let trapResult be Call(trap, handler, «target, P, Receiver»).
10. ReturnIfAbrupt(trapResult).
11. Let targetDesc be target.[[GetOwnProperty]](P).
12. ReturnIfAbrupt(targetDesc).
13. If targetDesc is not undefined, then
    1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
       1. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Get]] is undefined, then
       1. If trapResult is not undefined, throw a TypeError exception.
14. Return trapResult.

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

- The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.

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

# 9.5.9 [[Set]] ( P, V, Receiver)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"set"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[Set]](P, V, Receiver).
9. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target, P, V, Receiver»)).
10. ReturnIfAbrupt(booleanTrapResult).
11. If booleanTrapResult is false, return false.
12. Let targetDesc be target.[[GetOwnProperty]](P).
13. ReturnIfAbrupt(targetDesc).
14. If targetDesc is not undefined, then
    1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
       1. If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false, then
       1. If targetDesc.[[Set]] is undefined, throw a TypeError exception.
15. Return true.

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

- The result of [[Set]] is a Boolean value.

- Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.

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

# 9.5.10 [[Delete]] (P)

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

1. Assert: IsPropertyKey(P) is true.
2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
3. If handler is null, throw a TypeError exception.
4. Assert: Type(handler) is Object.
5. Let target be the value of the [[ProxyTarget]] internal slot of O.
6. Let trap be GetMethod(handler, `"deleteProperty"`).
7. ReturnIfAbrupt(trap).
8. If trap is undefined, then
   1. Return target.[[Delete]](P).
9. Let booleanTrapResult be ToBoolean(Call(trap, handler, «target, P»)).
10. ReturnIfAbrupt(booleanTrapResult).
11. If booleanTrapResult is false, return false.
12. Let targetDesc be target.[[GetOwnProperty]](P).
13. ReturnIfAbrupt(targetDesc).
14. If targetDesc is undefined, return true.
15. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
16. Return true.

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

- The result of [[Delete]] is a Boolean value.
- A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.

# 9.5.11 [[Enumerate]] ()

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"enumerate"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return target.[[Enumerate]]().
8. Let trapResult be Call(trap, handler, «target»).
9. ReturnIfAbrupt(trapResult).
10. If Type(trapResult) is not Object, throw a TypeError exception.
11. Return trapResult.

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

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

# 9.5.12 [[OwnPropertyKeys]] ( )

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"ownKeys"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return target.[[OwnPropertyKeys]]().
8. Let trapResultArray be Call(trap, handler, «target»).
9. Let trapResult be CreateListFromArrayLike(trapResultArray, «‍String, Symbol»).
10. ReturnIfAbrupt(trapResult).
11. Let extensibleTarget be IsExtensible(target).
12. ReturnIfAbrupt(extensibleTarget).
13. Let targetKeys be target.[[OwnPropertyKeys]]().
14. ReturnIfAbrupt(targetKeys).
15. Assert: targetKeys is a List containing only String and Symbol values.
16. Let targetConfigurableKeys be an empty List.
17. Let targetNonconfigurableKeys be an empty List.
18. Repeat, for each element key of targetKeys,
    1. Let desc be target.[[GetOwnProperty]](key).
    2. ReturnIfAbrupt(desc).
    3. If desc is not undefined and desc.[[Configurable]] is false, then
       1. Append key as an element of targetNonconfigurableKeys.
    4. Else,
       1. Append key as an element of targetConfigurableKeys.
19. If extensibleTarget is true and targetNonconfigurableKeys is empty, then
    1. Return trapResult.
20. Let uncheckedResultKeys be a new List which is a copy of trapResult.
21. Repeat, for each key that is an element of targetNonconfigurableKeys,
    1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
    2. Remove key from uncheckedResultKeys
22. If extensibleTarget is true, return trapResult.
23. Repeat, for each key that is an element of targetConfigurableKeys,
    1. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
    2. Remove key from uncheckedResultKeys
24. If uncheckedResultKeys is not empty, throw a TypeError exception.
25. Return trapResult.

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

- The result of [[OwnPropertyKeys]] is a List.

- The Type of each result List element is either String or Symbol.

- The result List must contain the keys of all non-configurable own properties of the target object.

- If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

# 9.5.13 [[Call]] (thisArgument, argumentsList)

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"apply"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Return Call(target, thisArgument, argumentsList).
8. Let argArray be CreateArrayFromList(argumentsList).
9. Return Call(trap, handler, «target, thisArgument, argArray»).

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

# 9.5.14 [[Construct]] ( argumentsList, newTarget)

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

1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
2. If handler is null, throw a TypeError exception.
3. Assert: Type(handler) is Object.
4. Let target be the value of the [[ProxyTarget]] internal slot of O.
5. Let trap be GetMethod(handler, `"construct"`).
6. ReturnIfAbrupt(trap).
7. If trap is undefined, then
   1. Assert: target has a [[Construct]] internal method.
   2. Return Construct(target, argumentsList, newTarget).
8. Let argArray be CreateArrayFromList(argumentsList).
9. Let newObj be Call(trap, handler, «target, argArray, newTarget »).
10. ReturnIfAbrupt(newObj).
11. If Type(newObj) is not Object, throw a TypeError exception.
12. Return newObj.

NOTE 1 A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

NOTE 2 [[Construct]] for proxy objects enforces the following invariants:

- The result of [[Construct]] must be an Object.

# 9.5.15 ProxyCreate(target, handler)

The abstract operation ProxyCreate with arguments target and handler is used to specify the creation of new Proxy exotic objects. It performs the following steps:

1. If Type(target) is not Object, throw a TypeError Exception.
2. If target is a Proxy exotic object and the value of the [[ProxyHandler]] internal slot of target is null, throw a TypeError exception.
3. If Type(handler) is not Object, throw a TypeError Exception.
4. If handler is a Proxy exotic object and the value of the [[ProxyHandler]] internal slot of handler is null, throw a TypeError exception.
5. Let P be a newly created object.
6. Set P’s essential internal methods (except for [[Call]] and [[Construct]]) to the definitions specified in 9.5.
7. If IsCallable(target) is true, then
   1. Set the [[Call]] internal method of P as specified in 9.5.13.
   2. If target has a [[Construct]] internal method, then
      1. Set the [[Construct]] internal method of P as specified in 9.5.14.
8. Set the [[ProxyTarget]] internal slot of P to target.
9. Set the [[ProxyHandler]] internal slot of P to handler.
10. Return P.

# 10 ECMAScript Language: Source Code

# 10.1 Source Text

## Syntax

SourceCharacter ::
any Unicode code point

ECMAScript code is expressed using Unicode, version 5.1 or later. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values. Each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

NOTE In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point’s numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

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

# 10.1.1 Static Semantics: UTF16Encoding ( cp )

The UTF16Encoding of a numeric code point value, cp, is determined as follows:

1. Assert: 0 ≤ cp ≤ 0x10FFFF.
2. If cp ≤ 65535, return cp.
3. Let cu1 be floor((cp – 65536) / 1024) + 0xD800.
4. Let cu2 be ((cp – 65536) modulo 1024) + 0xDC00.
5. Return the code unit sequence consisting of cu1 followed by cu2.

# 10.1.2 Static Semantics: UTF16Decode( lead, trail )

Two code units, lead and trail, that form a UTF-16 surrogate pair are converted to a code point by performing the following steps:

1. Assert: 0xD800 ≤ lead ≤ 0xDBFF and 0xDC00 ≤ trail ≤ 0xDFFF.
2. Let cp be (lead – 0xD800) × 1024 + (trail – 0xDC00) + 0x10000.
3. Return the code point cp.

# 10.2 Types of Source Code

There are four types of ECMAScript code:

- Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.

- Eval code is the source text supplied to the built-in `eval` function. More precisely, if the parameter to the built-in `eval` function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of `eval` is the global code portion of that Script.

- Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 9.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.

- Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.

NOTE Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3) and Generator Definitions (14.4). Function code is also derived from the arguments to the `Function` constructor (19.2.1.1) and the GeneratorFunction constructor (25.2.1.1).

# 10.2.1 Strict Mode Code

An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:

- Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).

- Module code is always strict mode code.

- All parts of a ClassDeclaration or a ClassExpression are strict mode code.

- Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see 12.3.4.1) that is contained in strict mode code.

- Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.

- Function code that is supplied as the arguments to the built-in `Function` and `Generator` constructors is strict mode code if the last argument is a String that when processed has a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

ECMAScript code that is not strict mode code is called non-strict code.

# 10.2.2 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of exotic function objects whose evaluative behaviour is expressed in some implementation defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.

# 11 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

NOTE The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:

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

where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

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

## Syntax

InputElementDiv ::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
RightBracePunctuator
InputElementRegExp ::
WhiteSpace
LineTerminator
Comment
CommonToken
RightBracePunctuator
RegularExpressionLiteral
InputElementRegExpOrTemplateTail ::
WhiteSpace
LineTerminator
Comment
CommonToken
RegularExpressionLiteral
TemplateSubstitutionTail
InputElementTemplateTail ::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
TemplateSubstitutionTail

# 11.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as left-to-right mark or right-to-left mark) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (Zero width non-joiner) and U+200D (Zero width joiner) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName (see 11.6.1) after the first character.

U+FEFF (Zero Width no-break space) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 11.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.

Table 31 — Format-Control Code Point Usage

Code Point | Name | Abbreviation | Usage
--- | --- | --- | ---
`U+200C` | Zero width non-joiner | <ZWNJ> | IdentifierPart
`U+200D` | Zero width joiner | <ZWJ> | IdentifierPart
`U+FEFF` | ZERO WIDTH NO-BREAK SPACE | <ZWNBSP> | WhiteSpace

# 11.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 32.

Table 32 — White Space Code Points

Code Point | Name | Abbreviation
--- | --- | ---
`U+0009` | Character Tabulation | <TAB>
`U+000B` | LINE TABULATION | <VT>
`U+000C` | Form Feed (ff) | <FF>
`U+0020` | Space | <SP>
`U+00A0` | No-break space | <NBSP>
`U+FEFF` | ZERO wIDTH nO-bREAK SPACE | <ZWNBSP>
Other category “Zs” | Any other Unicode “Separator, space” code point | <USP>

ECMAScript implementations must recognize as WhiteSpace code points listed in the “Separator, space” (Zs) category by Unicode 5.1. ECMAScript implementations may also recognize as WhiteSpace additional category Zs code points from subsequent editions of the Unicode Standard.

NOTE Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Zs”.

## Syntax

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<zwnbsp>
<USP>

# 11.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment (11.4) but cannot occur within a SingleLineComment.

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

The ECMAScript line terminator code points are listed in Table 33.

Table 33 — Line Terminator Code Points

Code Point | Unicode Name | Abbreviation
--- | --- | ---
`U+000A` | Line Feed (LF) | <LF>
`U+000D` | Carriage Return (CR) | <CR>
`U+2028` | Line separator | <LS>
`U+2029` | Paragraph separator | <PS>

Only the Unicode code points in Table 33 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

## Syntax

LineTerminator ::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence ::
<LF>
<CR> [lookahead ≠ <LF> ]
<LS>
<PS>
<CR> <LF>

# 11.4 Comments

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

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the `//` marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 11.9).

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

## Syntax

Comment ::
MultiLineComment
SingleLineComment
MultiLineComment ::
`/*` MultiLineCommentCharsopt `*/`
MultiLineCommentChars ::
MultiLineNotAsteriskChar MultiLineCommentCharsopt
`*` PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt
`*` PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::
SourceCharacter but not `*`
MultiLineNotForwardSlashOrAsteriskChar ::
SourceCharacter but not one of `/` or `*`
SingleLineComment ::
`//` SingleLineCommentCharsopt
SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::
SourceCharacter but not LineTerminator

# 11.5 Tokens

## Syntax

CommonToken ::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
Template

NOTE The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

# 11.6 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord (see 11.6.2). The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in version 5.1.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

NOTE This standard specifies specific code point additions: U+0024 (dollar sign) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (zero-width non-joiner) and U+200D (zero-width joiner) are permitted anywhere after the first code point of an IdentifierName.

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the HexDigits of the UnicodeEscapeSequence (see 11.8.4). The `\` preceding the UnicodeEscapeSequence and the `u` and `{ }` code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a `\` UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

## Syntax

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::
UnicodeIDStart
`$`
`_`
`\` UnicodeEscapeSequence
IdentifierPart ::
UnicodeIDContinue
`$`
`_`
`\` UnicodeEscapeSequence
<ZWNJ>
<ZWJ>
UnicodeIDStart ::
any Unicode code point with the Unicode property “ID_Start”
UnicodeIDContinue ::
any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 11.8.4.

NOTE The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

# 11.6.1 Identifier Names

# 11.6.1.1 Static Semantics: Early Errors

IdentifierStart :: `\` UnicodeEscapeSequence

- It is a Syntax Error if SV(UnicodeEscapeSequence) is none of `"$"`, or `"_"`, or the UTF16Encoding (10.1.1) of a code point that would be matched by the UnicodeIDStart lexical grammar production.

IdentifierPart :: `\` UnicodeEscapeSequence

- It is a Syntax Error if SV(UnicodeEscapeSequence) is none of `"$"`, or `"_"`, or the UTF16Encoding (10.1.1) of either <ZWNJ> or <ZWJ>, or the UTF16Encoding of a Unicode code point that would be matched by the UnicodeIDContinue lexical grammar production.

# 11.6.1.2 Static Semantics: StringValue

See also: 11.8.4.2, 12.1.4.

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart

1. Return the String value consisting of the sequence of code units corresponding to IdentifierName. In determining the sequence any occurrences of `\` UnicodeEscapeSequence are first replaced with the code point represented by the UnicodeEscapeSequence and then the code points of the entire IdentifierName are converted to code units by UTF16Encoding (10.1.1) each code point.

# 11.6.2 Reserved Words

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

## Syntax

ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

NOTE The ReservedWord definitions are specified as literal sequences of specific SourceCharacter elements. A code point in a ReservedWord cannot be expressed by a `\` UnicodeEscapeSequence.

# 11.6.2.1 Keywords

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

## Syntax

Keyword :: one of

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

NOTE In some contexts `yield` is given the semantics of an Identifier. See 12.1.1. In strict mode code, `let` and `static` are treated as reserved keywords through static semantic restrictions (see 12.1.1, 13.2.1.1, 13.6.4.1, and 14.5.1) rather than the lexical grammar.

# 11.6.2.2 Future Reserved Words

The following tokens are reserved for used as keywords in future language extensions.

## Syntax

FutureReservedWord ::

`enum await` |  |  |

`await` is only treated as a FutureReservedWord when Module is the goal symbol of the syntactic grammar.

NOTE Use of the following tokens within strict mode code (see 10.2.1) is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:

`implements` | `package` | `protected` |
`interface` | `private` | `public` |

# 11.7 Punctuators

## Syntax

Punctuator :: one of

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

DivPunctuator :: one of

`/` | `/=` |  |  |  |

RightBracePunctuator ::

`}` |  |  |  |  |

# 11.8 Literals

# 11.8.1 Null Literals

## Syntax

NullLiteral ::
`null`

# 11.8.2 Boolean Literals

## Syntax

BooleanLiteral ::
`true`
`false`

# 11.8.3 Numeric Literals

## Syntax

NumericLiteral ::
DecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral `.` DecimalDigitsopt ExponentPartopt
`.` DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::
`0`
NonZeroDigit DecimalDigitsopt
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit :: one of
`0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
NonZeroDigit :: one of
`1` `2` `3` `4` `5` `6` `7` `8` `9`
ExponentPart ::
ExponentIndicator SignedInteger
ExponentIndicator :: one of
`e` `E`
SignedInteger ::
DecimalDigits
`+` DecimalDigits
`-` DecimalDigits
BinaryIntegerLiteral ::
`0b` BinaryDigits
`0B` BinaryDigits
BinaryDigits ::
BinaryDigit
BinaryDigits BinaryDigit
BinaryDigit :: one of
`0` `1`
OctalIntegerLiteral ::
`0o` OctalDigits
`0O` OctalDigits
OctalDigits ::
OctalDigit
OctalDigits OctalDigit
OctalDigit :: one of
`0` `1` `2` `3` `4` `5` `6` `7`
HexIntegerLiteral ::
`0x` HexDigits
`0X` HexDigits
HexDigits ::
HexDigit
HexDigits HexDigit
HexDigit :: one of
`0` `1` `2` `3` `4` `5` `6` `7` `8` `9` `a` `b` `c` `d` `e` `f` `A` `B` `C` `D` `E` `F`

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

NOTE For example:

    3in

is an error and not the two input elements `3` and `in`.

A conforming implementation, when processing strict mode code (see 10.2.1), must not extend, as described in B.1.1, the syntax of NumericLiteral to include LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include NonOctalDecimalIntegerLiteral.

# 11.8.3.1 Static Semantics: MV’s

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

- The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.

- The MV of NumericLiteral :: BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.

- The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.

- The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.

- The MV of DecimalLiteral :: DecimalIntegerLiteral `.` is the MV of DecimalIntegerLiteral.

- The MV of DecimalLiteral :: DecimalIntegerLiteral `.` DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10–n), where n is the number of code points in DecimalDigits.

- The MV of DecimalLiteral :: DecimalIntegerLiteral `.` ExponentPart is the MV of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.

- The MV of DecimalLiteral :: DecimalIntegerLiteral `.` DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10–n)) × 10e, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.

- The MV of DecimalLiteral :: `.` DecimalDigits is the MV of DecimalDigits × 10–n, where n is the number of code points in DecimalDigits.

- The MV of DecimalLiteral :: `.` DecimalDigits ExponentPart is the MV of DecimalDigits × 10e–n, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.

- The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

- The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.

- The MV of DecimalIntegerLiteral :: `0` is 0.

- The MV of DecimalIntegerLiteral :: NonZeroDigit is the MV of NonZeroDigit.

- The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits, where n is the number of code points in DecimalDigits.

- The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.

- The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.

- The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.

- The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.

- The MV of SignedInteger :: `+` DecimalDigits is the MV of DecimalDigits.

- The MV of SignedInteger :: `-` DecimalDigits is the negative of the MV of DecimalDigits.

- The MV of DecimalDigit :: `0` or of HexDigit :: `0` or of OctalDigit :: `0` or of BinaryDigit :: `0` is 0.

- The MV of DecimalDigit :: `1` or of NonZeroDigit :: `1` or of HexDigit :: `1` or of OctalDigit :: `1` or
  of BinaryDigit :: `1` is 1.

- The MV of DecimalDigit :: `2` or of NonZeroDigit :: `2` or of HexDigit :: `2` or of OctalDigit :: `2` is 2.

- The MV of DecimalDigit :: `3` or of NonZeroDigit :: `3` or of HexDigit :: `3` or of OctalDigit :: `3` is 3.

- The MV of DecimalDigit :: `4` or of NonZeroDigit :: `4` or of HexDigit :: `4` or of OctalDigit :: `4` is 4.

- The MV of DecimalDigit :: `5` or of NonZeroDigit :: `5` or of HexDigit :: `5` or of OctalDigit :: `5` is 5.

- The MV of DecimalDigit :: `6` or of NonZeroDigit :: `6` or of HexDigit :: `6` or of OctalDigit :: `6` is 6.

- The MV of DecimalDigit :: `7` or of NonZeroDigit :: `7` or of HexDigit :: `7` or of OctalDigit :: `7` is 7.

- The MV of DecimalDigit :: `8` or of NonZeroDigit :: `8` or of HexDigit :: `8` is 8.

- The MV of DecimalDigit :: `9` or of NonZeroDigit :: `9` or of HexDigit :: `9` is 9.

- The MV of HexDigit :: `a` or of HexDigit :: `A` is 10.

- The MV of HexDigit :: `b` or of HexDigit :: `B` is 11.

- The MV of HexDigit :: `c` or of HexDigit :: `C` is 12.

- The MV of HexDigit :: `d` or of HexDigit :: `D` is 13.

- The MV of HexDigit :: `e` or of HexDigit :: `E` is 14.

- The MV of HexDigit :: `f` or of HexDigit :: `F` is 15.

- The MV of BinaryIntegerLiteral :: `0b` BinaryDigits is the MV of BinaryDigits.

- The MV of BinaryIntegerLiteral :: `0B` BinaryDigits is the MV of BinaryDigits.

- The MV of BinaryDigits :: BinaryDigit is the MV of BinaryDigit.

- The MV of BinaryDigits :: BinaryDigits BinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.

- The MV of OctalIntegerLiteral :: `0o` OctalDigits is the MV of OctalDigits.

- The MV of OctalIntegerLiteral :: `0O` OctalDigits is the MV of OctalDigits.

- The MV of OctalDigits :: OctalDigit is the MV of OctalDigit.

- The MV of OctalDigits :: OctalDigits OctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.

- The MV of HexIntegerLiteral :: `0x` HexDigits is the MV of HexDigits.

- The MV of HexIntegerLiteral :: `0X` HexDigits is the MV of HexDigits.

- The MV of HexDigits :: HexDigit is the MV of HexDigit.

- The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a `0` digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a `0` digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

- it is not `0`; or
- there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

# 11.8.4 String Literals

NOTE A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (carriage return), U+2028 (line separator), U+2029 (paragraph separator), and U+000A (line feed). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these string values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

## Syntax

StringLiteral ::
`"` DoubleStringCharactersopt `"`
`'` SingleStringCharactersopt `'`
DoubleStringCharacters ::
DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::
SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::
SourceCharacter but not one of `"` or `\` or LineTerminator
`\` EscapeSequence
LineContinuation
SingleStringCharacter ::
SourceCharacter but not one of `'` or `\` or LineTerminator
`\` EscapeSequence
LineContinuation
LineContinuation ::
`\` LineTerminatorSequence
EscapeSequence ::
CharacterEscapeSequence
`0` [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence

A conforming implementation, when processing strict mode code (see 10.2.1), must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence as described in B.1.2.

CharacterEscapeSequence ::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of
`'` `"` `\` `b` `f` `n` `r` `t` `v`
NonEscapeCharacter ::
SourceCharacter but not one of EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
DecimalDigit
`x`
`u`
HexEscapeSequence ::
`x` HexDigit HexDigit
UnicodeEscapeSequence ::
`u` Hex4Digits
`u{` HexDigits `}`
Hex4Digits ::
HexDigit HexDigit HexDigit HexDigit

The definition of the nonterminal HexDigit is given in 11.8.3. SourceCharacter is defined in 10.1.

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

# 11.8.4.1 Static Semantics: Early Errors

UnicodeEscapeSequence :: `u{` HexDigits `}`

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

# 11.8.4.2 Static Semantics: StringValue

See also: 11.6.1.2, 12.1.4.

StringLiteral ::
`"` DoubleStringCharactersopt `"`
`'` SingleStringCharactersopt `'`

1. Return the String value whose elements are the SV of this StringLiteral.

# 11.8.4.3 Static Semantics: SV’s

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.

- The SV of StringLiteral :: `""` is the empty code unit sequence.

- The SV of StringLiteral :: `''` is the empty code unit sequence.

- The SV of StringLiteral :: `"` DoubleStringCharacters `"` is the SV of DoubleStringCharacters.

- The SV of StringLiteral :: `'` SingleStringCharacters `'` is the SV of SingleStringCharacters.

- The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one or two code units that is the SV of DoubleStringCharacter.

- The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of one or two code units that is the SV of DoubleStringCharacter followed by all the code units in the SV of DoubleStringCharacters in order.

- The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one or two code units that is the SV of SingleStringCharacter.

- The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of one or two code units that is the SV of SingleStringCharacter followed by all the code units in the SV of SingleStringCharacters in order.

- The SV of DoubleStringCharacter :: SourceCharacter but not one of `"` or `\` or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.

- The SV of DoubleStringCharacter :: `\` EscapeSequence is the SV of the EscapeSequence.

- The SV of DoubleStringCharacter :: LineContinuation is the empty code unit sequence.

- The SV of SingleStringCharacter :: SourceCharacter but not one of `'` or `\` or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.

- The SV of SingleStringCharacter :: `\` EscapeSequence is the SV of the EscapeSequence.

- The SV of SingleStringCharacter :: LineContinuation is the empty code unit sequence.

- The SV of EscapeSequence :: CharacterEscapeSequence is the SV of the CharacterEscapeSequence.

- The SV of EscapeSequence :: `0` is the code unit value 0.

- The SV of EscapeSequence :: HexEscapeSequence is the SV of the HexEscapeSequence.

- The SV of EscapeSequence :: UnicodeEscapeSequence is the SV of the UnicodeEscapeSequence.

- The SV of CharacterEscapeSequence :: SingleEscapeCharacter is the code unit whose value is determined by the SingleEscapeCharacter according to { REF _Ref365803173 \h }Table 34.

Table 34 — String Single Character Escape Sequences

Escape Sequence | Code Unit Value | Unicode Character Name | Symbol
--- | --- | --- | ---
`\b` | `0x0008` | BACKSPACE | <BS>
`\t` | `0x0009` | CHARACTER TABULATION | <HT>
`\n` | `0x000A` | line feed (lf) | <LF>
`\v` | `0x000B` | LINE TABULATION | <VT>
`\f` | `0x000C` | form feed (ff) | <FF>
`\r` | `0x000D` | carriage return (cr) | <CR>
`\"` | `0x0022` | quotation Mark | `"`
`\'` | `0x0027` | apostrophe | `'`
`\\` | `0x005C` | REverse Solidus | `\`

- The SV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of the NonEscapeCharacter.

- The SV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.

- The SV of HexEscapeSequence :: `x` HexDigit HexDigit is the code unit value that is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

- The SV of UnicodeEscapeSequence :: `u` Hex4Digits is the SV of Hex4Digits.

- The SV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the code unit value that is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

- The SV of UnicodeEscapeSequence :: `u{` HexDigits `}` is the UTF16Encoding (10.1.1) of the MV of HexDigits.

# 11.8.5 Regular Expression Literals

NOTE A regular expression literal is an input element that is converted to a RegExp object (see 21.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as `===` to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by `new RegExp` or calling the `RegExp` constructor as a function (see 21.2.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (21.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

## Syntax

RegularExpressionLiteral ::
`/` RegularExpressionBody `/` RegularExpressionFlags
RegularExpressionBody ::
RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::
[empty]
RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::
RegularExpressionNonTerminator but not one of `*` or `\` or `/` or `[`
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar ::
RegularExpressionNonTerminator but not one of `\` or `/` or `[`
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence ::
`\` RegularExpressionNonTerminator
RegularExpressionNonTerminator ::
SourceCharacter but not LineTerminator
RegularExpressionClass ::
`[` RegularExpressionClassChars `]`
RegularExpressionClassChars ::
[empty]
RegularExpressionClassChars RegularExpressionClassChar
RegularExpressionClassChar ::
RegularExpressionNonTerminator but not one of `]` or `\`
RegularExpressionBackslashSequence
RegularExpressionFlags ::
[empty]
RegularExpressionFlags IdentifierPart

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

# 11.8.5.1 Static Semantics: Early Errors

RegularExpressionFlags :: RegularExpressionFlags IdentifierPart

- It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.

# 11.8.5.2 Static Semantics: BodyText

RegularExpressionLiteral :: `/` RegularExpressionBody `/` RegularExpressionFlags

1. Return the source text that was recognized as RegularExpressionBody.

# 11.8.5.3 Static Semantics: FlagText

RegularExpressionLiteral :: `/` RegularExpressionBody `/` RegularExpressionFlags

1. Return the source text that was recognized as RegularExpressionFlags.

# 11.8.6 Template Literal Lexical Components

## Syntax

Template ::
NoSubstitutionTemplate
TemplateHead
NoSubstitutionTemplate ::
``` TemplateCharactersopt ```
TemplateHead ::
``` TemplateCharactersopt `${`
TemplateSubstitutionTail ::
TemplateMiddle
TemplateTail
TemplateMiddle ::
`}` TemplateCharactersopt `${`
TemplateTail ::
`}` TemplateCharactersopt ```
TemplateCharacters ::
TemplateCharacter TemplateCharactersopt
TemplateCharacter ::
`$` [lookahead ≠ { ]
`\` EscapeSequence
LineContinuation
LineTerminatorSequence
SourceCharacter but not one of ``` or `\` or `$` or LineTerminator

A conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.

NOTE TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

# 11.8.6.1 Static Semantics: TV’s and TRV’s

A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.

- The TV and TRV of NoSubstitutionTemplate :: ```` is the empty code unit sequence.

- The TV and TRV of TemplateHead :: ``${` is the empty code unit sequence.

- The TV and TRV of TemplateMiddle :: `}${` is the empty code unit sequence.

- The TV and TRV of TemplateTail :: `}`` is the empty code unit sequence.

- The TV of NoSubstitutionTemplate :: ``` TemplateCharacters ``` is the TV of TemplateCharacters.

- The TV of TemplateHead :: ``` TemplateCharacters `${` is the TV of TemplateCharacters.

- The TV of TemplateMiddle :: `}` TemplateCharacters `${` is the TV of TemplateCharacters.

- The TV of TemplateTail :: `}` TemplateCharacters ``` is the TV of TemplateCharacters.

- The TV of TemplateCharacters :: TemplateCharacter is the TV of TemplateCharacter.

- The TV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TV of TemplateCharacter followed by all the code units in the TV of TemplateCharacters in order.

- The TV of TemplateCharacter :: SourceCharacter but not one of ``` or `\` or `$` or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.

- The TV of TemplateCharacter :: `$` is the code unit value 0x0024.

- The TV of TemplateCharacter :: `\` EscapeSequence is the SV of EscapeSequence.

- The TV of TemplateCharacter :: LineContinuation is the TV of LineContinuation.

- The TV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.

- The TV of LineContinuation :: `\` LineTerminatorSequence is the empty code unit sequence.

- The TRV of NoSubstitutionTemplate :: ``` TemplateCharacters ``` is the TRV of TemplateCharacters.

- The TRV of TemplateHead :: ``` TemplateCharacters `${` is the TRV of TemplateCharacters.

- The TRV of TemplateMiddle :: `}` TemplateCharacters `${` is the TRV of TemplateCharacters.

- The TRV of TemplateTail :: `}` TemplateCharacters ``` is the TRV of TemplateCharacters.

- The TRV of TemplateCharacters :: TemplateCharacter is the TRV of TemplateCharacter.

- The TRV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TRV of TemplateCharacter followed by all the code units in the TRV of TemplateCharacters, in order.

- The TRV of TemplateCharacter :: SourceCharacter but not one of ``` or `\` or `$` or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.

- The TRV of TemplateCharacter :: `$` is the code unit value 0x0024.

- The TRV of TemplateCharacter :: `\` EscapeSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of EscapeSequence.

- The TRV of TemplateCharacter :: LineContinuation is the TRV of LineContinuation.

- The TRV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.

- The TRV of EscapeSequence :: CharacterEscapeSequence is the TRV of the CharacterEscapeSequence.

- The TRV of EscapeSequence :: `0` is the code unit value 0x0030.

- The TRV of EscapeSequence :: HexEscapeSequence is the TRV of the HexEscapeSequence.

- The TRV of EscapeSequence :: UnicodeEscapeSequence is the TRV of the UnicodeEscapeSequence.

- The TRV of CharacterEscapeSequence :: SingleEscapeCharacter is the TRV of the SingleEscapeCharacter.

- The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of the NonEscapeCharacter.

- The TRV of SingleEscapeCharacter :: one of `'` `"` `\` `b` `f` `n` `r` `t` `v` is the SV of the SourceCharacter that is that single code point.

- The TRV of HexEscapeSequence :: `x` HexDigit HexDigit is the sequence consisting of code unit value 0x0078 followed by TRV of the first HexDigit followed by the TRV of the second HexDigit.

- The TRV of UnicodeEscapeSequence :: `u` Hex4Digits is the sequence consisting of code unit value 0x0075 followed by TRV of Hex4Digits.

- The TRV of UnicodeEscapeSequence :: `u{` HexDigits `}` is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit value 0x007D.

- The TRV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the sequence consisting of the TRV of the first HexDigit followed by the TRV of the second HexDigit followed by the TRV of the third HexDigit followed by the TRV of the fourth HexDigit.

- The TRV of HexDigits :: HexDigit is the TRV of HexDigit.

- The TRV of HexDigits :: HexDigits HexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.

- The TRV of a HexDigit is the SV of the SourceCharacter that is that HexDigit.

- The TRV of LineContinuation :: `\` LineTerminatorSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of LineTerminatorSequence.

- The TRV of LineTerminatorSequence :: <LF> is the code unit value 0x000A.

- The TRV of LineTerminatorSequence :: <CR> is the code unit value 0x000A.

- The TRV of LineTerminatorSequence :: <LS> is the code unit value 0x2028.

- The TRV of LineTerminatorSequence :: <PS> is the code unit value 0x2029.

- The TRV of LineTerminatorSequence :: <CR><LF> is the sequence consisting of the code unit value 0x000A.

NOTE TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.

# 11.9 Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, `let`, `const`, `import`, and `export` declarations, variable statement, expression statement, `debugger` 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.

# 11.9.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.

There are three basic rules of semicolon insertion:

1. When, as a Script or Module is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

   - The offending token is separated from the previous token by at least one LineTerminator.

   - The offending token is `}`.

   - The previous token is `)` and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (13.6.1).

2. When, as the Script or Module is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript Script or Module, then a semicolon is automatically inserted at the end of the input stream.
3. When, as the Script or Module is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a `for` statement (see 13.6.3).

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

PostfixExpression[Yield] :
LeftHandSideExpression[?Yield] [no LineTerminator here] `++`
LeftHandSideExpression[?Yield] [no LineTerminator here] `--`
ContinueStatement[Yield] :
`continue;`
`continue` [no LineTerminator here] LabelIdentifier[?Yield] `;`
BreakStatement[Yield] :
`break` `;`
`break` [no LineTerminator here] LabelIdentifier[?Yield] `;`
ReturnStatement[Yield] :
`return` [no LineTerminator here] Expression `;`
`return` [no LineTerminator here] Expression[In, ?Yield] `;`
ThrowStatement[Yield] :
`throw` [no LineTerminator here] Expression[In, ?Yield] `;`
ArrowFunction[In, Yield] :
ArrowParameters[?Yield] [no LineTerminator here] `=>` ConciseBody[?In]
YieldExpression[In] :
`yield` [no LineTerminator here] `*` AssignmentExpression[?In, Yield]
`yield` [no LineTerminator here] AssignmentExpression[?In, Yield]

The practical effect of these restricted productions is as follows:

When a `++` or `--` token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the `++` or `--` token, then a semicolon is automatically inserted before the `++` or `--` token.

When a `continue`, `break`, `return`, `throw`, or `yield` token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the `continue`, `break`, `return`, `throw`, or `yield` token.

The resulting practical advice to ECMAScript programmers is:

A postfix `++` or `--` operator should appear on the same line as its operand.

An Expression in a `return` or `throw` statement or an AssignmentExpression in a `yield` expression should start on the same line as the `return`, `throw`, or `yield` token.

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

# 11.9.2 Examples of Automatic Semicolon Insertion

The source

    { 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

    { 1
    2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

    { 1
    ;2 ;} 3;

which is a valid ECMAScript sentence.

The source

    for (a; b
    )

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a `for` statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a `for` statement.

The source

    return
    a + b

is transformed by automatic semicolon insertion into the following:

    return;
    a + b;

NOTE The expression `a + b` is not treated as a value to be returned by the `return` statement, because a LineTerminator separates it from the token `return`.

The source

    a = b
    ++c

is transformed by automatic semicolon insertion into the following:

    a = b;
    ++c;

NOTE The token `++` is not treated as a postfix operator applying to the variable `b`, because a LineTerminator occurs between `b` and `++`.

The source

    if (a > b)
    else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the `else` token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

    a = b + c
    (d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

    a = b + c(d + e).print()

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

# 12 ECMAScript Language: Expressions

# 12.1 Identifiers

Syntax

IdentifierReference[Yield] :
Identifier
[~Yield] `yield`

BindingIdentifier[Yield] :

Identifier
[~Yield] `yield`

LabelIdentifier[Yield] :
Identifier
[~Yield] `yield`
Identifier :
IdentifierName but not ReservedWord

# 12.1.1 Static Semantics: Early Errors

BindingIdentifier : Identifier

- It is a Syntax Error if the code matched by this production is contained in strict mode code and the StringValue of Identifier is `"arguments"` or `"eval"`.

IdentifierReference : `yield`

BindingIdentifier : `yield`

LabelIdentifier : `yield`

- It is a Syntax Error if the code matched by this production is contained in strict code .

IdentifierReference[Yield] : Identifier

BindingIdentifier[Yield] : Identifier

LabelIdentifier[Yield] : Identifier

- It is a Syntax Error if this production has a [Yield] parameter and StringValue of Identifier is `"yield"`.

Identifier : IdentifierName but not ReservedWord

- It is a Syntax Error if this phrase is contained in strict mode code and the StringValue of IdentifierName is: `"implements"`, `"interface"`, `"let"`, `"package"`, `"private"`, `"protected"`, `"public"`, `"static"`, or `"yield"`.

- It is a Syntax Error if StringValue of IdentifierName is the same string value as the StringValue of any ReservedWord except for `yield`.

NOTE StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

# 12.1.2 Static Semantics: BoundNames

See also: 13.2.1.2, 13.2.2.1, 13.2.3.1, 13.6.4.2, 14.1.3, 14.2.2, 14.4.2, 14.5.2, 15.2.2.2, 15.2.3.2.

BindingIdentifier : Identifier

1. Return a new List containing the StringValue of Identifier.

BindingIdentifier : `yield`

1. Return a new List containing `"yield"`.

# 12.1.3 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.2.0.4, 12.2.9.3, 12.3.1.5, 12.4.3, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

IdentifierReference : Identifier

1. If this IdentifierReference is contained in strict mode code and StringValue of Identifier is `"eval"` or `"arguments"`, return false.
2. Return true.

IdentifierReference : `yield`

1. Return true.

# 12.1.4 Static Semantics: StringValue

See also: 11.6.1.2, 11.8.4.2.

IdentifierReference : `yield`

BindingIdentifier : `yield`

LabelIdentifier : `yield`

1. Return `"yield"`.

Identifier : IdentifierName but not ReservedWord

1. Return the StringValue of IdentifierName.

# 12.1.5 Runtime Semantics: BindingInitialization

With arguments value and environment.

See also: 13.2.3.5, 13.6.4.9.

NOTE undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for `var` statements and formal parameter lists of some non-strict functions (See 9.2.12). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

BindingIdentifier : Identifier

1. Let name be StringValue of Identifier.
2. Return InitializeBoundName( name, value, environment).

BindingIdentifier : `yield`

1. Return InitializeBoundName(`"yield"`, value, environment).

# 12.1.5.1 Runtime Semantics: InitializeBoundName(name, value, environment)

1. Assert: Type(name) is String.
2. If environment is not undefined, then
   1. Let env be the EnvironmentRecord component of environment.
   2. Perform env.InitializeBinding(name, value).
   3. Return NormalCompletion(undefined).
3. Else
   1. Let lhs be ResolveBinding(name).
   2. Return PutValue(lhs, value).

# 12.1.6 Runtime Semantics: Evaluation

IdentifierReference : Identifier

1. Return ResolveBinding(StringValue of Identifier).

IdentifierReference : `yield`

1. Return ResolveBinding(`"yield"`).

NOTE 1: The result of evaluating an IdentifierReference is always a value of type Reference.

NOTE 2: In non-strict code, the keyword `yield` may be used as an identifier. Evaluating the IdentifierReference production resolves the binding of `yield` as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code. See 13.2.1 for the handling of `yield` in binding creation contexts.

# 12.2 Primary Expression

## Syntax

PrimaryExpression[Yield] :
`this`
IdentifierReference[?Yield]
Literal
ArrayLiteral[?Yield]
ObjectLiteral[?Yield]
FunctionExpression
ClassExpression
GeneratorExpression
RegularExpressionLiteral
TemplateLiteral[?Yield]
CoverParenthesizedExpressionAndArrowParameterList[?Yield]
CoverParenthesizedExpressionAndArrowParameterList[Yield] :
`(` Expression[In, ?Yield] `)`
`(` `)`
`(` `...` BindingIdentifier[?Yield] `)`
`(` Expression[In, ?Yield] `,` `...` BindingIdentifier[?Yield] `)`

## Supplemental Syntax

When processing the production

PrimaryExpression[Yield] : CoverParenthesizedExpressionAndArrowParameterList[?Yield]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression[Yield] :
`(` Expression[In, ?Yield] `)`

# 12.2.0 Semantics

# 12.2.0.1 Static Semantics: CoveredParenthesizedExpression

CoverParenthesizedExpressionAndArrowParameterList[Yield] : `(` Expression[In, ?Yield] `)`

1. Return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList[Yield] using either ParenthesizedExpression or ParenthesizedExpression[Yield] as the goal symbol depending upon whether the [Yield] grammar parameter was present when CoverParenthesizedExpressionAndArrowParameterList was matched.

# 12.2.0.2 Static Semantics: IsFunctionDefinition

See also: 12.2.9.2, 12.3.1.2, 12.4.2, 12.5.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8.

PrimaryExpression :
`this`
IdentifierReference
Literal
ArrayLiteral
ObjectLiteral
RegularExpressionLiteral
TemplateLiteral

1. Return false.

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

1. Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
2. Return IsFunctionDefinition of expr.

# 12.2.0.3 Static Semantics: IsIdentifierRef

See also: 12.3.1.4.

PrimaryExpression :
IdentifierReference

1. Return true.

PrimaryExpression :
`this`
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
RegularExpressionLiteral
TemplateLiteral
CoverParenthesizedExpressionAndArrowParameterList

1. Return false.

# 12.2.0.4 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.9.3, 12.3.1.5, 12.4.3, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

PrimaryExpression :
`this`
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
RegularExpressionLiteral
TemplateLiteral

1. Return false.

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

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

# 12.2.1 The `this` Keyword

# 12.2.1.1 Runtime Semantics: Evaluation

PrimaryExpression : `this`

1. Return ResolveThisBinding( ) .

# 12.2.2 Identifier Reference

See 12.1 for IdentifierReference.

# 12.2.3 Literals

## Syntax

Literal :
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral

# 12.2.3.1 Runtime Semantics: Evaluation

Literal : NullLiteral

1. Return null.

Literal : BooleanLiteral

1. Return false if BooleanLiteral is the token `false`.
2. Return true if BooleanLiteral is the token `true`.

Literal : NumericLiteral

1. Return the number whose value is MV of NumericLiteral as defined in 11.8.3.

Literal : StringLiteral

1. Return the StringValue of StringLiteral as defined in 11.8.4.2.

# 12.2.4 Array Initializer

NOTE An ArrayLiteral is an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

## Syntax

ArrayLiteral[Yield] :
`[` Elisionopt `]`
`[` ElementList[?Yield] `]`
`[` ElementList[?Yield] `,` Elisionopt `]`
ElementList[Yield] :
Elisionopt AssignmentExpression[In, ?Yield]
Elisionopt SpreadElement[?Yield]
ElementList[?Yield] `,` Elisionopt AssignmentExpression[In, ?Yield]
ElementList[?Yield] `,` Elisionopt SpreadElement[?Yield]
Elision :
`,`
Elision `,`
SpreadElement[Yield] :
`...` AssignmentExpression[In, ?Yield]

# 12.2.4.1 Static Semantics: ElisionWidth

Elision : `,`

1. Return the numeric value 1.

Elision : Elision `,`

1. Let preceding be the ElisionWidth of Elision.
2. Return preceding+1.

# 12.2.4.2 Runtime Semantics: ArrayAccumulation

With parameters array and nextIndex.

ElementList : Elisionopt AssignmentExpression

1. Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
2. Let initResult be the result of evaluating AssignmentExpression.
3. Let initValue be GetValue(initResult).
4. ReturnIfAbrupt(initValue).
5. Let created be CreateDataProperty(array, ToString(ToUint32(nextIndex+padding)), initValue).
6. Assert: created is true.
7. Return nextIndex+padding+1.

ElementList : Elisionopt SpreadElement

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

ElementList : ElementList `,` Elisionopt AssignmentExpression

1. Let postIndex be the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
2. ReturnIfAbrupt(postIndex).
3. Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
4. Let initResult be the result of evaluating AssignmentExpression.
5. Let initValue be GetValue(initResult).
6. ReturnIfAbrupt(initValue).
7. Let created be CreateDataProperty(array, ToString(ToUint32(postIndex+padding)), initValue).
8. Assert: created is true.
9. Return postIndex+padding+1.

ElementList : ElementList `,` Elisionopt SpreadElement

1. Let postIndex be the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex.
2. ReturnIfAbrupt(postIndex).
3. Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
4. Return the result of performing ArrayAccumulation for SpreadElement with arguments array and postIndex+padding.

SpreadElement : `...` AssignmentExpression

1. Let spreadRef be the result of evaluating AssignmentExpression.
2. Let spreadObj be GetValue(spreadRef).
3. Let iterator be GetIterator(spreadObj).
4. ReturnIfAbrupt(iterator).
5. Repeat
   1. Let next be IteratorStep(iterator).
   2. ReturnIfAbrupt(next).
   3. If next is false, return nextIndex.
   4. Let nextValue be IteratorValue(next).
   5. ReturnIfAbrupt(nextValue).
   6. Let status be CreateDataProperty(array, ToString(nextIndex), nextValue).
   7. Assert: status is true .
   8. Let nextIndex be nextIndex + 1.

NOTE CreateDataProperty is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].

# 12.2.4.3 Runtime Semantics: Evaluation

ArrayLiteral : `[` Elisionopt `]`

1. Let array be ArrayCreate(0).
2. Let pad be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
3. Perform Set(array, `"length"`, pad, false).
4. NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
5. Return array.

ArrayLiteral : `[` ElementList `]`

1. Let array be ArrayCreate(0).
2. Let len be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
3. ReturnIfAbrupt(len).
4. Perform Set(array, `"length"`, len, false).
5. NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
6. Return array.

ArrayLiteral : `[` ElementList `,` Elisionopt `]`

1. Let array be ArrayCreate(0).
2. Let len be the result of performing ArrayAccumulation for ElementList with arguments array and 0.
3. ReturnIfAbrupt(len).
4. Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
5. Perform Set(array, `"length"`, ToUint32(padding+len), false).
6. NOTE: The above Set cannot fail because of the nature of the object returned by ArrayCreate.
7. Return array.

# 12.2.5 Object Initializer

NOTE 1 An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

## Syntax

ObjectLiteral[Yield] :
`{` `}`
`{` PropertyDefinitionList[?Yield] `}`
`{` PropertyDefinitionList[?Yield] `,` `}`
PropertyDefinitionList[Yield] :
PropertyDefinition[?Yield]
PropertyDefinitionList[?Yield] `,` PropertyDefinition[?Yield]
PropertyDefinition[Yield] :
IdentifierReference[?Yield]
CoverInitializedName[?Yield]
PropertyName[?Yield] `:` AssignmentExpression[In, ?Yield]
MethodDefinition[?Yield]
PropertyName[Yield,GeneratorParameter] :
LiteralPropertyName
[+GeneratorParameter] ComputedPropertyName
[~GeneratorParameter] ComputedPropertyName[?Yield]
LiteralPropertyName :
IdentifierName
StringLiteral
NumericLiteral
ComputedPropertyName[Yield] :
`[` AssignmentExpression[In, ?Yield] `]`
CoverInitializedName[Yield] :
IdentifierReference[?Yield] Initializer[In, ?Yield]
Initializer[In, Yield] :
`=` AssignmentExpression[?In, ?Yield]

NOTE 2 MethodDefinition is defined in 14.3.

NOTE 3 In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.

# 12.2.5.1 Static Semantics: Early Errors

PropertyDefinition : MethodDefinition

- It is a Syntax Error if HasDirectSuper of MethodDefinition is true.

In addition to describing an actual object initializer the ObjectLiteral productions are also used as a cover grammar for ObjectAssignmentPattern (12.14.5). and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList.

PropertyDefinition : CoverInitializedName

- Always throw a Syntax Error if code matches this production.

NOTE This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern (12.14.5). It cannot occur in an actual object initializer.

# 12.2.5.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

See also: 14.3.2, 14.4.3, 14.5.5.

PropertyName : LiteralPropertyName

1. Return false.

PropertyName : ComputedPropertyName

1. Return the result of ComputedPropertyName Contains symbol.

# 12.2.5.3 Static Semantics: Contains

With parameter symbol.

See also: 5.3, 12.3.1.1, 14.1.4, 14.2.3, 14.4.4, 14.5.4.

PropertyDefinition : MethodDefinition

1. If symbol is MethodDefinition, return true.
2. Return the result of ComputedPropertyContains for MethodDefinition with argument symbol.

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

LiteralPropertyName : IdentifierName

1. If symbol is a ReservedWord, return false.
2. If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true;
3. Return false.

# 12.2.5.4 Static Semantics: HasComputedPropertyKey

See also: 14.3.4, 14.4.5

PropertyDefinitionList : PropertyDefinitionList `,` PropertyDefinition

1. If HasComputedPropertyKey of PropertyDefinitionList is true, return true.
2. Return HasComputedPropertyKey of PropertyDefinition.

PropertyDefinition : IdentifierReference

1. Return false.

PropertyDefinition : PropertyName `:` AssignmentExpression

1. Return IsComputedPropertyKey of PropertyName.

# 12.2.5.5 Static Semantics: IsComputedPropertyKey

PropertyName : LiteralPropertyName

1. Return false.

PropertyName : ComputedPropertyName

1. Return true.

# 12.2.5.6 Static Semantics: PropName

See also: 14.3.6, 14.4.10, 14.5.12

PropertyDefinition : IdentifierReference

1. Return StringValue of IdentifierReference.

PropertyDefinition : PropertyName `:` AssignmentExpression

1. Return PropName of PropertyName.

LiteralPropertyName : IdentifierName

1. Return StringValue of IdentifierName.

LiteralPropertyName : StringLiteral

1. Return a String value whose code units are the SV of the StringLiteral.

LiteralPropertyName : NumericLiteral

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

ComputedPropertyName : `[` AssignmentExpression `]`

1. Return empty.

# 12.2.5.7 Static Semantics: PropertyNameList

PropertyDefinitionList : PropertyDefinition

1. If PropName of PropertyDefinition is empty, return a new empty List.
2. Return a new List containing PropName of PropertyDefinition.

PropertyDefinitionList : PropertyDefinitionList `,` PropertyDefinition

1. Let list be PropertyNameList of PropertyDefinitionList.
2. If PropName of PropertyDefinition is empty, return list.
3. Append PropName of PropertyDefinition to the end of list.
4. Return list.

# 12.2.5.8 Runtime Semantics: Evaluation

ObjectLiteral : `{` `}`

1. Return ObjectCreate(%ObjectPrototype%).

ObjectLiteral :
`{` PropertyDefinitionList `}`
`{` PropertyDefinitionList `,` `}`

1. Let obj be ObjectCreate(%ObjectPrototype%).
2. Let status be the result of performing PropertyDefinitionEvaluation of PropertyDefinitionList with arguments obj and true.
3. ReturnIfAbrupt(status).
4. Return obj.

LiteralPropertyName : IdentifierName

1. Return StringValue of IdentifierName.

LiteralPropertyName : StringLiteral

1. Return a String value whose code units are the SV of the StringLiteral.

LiteralPropertyName : NumericLiteral

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

ComputedPropertyName : `[` AssignmentExpression `]`

1. Let exprValue be the result of evaluating AssignmentExpression.
2. Let propName be GetValue(exprValue).
3. ReturnIfAbrupt(propName).
4. Return ToPropertyKey(propName).

# 12.2.5.9 Runtime Semantics: PropertyDefinitionEvaluation

With parameter object and enumerable.

See also: 14.3.9, 14.4.13, B.3.1

PropertyDefinitionList : PropertyDefinitionList `,` PropertyDefinition

1. Let status be the result of performing PropertyDefinitionEvaluation of PropertyDefinitionList with arguments object and enumerable.
2. ReturnIfAbrupt(status).
3. Return the result of performing PropertyDefinitionEvaluation of PropertyDefinition with arguments object and enumerable.

PropertyDefinition : IdentifierReference

1. Let propName be StringValue of IdentifierReference.
2. Let exprValue be the result of evaluating IdentifierReference.
3. ReturnIfAbrupt(exprValue).
4. Let propValue be GetValue(exprValue).
5. ReturnIfAbrupt(propValue).
6. Assert: enumerable is true.
7. Return CreateDataPropertyOrThrow(object, propName, propValue).

PropertyDefinition : PropertyName `:` AssignmentExpression

1. Let propKey be the result of evaluating PropertyName.
2. ReturnIfAbrupt(propKey).
3. Let exprValueRef be the result of evaluating AssignmentExpression.
4. Let propValue be GetValue(exprValueRef).
5. ReturnIfAbrupt(propValue).
6. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
   1. Let hasNameProperty be HasOwnProperty(propValue, `"name"`).
   2. ReturnIfAbrupt(hasNameProperty).
   3. If hasNameProperty is false, perform SetFunctionName(propValue, propKey).
7. Assert: enumerable is true.
8. Return CreateDataPropertyOrThrow(object, propKey, propValue).

NOTE An alternative semantics for this production is given in B.3.1.

# 12.2.6 Function Defining Expressions

See 14.1 for PrimaryExpression : FunctionExpression .

See 14.4 for PrimaryExpression : GeneratorExpression .

See 14.5 for PrimaryExpression : ClassExpression .

# 12.2.7 Regular Expression Literals

## Syntax

See 11.8.4.

# 12.2.7.1 Static Semantics: Early Errors

PrimaryExpression : RegularExpressionLiteral

- It is a Syntax Error if BodyText of RegularExpressionLiteral cannot be recognized using the goal symbol Pattern of the ECMAScript RegExp grammar specified in 21.2.1 .

- It is a Syntax Error if FlagText of RegularExpressionLiteral contains any code points other than `"g"`, `"i"`, `"m"`, `"u"`, or `"y"`, or if it contains the same code point more than once.

# 12.2.7.2 Runtime Semantics: Evaluation

PrimaryExpression : RegularExpressionLiteral

1. Let pattern be the string value consisting of the UTF16Encoding of each code point of BodyText of RegularExpressionLiteral.
2. Let flags be the string value consisting of the UTF16Encoding of each code point of FlagText of RegularExpressionLiteral.
3. Return RegExpCreate(pattern, flags).

# 12.2.8 Template Literals

## Syntax

TemplateLiteral[Yield] :
NoSubstitutionTemplate
TemplateHead Expression[In, ?Yield] TemplateSpans[?Yield]
TemplateSpans[Yield] :
TemplateTail
TemplateMiddleList[?Yield] TemplateTail
TemplateMiddleList[Yield] :
TemplateMiddle Expression[In, ?Yield]
TemplateMiddleList[?Yield] TemplateMiddle Expression[In, ?Yield]

# 12.2.8.1 Static Semantics: TemplateStrings

With parameter raw.

TemplateLiteral : NoSubstitutionTemplate

1. If raw is false, then
   1. Let string be the TV of NoSubstitutionTemplate.
2. Else,
   1. Let string be the TRV of NoSubstitutionTemplate.
3. Return a List containing the single element, string.

TemplateLiteral : TemplateHead Expression TemplateSpans

1. If raw is false, then
   1. Let head be the TV of TemplateHead.
2. Else,
   1. Let head be the TRV of TemplateHead.
3. Let tail be TemplateStrings of TemplateSpans with argument raw.
4. Return a List containing head followed by the element, in order of tail.

TemplateSpans : TemplateTail

1. If raw is false, then
   1. Let tail be the TV of TemplateTail.
2. Else,
   1. Let tail be the TRV of TemplateTail.
3. Return a List containing the single element, tail.

TemplateSpans : TemplateMiddleList TemplateTail

1. Let middle be TemplateStrings of TemplateMiddleList with argument raw.
2. If raw is false, then
   1. Let tail be the TV of TemplateTail.
3. Else,
   1. Let tail be the TRV of TemplateTail.
4. Return a List containing the elements, in order, of middle followed by tail.

TemplateMiddleList : TemplateMiddle Expression

1. If raw is false, then
   1. Let string be the TV of TemplateMiddle.
2. Else,
   1. Let string be the TRV of TemplateMiddle.
3. Return a List containing the single element, string.

TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression

1. Let front be TemplateStrings of TemplateMiddleList with argument raw.
2. If raw is false, then
   1. Let last be the TV of TemplateMiddle.
3. Else,
   1. Let last be the TRV of TemplateMiddle.
4. Append last as the last element of the List front.
5. Return front.

# 12.2.8.2 Runtime Semantics: ArgumentListEvaluation

See also: 12.3.6.1

TemplateLiteral : NoSubstitutionTemplate

1. Let templateLiteral be this TemplateLiteral.
2. Let siteObj be GetTemplateObject(templateLiteral).
3. Return a List containing the one element which is siteObj.

TemplateLiteral : TemplateHead Expression TemplateSpans

1. Let templateLiteral be this TemplateLiteral.
2. Let siteObj be GetTemplateObject(templateLiteral).
3. Let firstSub be the result of evaluating Expression.
4. ReturnIfAbrupt(firstSub).
5. Let restSub be SubstitutionEvaluation of TemplateSpans.
6. ReturnIfAbrupt(restSub).
7. Assert: restSub is a List.
8. Return a List whose first element is siteObj, whose second elements is firstSub, and whose subsequent elements are the elements of restSub, in order. restSub may contain no elements.

# 12.2.8.3 Runtime Semantics: GetTemplateObject ( templateLiteral )

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

1. Let rawStrings be TemplateStrings of templateLiteral with argument true.
2. Let ctx be the running execution context.
3. Let realm be the ctx’s Realm.
4. Let templateRegistry be realm.[[templateMap]].
5. For each element e of templateRegistry, do
   1. If e.[[strings]] and rawStrings contain the same values in the same order, then
      1. Return e.[[array]].
6. Let cookedStrings be TemplateStrings of templateLiteral with argument false.
7. Let count be the number of elements in the List cookedStrings.
8. Let template be ArrayCreate(count).
9. Let rawObj be ArrayCreate(count).
10. Let index be 0.
11. Repeat while index < count
    1. Let prop be ToString(index).
    2. Let cookedValue be the string value cookedStrings[index].
    3. Call template.[[DefineOwnProperty]](prop, PropertyDescriptor{[[Value]]: cookedValue, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false}).
    4. Let rawValue be the string value rawStrings[index].
    5. Call rawObj.[[DefineOwnProperty]](prop, PropertyDescriptor{[[Value]]: rawValue, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false}).
    6. Let index be index+1.
12. Perform SetIntegrityLevel(rawObj, `"frozen"`).
13. Call template.[[DefineOwnProperty]](`"raw"`, PropertyDescriptor{[[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}).
14. Perform SetIntegrityLevel(template, `"frozen"`).
15. Append the Record{[[strings]]: rawStrings, [[array]]: template} to templateRegistry.
16. Return template.

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

NOTE 2 Each TemplateLiteral in the program code of a Realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.8.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

NOTE 3 Future editions of this specification may define additional non-enumerable properties of template objects.

# 12.2.8.4 Runtime Semantics: SubstitutionEvaluation

TemplateSpans : TemplateTail

1. Return an empty List.

TemplateSpans : TemplateMiddleList TemplateTail

1. Return the result of SubstitutionEvaluation of TemplateMiddleList.

TemplateMiddleList : TemplateMiddle Expression

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

TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression

1. Let preceding be the result of SubstitutionEvaluation of TemplateMiddleList .
2. ReturnIfAbrupt(preceding).
3. Let next be the result of evaluating Expression.
4. ReturnIfAbrupt(next).
5. Append next as the last element of the List preceding.
6. Return preceding.

# 12.2.8.5 Runtime Semantics: Evaluation

TemplateLiteral : NoSubstitutionTemplate

1. Return the string value whose code units are the elements of the TV of NoSubstitutionTemplate as defined in 11.8.6.

TemplateLiteral : TemplateHead Expression TemplateSpans

1. Let head be the TV of TemplateHead as defined in 11.8.6.
2. Let sub be the result of evaluating Expression.
3. Let middle be ToString(sub).
4. ReturnIfAbrupt(middle).
5. Let tail be the result of evaluating TemplateSpans .
6. ReturnIfAbrupt(tail).
7. Return the string value whose code units are the elements of head followed by the elements of middle followed by the elements of tail.

NOTE The string conversion semantics applied to the Expression value are like `String.prototype.concat` rather than the `+` operator.

TemplateSpans : TemplateTail

1. Let tail be the TV of TemplateTail as defined in 11.8.6.
2. Return the string consisting of the code units of tail.

TemplateSpans : TemplateMiddleList TemplateTail

1. Let head be the result of evaluating TemplateMiddleList.
2. ReturnIfAbrupt(head).
3. Let tail be the TV of TemplateTail as defined in 11.8.6.
4. Return the string whose code units are the elements of head followed by the elements of tail.

TemplateMiddleList : TemplateMiddle Expression

1. Let head be the TV of TemplateMiddle as defined in 11.8.6.
2. Let sub be the result of evaluating Expression.
3. Let middle be ToString(sub).
4. ReturnIfAbrupt(middle).
5. Return the sequence of code units consisting of the code units of head followed by the elements of middle.

NOTE The string conversion semantics applied to the Expression value are like `String.prototype.concat` rather than the `+` operator.

TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression

1. Let rest be the result of evaluating TemplateMiddleList .
2. ReturnIfAbrupt(rest).
3. Let middle be the TV of TemplateMiddle as defined in 11.8.6.
4. Let sub be the result of evaluating Expression.
5. Let last be ToString(sub).
6. ReturnIfAbrupt(last).
7. Return the sequence of code units consisting of the elements of rest followed by the code units of middle followed by the elements of last.

NOTE The string conversion semantics applied to the Expression value are like `String.prototype.concat` rather than the `+` operator.

# 12.2.9 The Grouping Operator

# 12.2.9.1 Static Semantics: Early Errors

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

- It is a Syntax Error if the lexical token sequence matched by CoverParenthesizedExpressionAndArrowParameterList cannot be parsed with no tokens left over using ParenthesizedExpression as the goal symbol.

- All Early Errors rules for ParenthesizedExpression and its derived productions also apply to CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.

# 12.2.9.2 Static Semantics: IsFunctionDefinition

See also: 12.2.0.2, 12.3.1.2, 12.4.2, 12.5.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8.

ParenthesizedExpression : `(` Expression `)`

1. Return IsFunctionDefinition of Expression.

# 12.2.9.3 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.0.4, 12.3.1.5, 12.4.3, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

ParenthesizedExpression : `(` Expression `)`

1. Return IsValidSimpleAssignmentTarget of Expression.

# 12.2.9.4 Runtime Semantics: Evaluation

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

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

ParenthesizedExpression : `(` Expression `)`

1. Return the result of evaluating Expression. This may be of type Reference.

NOTE This algorithm does not apply GetValue to the result of evaluating Expression. The principal motivation for this is so that operators such as `delete` and `typeof` may be applied to parenthesized expressions.

# 12.3 Left-Hand-Side Expressions

## Syntax

MemberExpression[Yield] :
PrimaryExpression[?Yield]
MemberExpression[?Yield] `[` Expression[In, ?Yield] `]`
MemberExpression[?Yield] `.` IdentifierName
MemberExpression[?Yield] TemplateLiteral[?Yield]
SuperProperty[?Yield]
MetaProperty
`new` MemberExpression[?Yield] Arguments[?Yield]
SuperProperty[Yield] :
`super` `[` Expression[In, ?Yield] `]`
`super` `.` IdentifierName
MetaProperty :
NewTarget
NewTarget :
`new` `.` `target`
NewExpression[Yield] :
MemberExpression[?Yield]
`new` NewExpression[?Yield]
CallExpression[Yield] :
MemberExpression[?Yield] Arguments[?Yield]
SuperCall[?Yield]
CallExpression[?Yield] Arguments[?Yield]
CallExpression[?Yield] `[` Expression[In, ?Yield] `]`
CallExpression[?Yield] `.` IdentifierName
CallExpression[?Yield] TemplateLiteral[?Yield]
SuperCall[Yield] :
`super` Arguments[?Yield]
Arguments[Yield] :
`(` `)`
`(` ArgumentList[?Yield] `)`
ArgumentList[Yield] :
AssignmentExpression[In, ?Yield]
`...` AssignmentExpression[In, ?Yield]
ArgumentList[?Yield] `,` AssignmentExpression[In, ?Yield]
ArgumentList[?Yield] `,` `...` AssignmentExpression[In, ?Yield]
LeftHandSideExpression[Yield] :
NewExpression[?Yield]
CallExpression[?Yield]

# 12.3.1 Static Semantics

# 12.3.1.1 Static Semantics: Contains

With parameter symbol.

See also: 5.3, 12.2.5.3, 14.1.4, 14.2.3, 14.4.4, 14.5.4

MemberExpression : MemberExpression `.` IdentifierName

1. If MemberExpression Contains symbol is true, return true.
2. If symbol is a ReservedWord, return false.
3. If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true;
4. Return false.

SuperProperty : `super` `.` IdentifierName

1. If symbol is the ReservedWord `super`, return true.
2. If symbol is a ReservedWord, return false.
3. If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true;
4. Return false.

CallExpression : CallExpression `.` IdentifierName

1. If CallExpression Contains symbol is true, return true.
2. If symbol is a ReservedWord, return false.
3. If symbol is an Identifier and StringValue of symbol is the same value as the StringValue of IdentifierName, return true;
4. Return false.

# 12.3.1.2 Static Semantics: IsFunctionDefinition

See also: 12.2.0.2, 12.2.9.2, 12.4.2, 12.5.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8.

MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
NewExpression :
`new` NewExpression
CallExpression :
MemberExpression Arguments
SuperCall
CallExpression Arguments
CallExpression `[` Expression `]`
CallExpression `.` IdentifierName
CallExpression TemplateLiteral

1. Return false.

# 12.3.1.3 Static Semantics: IsDestructuring

See also: 13.6.4.6.

MemberExpression : PrimaryExpression

1. If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral, return true.
2. Return false.

MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
NewExpression :
`new` NewExpression
CallExpression :
MemberExpression Arguments
SuperCall
CallExpression Arguments
CallExpression `[` Expression `]`
CallExpression `.` IdentifierName
CallExpression TemplateLiteral

1. Return false.

# 12.3.1.4 Static Semantics: IsIdentifierRef

See also: 12.2.0.3.

LeftHandSideExpression :
CallExpression
MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
NewExpression :
`new` NewExpression

1. Return false.

# 12.3.1.5 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.0.4, 12.2.9.3, 12.4.3, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

CallExpression :
CallExpression `[` Expression `]`
CallExpression `.` IdentifierName
MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
SuperProperty

1. Return true.

CallExpression :
MemberExpression Arguments
SuperCall
CallExpression Arguments
CallExpression TemplateLiteral
NewExpression :
`new` NewExpression
MemberExpression :
MemberExpression TemplateLiteral
`new` MemberExpression Arguments
NewTarget :
`new` `.` `target`

1. Return false.

# 12.3.2 Property Accessors

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

MemberExpression `.` IdentifierName
CallExpression `.` IdentifierName

or the bracket notation:

MemberExpression `[` Expression `]`
CallExpression `[` Expression `]`

The dot notation is explained by the following syntactic conversion:

MemberExpression `.` IdentifierName

is identical in its behaviour to

MemberExpression `[` <identifier-name-string> `]`

and similarly

CallExpression `.` IdentifierName

is identical in its behaviour to

CallExpression `[` <identifier-name-string> `]`

where <identifier-name-string> is the result of evaluating StringValue of IdentifierName.

# 12.3.2.1 Runtime Semantics: Evaluation

MemberExpression : MemberExpression `[` Expression `]`

1. Let baseReference be the result of evaluating MemberExpression.
2. Let baseValue be GetValue(baseReference).
3. ReturnIfAbrupt(baseValue).
4. Let propertyNameReference be the result of evaluating Expression.
5. Let propertyNameValue be GetValue(propertyNameReference).
6. ReturnIfAbrupt(propertyNameValue).
7. Let bv be RequireObjectCoercible(baseValue).
8. ReturnIfAbrupt(bv).
9. Let propertyKey be ToPropertyKey(propertyNameValue).
10. ReturnIfAbrupt(propertyKey).
11. If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
12. Return a value of type Reference whose base value is bv and whose referenced name is propertyKey, and whose strict reference flag is strict.

MemberExpression : MemberExpression `.` IdentifierName

1. Let baseReference be the result of evaluating MemberExpression.
2. Let baseValue be GetValue(baseReference).
3. ReturnIfAbrupt(baseValue).
4. Let bv be RequireObjectCoercible(baseValue).
5. ReturnIfAbrupt(bv).
6. Let propertyNameString be StringValue of IdentifierName
7. If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
8. Return a value of type Reference whose base value is bv and whose referenced name is propertyNameString, and whose strict reference flag is strict.

CallExpression : CallExpression `[` Expression `]`

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

CallExpression : CallExpression `.` IdentifierName

Is evaluated in exactly the same manner as MemberExpression : MemberExpression `.` IdentifierName except that the contained CallExpression is evaluated in step 1.

# 12.3.3 The `new` Operator

# 12.3.3.1 Runtime Semantics: Evaluation

NewExpression : `new` NewExpression

1. Return EvaluateNew(NewExpression, empty).

MemberExpression : `new` MemberExpression Arguments

1. Return EvaluateNew(MemberExpression, Arguments).

# 12.3.3.1.1 Runtime Semantics: EvaluateNew(constructProduction, arguments)

The abstract operation EvaluateNew with arguments constructProduction, and arguments performs the following steps:

1. Assert: constructProduction is either a NewExpression or a MemberExpression.
2. Assert: arguments is either empty or an Arguments production.
3. Let ref be the result of evaluating constructProduction.
4. Let constructor be GetValue(ref).
5. ReturnIfAbrupt(constructor).
6. If arguments is empty, let argList be an empty List.
7. Else,
   1. Let argList be ArgumentListEvaluation of arguments.
   2. ReturnIfAbrupt(argList).
8. If IsConstructor (constructor) is false, throw a TypeError exception.
9. Return Construct(constructor, argList).

# 12.3.4 Function Calls

# 12.3.4.1 Runtime Semantics: Evaluation

CallExpression : MemberExpression Arguments

1. Let ref be the result of evaluating MemberExpression.
2. Let func be GetValue(ref).
3. ReturnIfAbrupt(func).
4. If Type(ref) is Reference and IsPropertyReference(ref) is false and GetReferencedName(ref) is `"eval"`, then
   1. If SameValue(func, %eval%) is true, then
      1. Let argList be ArgumentListEvaluation(Arguments).
      2. ReturnIfAbrupt(argList).
      3. If argList has no elements, return undefined.
      4. Let evalText be the first element of argList.
      5. If the source code matching this CallExpression is strict code, let strictCaller be true. Otherwise let strictCaller be false.
      6. Let evalRealm be the running execution context’s Realm.
      7. Return PerformEval(evalText, evalRealm, strictCaller, true). .
5. If Type(ref) is Reference, then
   1. If IsPropertyReference(ref) is true, then
      1. Let thisValue be GetThisValue(ref).
   2. Else, the base of ref is an Environment Record
      1. Let refEnv be GetBase(ref).
      2. Let thisValue be refEnv.WithBaseObject().
6. Else Type(ref) is not Reference,
   1. Let thisValue be undefined.
7. Let thisCall be this CallExpression.
8. Let tailCall be IsInTailPosition(thisCall). (See 14.6.1)
9. Return EvaluateDirectCall(func, thisValue, Arguments, tailCall).

A CallExpression whose evaluation executes step 4.a.vii is a direct eval.

CallExpression : CallExpression Arguments

1. Let ref be the result of evaluating CallExpression.
2. Let thisCall be this CallExpression
3. Let tailCall be IsInTailPosition(thisCall). (See 14.6.1)
4. Return EvaluateCall(ref, Arguments, tailCall).

# 12.3.4.2 Runtime Semantics: EvaluateCall( ref, arguments, tailPosition )

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

1. Let func be GetValue(ref).
2. ReturnIfAbrupt(func).
3. If Type(ref) is Reference, then
   1. If IsPropertyReference(ref) is true, then
      1. Let thisValue be GetThisValue(ref).
   2. Else, the base of ref is an Environment Record
      1. Let refEnv be GetBase(ref).
      2. Let thisValue be refEnv.WithBaseObject().
4. Else Type(ref) is not Reference,
   1. Let thisValue be undefined.
5. Return EvaluateDirectCall(func, thisValue, arguments, tailPosition).

# 12.3.4.3 Runtime Semantics: EvaluateDirectCall( func, thisValue, arguments, tailPosition )

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

1. Let argList be ArgumentListEvaluation(arguments).
2. ReturnIfAbrupt(argList).
3. If Type(func) is not Object, throw a TypeError exception.
4. If IsCallable(func) is false, throw a TypeError exception.
5. If tailPosition is true, perform PrepareForTailCall().
6. Let result be Call(func, thisValue, argList).
7. Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
8. Assert: If result is not an abrupt completion then Type(result) is an ECMAScript language type.
9. Return result.

# 12.3.5 The `super` Keyword

# 12.3.5.1 Runtime Semantics: Evaluation

SuperProperty : `super` `[` Expression `]`

1. Let propertyNameReference be the result of evaluating Expression.
2. Let propertyNameValue be GetValue(propertyNameReference).
3. Let propertyKey be ToPropertyKey(propertyNameValue).
4. ReturnIfAbrupt(propertyKey).
5. If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.
6. Return MakeSuperPropertyReference(propertyKey, strict).

SuperProperty : `super` `.` IdentifierName

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

SuperCall : `super` Arguments

1. Let newTarget be GetNewTarget().
2. If newTarget is undefined, throw a ReferenceError exception.
3. Let func be GetSuperConstructor().
4. ReturnIfAbrupt(func).
5. Let argList be ArgumentListEvaluation of Arguments.
6. ReturnIfAbrupt(argList).
7. Let result be Construct(func, argList, newTarget).
8. ReturnIfAbrupt(result).
9. Let thisER be GetThisEnvironment( ).
10. Return thisER.BindThisValue(result).

# 12.3.5.2 Runtime Semantics: GetSuperConstructor ( )

The abstract operation GetSuperConstructor performs the following steps:

1. Let envRec be GetThisEnvironment( ).
2. Assert: envRec is a function Environment Record.
3. Let activeFunction be envRec.[[FunctionObject]].
4. Let superConstructor be activeFunction.[[GetPrototypeOf]]().
5. ReturnIfAbrupt(superConstructor).
6. If IsConstructor(superConstructor) is false, throw a TypeError exception.
7. Return superConstructor.

# 12.3.5.3 Runtime Semantics: MakeSuperPropertyReference(propertyKey, strict)

The abstract operation MakeSuperPropertyReference with arguments propertyKey and strict performs the following steps:

1. Let env be GetThisEnvironment( ).
2. If env.HasSuperBinding() is false, throw a ReferenceError exception.
3. Let actualThis be env.GetThisBinding().
4. ReturnIfAbrupt(actualThis).
5. Let baseValue be env.GetSuperBase().
6. Let bv be RequireObjectCoercible(baseValue).
7. ReturnIfAbrupt(bv).
8. Return a value of type Reference that is a Super Reference whose base value is bv, whose referenced name is propertyKey, whose thisValue is actualThis, and whose strict reference flag is strict.

# 12.3.6 Argument Lists

NOTE The evaluation of an argument list produces a List of values (see 6.2.1).

# 12.3.6.1 Runtime Semantics: ArgumentListEvaluation

See also: 12.2.8.2

Arguments : `(` `)`

1. Return an empty List.

ArgumentList : AssignmentExpression

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

ArgumentList : `...` AssignmentExpression

1. Let list be an empty List.
2. Let spreadRef be the result of evaluating AssignmentExpression.
3. Let spreadObj be GetValue(spreadRef).
4. Let iterator be GetIterator(spreadObj).
5. ReturnIfAbrupt(iterator).
6. Repeat
   1. Let next be IteratorStep(iterator).
   2. ReturnIfAbrupt(next).
   3. If next is false, return list.
   4. Let nextArg be IteratorValue(next).
   5. ReturnIfAbrupt(nextArg).
   6. Append nextArg as the last element of list.

ArgumentList : ArgumentList `,` AssignmentExpression

1. Let precedingArgs be the result of evaluating ArgumentList.
2. ReturnIfAbrupt(precedingArgs).
3. Let ref be the result of evaluating AssignmentExpression.
4. Let arg be GetValue(ref).
5. ReturnIfAbrupt(arg).
6. Append arg to the end of precedingArgs.
7. Return precedingArgs.

ArgumentList : ArgumentList `,` `...` AssignmentExpression

1. Let precedingArgs be the result of evaluating ArgumentList.
2. Let spreadRef be the result of evaluating AssignmentExpression.
3. Let iterator be GetIterator(GetValue(spreadRef) ).
4. ReturnIfAbrupt(iterator).
5. Repeat
   1. Let next be IteratorStep(iterator).
   2. ReturnIfAbrupt(next).
   3. If next is false, return precedingArgs.
   4. Let nextArg be IteratorValue(next).
   5. ReturnIfAbrupt(nextArg).
   6. Append nextArg as the last element of precedingArgs.

# 12.3.7 Tagged Templates

NOTE A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (12.2.8). The actual arguments include a template object (12.2.8.3) and the values produced by evaluating the expressions embedded within the TemplateLiteral.

# 12.3.7.1 Runtime Semantics: Evaluation

MemberExpression : MemberExpression TemplateLiteral

1. Let tagRef be the result of evaluating MemberExpression.
2. Let thisCall be this MemberExpression.
3. Let tailCall be IsInTailPosition(thisCall). (See 14.6.1)
4. Return EvaluateCall(tagRef, TemplateLiteral, tailCall).

CallExpression : CallExpression TemplateLiteral

1. Let tagRef be the result of evaluating CallExpression.
2. Let thisCall be this CallExpression.
3. Let tailCall be IsInTailPosition(thisCall). (See 14.6.1)
4. Return EvaluateCall(tagRef, TemplateLiteral, tailCall).

# 12.3.8 Meta Properties

# 12.3.8.1 Runtime Semantics: Evaluation

NewTarget : `new` `.` `target`

1. Return GetNewTarget().

# 12.4 Postfix Expressions

## Syntax

PostfixExpression[Yield] :
LeftHandSideExpression[?Yield]
LeftHandSideExpression[?Yield] [no LineTerminator here] `++`
LeftHandSideExpression[?Yield] [no LineTerminator here] `--`

# 12.4.1 Static Semantics: Early Errors

PostfixExpression :
LeftHandSideExpression `++`
LeftHandSideExpression `--`

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

# 12.4.2 Static Semantics: IsFunctionDefinition

See also: 12.2.0.2, 12.2.9.2, 12.3.1.2, 12.5.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8

PostfixExpression :
LeftHandSideExpression `++`
LeftHandSideExpression `--`

1. Return false.

# 12.4.3 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.0.4, 12.2.9.3, 12.3.1.5, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

PostfixExpression :
LeftHandSideExpression `++`
LeftHandSideExpression `--`

1. Return false.

# 12.4.4 Postfix Increment Operator

# 12.4.4.1 Runtime Semantics: Evaluation

PostfixExpression : LeftHandSideExpression `++`

1. Let lhs be the result of evaluating LeftHandSideExpression.
2. Let oldValue be ToNumber(GetValue(lhs)).
3. ReturnIfAbrupt(oldValue).
4. Let newValue be the result of adding the value `1` to oldValue, using the same rules as for the `+` operator (see 12.7.5).
5. Let status be PutValue(lhs, newValue).
6. ReturnIfAbrupt(status).
7. Return oldValue.

# 12.4.5 Postfix Decrement Operator

# 12.4.5.1 Runtime Semantics: Evaluation

PostfixExpression : LeftHandSideExpression `--`

1. Let lhs be the result of evaluating LeftHandSideExpression.
2. Let oldValue be ToNumber(GetValue(lhs)).
3. ReturnIfAbrupt(oldValue).
4. Let newValue be the result of subtracting the value `1` from oldValue, using the same rules as for the `-` operator (12.7.5).
5. Let status be PutValue(lhs, newValue).
6. ReturnIfAbrupt(status).
7. Return oldValue.

# 12.5 Unary Operators

## Syntax

UnaryExpression[Yield] :
PostfixExpression[?Yield]
`delete` UnaryExpression[?Yield]
`void` UnaryExpression[?Yield]
`typeof` UnaryExpression[?Yield]
`++` UnaryExpression[?Yield]
`--` UnaryExpression[?Yield]
`+` UnaryExpression[?Yield]
`-` UnaryExpression[?Yield]
`~` UnaryExpression[?Yield]
`!` UnaryExpression[?Yield]

# 12.5.1 Static Semantics: Early Errors

UnaryExpression :
`++` UnaryExpression
`--` UnaryExpression

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

# 12.5.2 Static Semantics: IsFunctionDefinition

See also: 12.2.0.2, 12.2.9.2, 12.3.1.2, 12.4.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8.

UnaryExpression :
`delete` UnaryExpression
`void` UnaryExpression
`typeof` UnaryExpression
`++` UnaryExpression
`--` UnaryExpression
`+` UnaryExpression
`-` UnaryExpression
`~` UnaryExpression
`!` UnaryExpression

1. Return false.

# 12.5.3 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.0.4, 12.2.9.3, 12.3.1.5, 12.4.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

UnaryExpression :
`delete` UnaryExpression
`void` UnaryExpression
`typeof` UnaryExpression
`++` UnaryExpression
`--` UnaryExpression
`+` UnaryExpression
`-` UnaryExpression
`~` UnaryExpression
`!` UnaryExpression

1. Return false.

# 12.5.4 The `delete` Operator

# 12.5.4.1 Static Semantics: Early Errors

UnaryExpression : `delete` UnaryExpression

- It is a Syntax Error if the UnaryExpression is contained in strict mode code and the derived UnaryExpression is PrimaryExpression : IdentifierReference.

- It is a Syntax Error if the derived UnaryExpression is
  PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  and CoverParenthesizedExpressionAndArrowParameterList ultimately derives a phrase that, if used in place of UnaryExpression, would produce a Syntax Error according to these rules. This rule is recursively applied.

NOTE The last rule means that expressions such as
`delete (((foo)))`
produce early errors because of recursive application of the first rule.

# 12.5.4.2 Runtime Semantics: Evaluation

UnaryExpression : `delete` UnaryExpression

1. Let ref be the result of evaluating UnaryExpression.
2. ReturnIfAbrupt(ref).
3. If Type(ref) is not Reference, return true.
4. If IsUnresolvableReference(ref) is true, then
   1. Assert: IsStrictReference(ref) is false.
   2. Return true.
5. If IsPropertyReference(ref) is true, then
   1. If IsSuperReference(ref), throw a ReferenceError exception.
   2. Let baseObj be ToObject(GetBase(ref)).
   3. Let deleteStatus be baseObj.[[Delete]](GetReferencedName(ref)).
   4. ReturnIfAbrupt(deleteStatus).
   5. If deleteStatus is false and IsStrictReference(ref) is true, throw a TypeError exception.
   6. Return deleteStatus.
6. Else ref is a Reference to an Environment Record binding,
   1. Let bindings be GetBase(ref).
   2. Return bindings.DeleteBinding(GetReferencedName(ref)).

NOTE When a `delete` operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a `delete` operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.

# 12.5.5 The `void` Operator

# 12.5.5.1 Runtime Semantics: Evaluation

UnaryExpression : `void` UnaryExpression

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

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

# 12.5.6 The `typeof` Operator

# 12.5.6.1 Runtime Semantics: Evaluation

UnaryExpression : `typeof` UnaryExpression

1. Let val be the result of evaluating UnaryExpression.
2. If Type(val) is Reference, then
   1. If IsUnresolvableReference(val) is true, return `"undefined"`.
3. Let val be GetValue(val).
4. ReturnIfAbrupt(val).
5. Return a String according to Table 35.

Table 35 — typeof Operator Results

Type of val | Result
--- | ---
Undefined | `"undefined"`
Null | `"object"`
Boolean | `"boolean"`
Number | `"number"`
String | `"string"`
Symbol | `"symbol"`
Object (ordinary and does not implement [[Call]]) | `"object"`
Object (standard exotic and does not implement [[Call]]) | `"object"`
Object (implements [[Call]]) | `"function"`
Object (non-standard exotic and does not implement [[Call]]) | Implementation-defined. Must not be `"undefined"`, `"boolean"`, `"function"`, `"number"`, `"symbol"`, or `"string".`

NOTE Implementations are discouraged from defining new `typeof` result values for non-standard exotic objects. If possible `"object"`should be used for such objects.

# 12.5.7 Prefix Increment Operator

# 12.5.7.1 Runtime Semantics: Evaluation

UnaryExpression : `++` UnaryExpression

1. Let expr be the result of evaluating UnaryExpression.
2. Let oldValue be ToNumber(GetValue(expr)).
3. ReturnIfAbrupt(oldValue).
4. Let newValue be the result of adding the value `1` to oldValue, using the same rules as for the `+` operator (see 12.7.5).
5. Let status be PutValue(expr, newValue).
6. ReturnIfAbrupt(status).
7. Return newValue.

# 12.5.8 Prefix Decrement Operator

# 12.5.8.1 Runtime Semantics: Evaluation

UnaryExpression : `--` UnaryExpression

1. Let expr be the result of evaluating UnaryExpression.
2. Let oldValue be ToNumber(GetValue(expr)).
3. ReturnIfAbrupt(oldValue).
4. Let newValue be the result of subtracting the value `1` from oldValue, using the same rules as for the `-` operator (see 12.7.5).
5. Let status be PutValue(expr, newValue).
6. ReturnIfAbrupt(status).
7. Return newValue.

# 12.5.9 Unary `+` Operator

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

# 12.5.9.1 Runtime Semantics: Evaluation

UnaryExpression : `+` UnaryExpression

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

# 12.5.10 Unary `-` Operator

NOTE The unary `-` operator converts its operand to Number type and then negates it. Negating +0 produces −0, and negating −0 produces +0.

# 12.5.10.1 Runtime Semantics: Evaluation

UnaryExpression : `-` UnaryExpression

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

# 12.5.11 Bitwise NOT Operator ( `~` )

# 12.5.11.1 Runtime Semantics: Evaluation

UnaryExpression : `~` UnaryExpression

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

# 12.5.12 Logical NOT Operator ( `!` )

# 12.5.12.1 Runtime Semantics: Evaluation

UnaryExpression : `!` UnaryExpression

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

# 12.6 Multiplicative Operators

## Syntax

MultiplicativeExpression[Yield] :
UnaryExpression[?Yield]
MultiplicativeExpression[?Yield] MultiplicativeOperator UnaryExpression[?Yield]
MultiplicativeOperator : one of
`*` `/` `%`

# 12.6.1 Static Semantics: IsFunctionDefinition

See also: 12.2.0.2, 12.2.9.2, 12.3.1.2, 12.4.2, 12.5.2, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.9, 14.5.8.

MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator UnaryExpression

1. Return false.

# 12.6.2 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.1.3, 12.2.0.4, 12.2.9.3, 12.3.1.5, 12.4.3, 12.5.3, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator UnaryExpression

1. Return false.

# 12.6.3 Runtime Semantics: Evaluation

MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator UnaryExpression

1. Let left be the result of evaluating MultiplicativeExpression.
2. Let leftValue be GetValue(left).
3. ReturnIfAbrupt(leftValue).
4. Let right be the result of evaluating UnaryExpression.
5. Let rightValue be GetValue(right).
6. Let lnum be ToNumber(leftValue).
7. ReturnIfAbrupt(lnum).
8. Let rnum be ToNumber(rightValue).
9. ReturnIfAbrupt(rnum).
10. Return the result of applying the MultiplicativeOperator (*, /, or %) to lnum and rnum as specified in 12.6.3.1, 12.6.3.2, or 12.6.3.3.

# 12.6.3.1 Applying the `*` Operator

The `*` MultiplicativeOperator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision arithmetic:

- If either operand is NaN, the result is NaN.

- The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

- Multiplication of an infinity by a zero results in NaN.

- Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.

- Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.

- In the remaining cases, where neither an infinity nor NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

# 12.6.3.2 Applying the `/` Operator

The `/` MultiplicativeOperator performs division, producing the quotient of its operands. The left operand is the dividend and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754 arithmetic:

- If either operand is NaN, the result is NaN.

- The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

- Division of an infinity by an infinity results in NaN.

- Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.

- Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.

- Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.

- Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.

- Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.

- In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

# 12.6.3.3 Applying the `%` Operator

The `%` MultiplicativeOperator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.

NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the `%` operator is not the same as the “remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines `%` on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

- If either operand is NaN, the result is NaN.

- The sign of the result equals the sign of the dividend.

- If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.

- If the dividend is finite and the divisor is an infinity, the result equals the dividend.

- If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.

- In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n − (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode.