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Standard ECMA-262

5.1 Edition / June 2011

ECMAScript® Language Specification

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Contents

  1. Copyright notice
  2. Introduction
  3. 1 Scope
  4. 2 Conformance
  5. 3 Normative references
  6. 4 Overview
    1. 4.1 Web Scripting
    2. 4.2 Language Overview
      1. 4.2.1 Objects
      2. 4.2.2 The Strict Variant of ECMAScript
    3. 4.3 Terms and definitions
      1. 4.3.1 type
      2. 4.3.2 primitive value
      3. 4.3.3 object
      4. 4.3.4 constructor
      5. 4.3.5 prototype
      6. 4.3.6 native object
      7. 4.3.7 built-in object
      8. 4.3.8 host object
      9. 4.3.9 undefined value
      10. 4.3.10 Undefined type
      11. 4.3.11 null value
      12. 4.3.12 Null type
      13. 4.3.13 Boolean value
      14. 4.3.14 Boolean type
      15. 4.3.15 Boolean object
      16. 4.3.16 String value
      17. 4.3.17 String type
      18. 4.3.18 String object
      19. 4.3.19 Number value
      20. 4.3.20 Number type
      21. 4.3.21 Number object
      22. 4.3.22 Infinity
      23. 4.3.23 NaN
      24. 4.3.24 function
      25. 4.3.25 built-in function
      26. 4.3.26 property
      27. 4.3.27 method
      28. 4.3.28 built-in method
      29. 4.3.29 attribute
      30. 4.3.30 own property
      31. 4.3.31 inherited property
  7. 5 Notational Conventions
    1. 5.1 Syntactic and Lexical Grammars
      1. 5.1.1 Context-Free Grammars
      2. 5.1.2 The Lexical and RegExp Grammars
      3. 5.1.3 The Numeric String Grammar
      4. 5.1.4 The Syntactic Grammar
      5. 5.1.5 The JSON Grammar
      6. 5.1.6 Grammar Notation
    2. 5.2 Algorithm Conventions
  8. 6 Source Text
  9. 7 Lexical Conventions
    1. 7.1 Unicode Format-Control Characters
    2. 7.2 White Space
    3. 7.3 Line Terminators
    4. 7.4 Comments
    5. 7.5 Tokens
    6. 7.6 Identifier Names and Identifiers
      1. 7.6.1 Reserved Words
    7. 7.7 Punctuators
    8. 7.8 Literals
      1. 7.8.1 Null Literals
      2. 7.8.2 Boolean Literals
      3. 7.8.3 Numeric Literals
      4. 7.8.4 String Literals
      5. 7.8.5 Regular Expression Literals
    9. 7.9 Automatic Semicolon Insertion
      1. 7.9.1 Rules of Automatic Semicolon Insertion
      2. 7.9.2 Examples of Automatic Semicolon Insertion
  10. 8 Types
    1. 8.1 The Undefined Type
    2. 8.2 The Null Type
    3. 8.3 The Boolean Type
    4. 8.4 The String Type
    5. 8.5 The Number Type
    6. 8.6 The Object Type
      1. 8.6.1 Property Attributes
      2. 8.6.2 Object Internal Properties and Methods
    7. 8.7 The Reference Specification Type
      1. 8.7.1 GetValue (V)
      2. 8.7.2 PutValue (V, W)
    8. 8.8 The List Specification Type
    9. 8.9 The Completion Specification Type
    10. 8.10 The Property Descriptor and Property Identifier Specification Types
      1. 8.10.1 IsAccessorDescriptor ( Desc )
      2. 8.10.2 IsDataDescriptor ( Desc )
      3. 8.10.3 IsGenericDescriptor ( Desc )
      4. 8.10.4 FromPropertyDescriptor ( Desc )
      5. 8.10.5 ToPropertyDescriptor ( Obj )
    11. 8.11 The Lexical Environment and Environment Record Specification Types
    12. 8.12 Algorithms for Object Internal Methods
      1. 8.12.1 [[GetOwnProperty]] (P)
      2. 8.12.2 [[GetProperty]] (P)
      3. 8.12.3 [[Get]] (P)
      4. 8.12.4 [[CanPut]] (P)
      5. 8.12.5 [[Put]] ( P, V, Throw )
      6. 8.12.6 [[HasProperty]] (P)
      7. 8.12.7 [[Delete]] (P, Throw)
      8. 8.12.8 [[DefaultValue]] (hint)
      9. 8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)
  11. 9 Type Conversion and Testing
    1. 9.1 ToPrimitive
    2. 9.2 ToBoolean
    3. 9.3 ToNumber
      1. 9.3.1 ToNumber Applied to the String Type
    4. 9.4 ToInteger
    5. 9.5 ToInt32: (Signed 32 Bit Integer)
    6. 9.6 ToUint32: (Unsigned 32 Bit Integer)
    7. 9.7 ToUint16: (Unsigned 16 Bit Integer)
    8. 9.8 ToString
      1. 9.8.1 ToString Applied to the Number Type
    9. 9.9 ToObject
    10. 9.10 CheckObjectCoercible
    11. 9.11 IsCallable
    12. 9.12 The SameValue Algorithm
  12. 10 Executable Code and Execution Contexts
    1. 10.1 Types of Executable Code
      1. 10.1.1 Strict Mode Code
    2. 10.2 Lexical Environments
      1. 10.2.1 Environment Records
      2. 10.2.2 Lexical Environment Operations
      3. 10.2.3 The Global Environment
    3. 10.3 Execution Contexts
      1. 10.3.1 Identifier Resolution
    4. 10.4 Establishing an Execution Context
      1. 10.4.1 Entering Global Code
      2. 10.4.2 Entering Eval Code
      3. 10.4.3 Entering Function Code
    5. 10.5 Declaration Binding Instantiation
    6. 10.6 Arguments Object
  13. 11 Expressions
    1. 11.1 Primary Expressions
      1. 11.1.1 The this Keyword
      2. 11.1.2 Identifier Reference
      3. 11.1.3 Literal Reference
      4. 11.1.4 Array Initialiser
      5. 11.1.5 Object Initialiser
      6. 11.1.6 The Grouping Operator
    2. 11.2 Left-Hand-Side Expressions
      1. 11.2.1 Property Accessors
      2. 11.2.2 The new Operator
      3. 11.2.3 Function Calls
      4. 11.2.4 Argument Lists
      5. 11.2.5 Function Expressions
    3. 11.3 Postfix Expressions
      1. 11.3.1 Postfix Increment Operator
      2. 11.3.2 Postfix Decrement Operator
    4. 11.4 Unary Operators
      1. 11.4.1 The delete Operator
      2. 11.4.2 The void Operator
      3. 11.4.3 The typeof Operator
      4. 11.4.4 Prefix Increment Operator
      5. 11.4.5 Prefix Decrement Operator
      6. 11.4.6 Unary + Operator
      7. 11.4.7 Unary - Operator
      8. 11.4.8 Bitwise NOT Operator ( ~ )
      9. 11.4.9 Logical NOT Operator ( ! )
    5. 11.5 Multiplicative Operators
      1. 11.5.1 Applying the * Operator
      2. 11.5.2 Applying the / Operator
      3. 11.5.3 Applying the % Operator
    6. 11.6 Additive Operators
      1. 11.6.1 The Addition operator ( + )
      2. 11.6.2 The Subtraction Operator ( - )
      3. 11.6.3 Applying the Additive Operators to Numbers
    7. 11.7 Bitwise Shift Operators
      1. 11.7.1 The Left Shift Operator ( << )
      2. 11.7.2 The Signed Right Shift Operator ( >> )
      3. 11.7.3 The Unsigned Right Shift Operator ( >>> )
    8. 11.8 Relational Operators
      1. 11.8.1 The Less-than Operator ( < )
      2. 11.8.2 The Greater-than Operator ( > )
      3. 11.8.3 The Less-than-or-equal Operator ( <= )
      4. 11.8.4 The Greater-than-or-equal Operator ( >= )
      5. 11.8.5 The Abstract Relational Comparison Algorithm
      6. 11.8.6 The instanceof operator
      7. 11.8.7 The in operator
    9. 11.9 Equality Operators
      1. 11.9.1 The Equals Operator ( == )
      2. 11.9.2 The Does-not-equals Operator ( != )
      3. 11.9.3 The Abstract Equality Comparison Algorithm
      4. 11.9.4 The Strict Equals Operator ( === )
      5. 11.9.5 The Strict Does-not-equal Operator ( !== )
      6. 11.9.6 The Strict Equality Comparison Algorithm
    10. 11.10 Binary Bitwise Operators
    11. 11.11 Binary Logical Operators
    12. 11.12 Conditional Operator ( ? : )
    13. 11.13 Assignment Operators
      1. 11.13.1 Simple Assignment ( = )
      2. 11.13.2 Compound Assignment ( op= )
    14. 11.14 Comma Operator ( , )
  14. 12 Statements
    1. 12.1 Block
    2. 12.2 Variable Statement
      1. 12.2.1 Strict Mode Restrictions
    3. 12.3 Empty Statement
    4. 12.4 Expression Statement
    5. 12.5 The if Statement
    6. 12.6 Iteration Statements
      1. 12.6.1 The do-while Statement
      2. 12.6.2 The while Statement
      3. 12.6.3 The for Statement
      4. 12.6.4 The for-in Statement
    7. 12.7 The continue Statement
    8. 12.8 The break Statement
    9. 12.9 The return Statement
    10. 12.10 The with Statement
      1. 12.10.1 Strict Mode Restrictions
    11. 12.11 The switch Statement
    12. 12.12 Labelled Statements
    13. 12.13 The throw Statement
    14. 12.14 The try Statement
      1. 12.14.1 Strict Mode Restrictions
    15. 12.15 The debugger statement
  15. 13 Function Definition
    1. 13.1 Strict Mode Restrictions
    2. 13.2 Creating Function Objects
      1. 13.2.1 [[Call]]
      2. 13.2.2 [[Construct]]
      3. 13.2.3 The [[ThrowTypeError]] Function Object
  16. 14 Program
    1. 14.1 Directive Prologues and the Use Strict Directive
  17. 15 Standard Built-in ECMAScript Objects
    1. 15.1 The Global Object
      1. 15.1.1 Value Properties of the Global Object
      2. 15.1.2 Function Properties of the Global Object
      3. 15.1.3 URI Handling Function Properties
      4. 15.1.4 Constructor Properties of the Global Object
      5. 15.1.5 Other Properties of the Global Object
    2. 15.2 Object Objects
      1. 15.2.1 The Object Constructor Called as a Function
      2. 15.2.2 The Object Constructor
      3. 15.2.3 Properties of the Object Constructor
      4. 15.2.4 Properties of the Object Prototype Object
      5. 15.2.5 Properties of Object Instances
    3. 15.3 Function Objects
      1. 15.3.1 The Function Constructor Called as a Function
      2. 15.3.2 The Function Constructor
      3. 15.3.3 Properties of the Function Constructor
      4. 15.3.4 Properties of the Function Prototype Object
      5. 15.3.5 Properties of Function Instances
    4. 15.4 Array Objects
      1. 15.4.1 The Array Constructor Called as a Function
      2. 15.4.2 The Array Constructor
      3. 15.4.3 Properties of the Array Constructor
      4. 15.4.4 Properties of the Array Prototype Object
      5. 15.4.5 Properties of Array Instances
    5. 15.5 String Objects
      1. 15.5.1 The String Constructor Called as a Function
      2. 15.5.2 The String Constructor
      3. 15.5.3 Properties of the String Constructor
      4. 15.5.4 Properties of the String Prototype Object
      5. 15.5.5 Properties of String Instances
    6. 15.6 Boolean Objects
      1. 15.6.1 The Boolean Constructor Called as a Function
      2. 15.6.2 The Boolean Constructor
      3. 15.6.3 Properties of the Boolean Constructor
      4. 15.6.4 Properties of the Boolean Prototype Object
      5. 15.6.5 Properties of Boolean Instances
    7. 15.7 Number Objects
      1. 15.7.1 The Number Constructor Called as a Function
      2. 15.7.2 The Number Constructor
      3. 15.7.3 Properties of the Number Constructor
      4. 15.7.4 Properties of the Number Prototype Object
      5. 15.7.5 Properties of Number Instances
    8. 15.8 The Math Object
      1. 15.8.1 Value Properties of the Math Object
      2. 15.8.2 Function Properties of the Math Object
    9. 15.9 Date Objects
      1. 15.9.1 Overview of Date Objects and Definitions of Abstract Operators
      2. 15.9.2 The Date Constructor Called as a Function
      3. 15.9.3 The Date Constructor
      4. 15.9.4 Properties of the Date Constructor
      5. 15.9.5 Properties of the Date Prototype Object
      6. 15.9.6 Properties of Date Instances
    10. 15.10 RegExp (Regular Expression) Objects
      1. 15.10.1 Patterns
      2. 15.10.2 Pattern Semantics
      3. 15.10.3 The RegExp Constructor Called as a Function
      4. 15.10.4 The RegExp Constructor
      5. 15.10.5 Properties of the RegExp Constructor
      6. 15.10.6 Properties of the RegExp Prototype Object
      7. 15.10.7 Properties of RegExp Instances
    11. 15.11 Error Objects
      1. 15.11.1 The Error Constructor Called as a Function
      2. 15.11.2 The Error Constructor
      3. 15.11.3 Properties of the Error Constructor
      4. 15.11.4 Properties of the Error Prototype Object
      5. 15.11.5 Properties of Error Instances
      6. 15.11.6 Native Error Types Used in This Standard
      7. 15.11.7 NativeError Object Structure
    12. 15.12 The JSON Object
      1. 15.12.1 The JSON Grammar
      2. 15.12.2 parse ( text [ , reviver ] )
      3. 15.12.3 stringify ( value [ , replacer [ , space ] ] )
  18. 16 Errors
  19. Annex A (informative) Grammar Summary
    1. A.1 Lexical Grammar
    2. A.2 Number Conversions
    3. A.3 Expressions
    4. A.4 Statements
    5. A.5 Functions and Programs
    6. A.6 Universal Resource Identifier Character Classes
    7. A.7 Regular Expressions
    8. A.8 JSON
      1. A.8.1 JSON Lexical Grammar
      2. A.8.2 JSON Syntactic Grammar
  20. Annex B (informative) Compatibility
    1. B.1 Additional Syntax
      1. B.1.1 Numeric Literals
      2. B.1.2 String Literals
    2. B.2 Additional Properties
      1. B.2.1 escape (string)
      2. B.2.2 unescape (string)
      3. B.2.3 String.prototype.substr (start, length)
      4. B.2.4 Date.prototype.getYear ( )
      5. B.2.5 Date.prototype.setYear (year)
      6. B.2.6 Date.prototype.toGMTString ( )
  21. Annex C (informative) The Strict Mode of ECMAScript
  22. Annex D (informative) Corrections and Clarifications in the 5th Edition with Possible 3rd Edition Compatibility Impact
  23. Annex E (informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities with the 3rd Edition
  24. Annex F (informative) Technically Significant Corrections and Clarifications in the 5.1 Edition
  25. Bibliography

Introduction

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

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

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

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

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

This present edition 5.1 of the ECMAScript Standard is fully aligned with third edition of the international standard ISO/IEC 16262:2011.

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

This Ecma Standard has been adopted by the General Assembly of June 2011.

ECMAScript Language Specification

1 Scope

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

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

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

3 Normative references

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

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

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

4 Overview

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

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

A scripting language is a programming language that is used to manipulate, customise, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and 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 can provide core scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this document apart from any particular host environment.

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

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

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

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

4.1 Web Scripting

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

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

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

4.2 Language Overview

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

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

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object, the Object object, the Function object, the Array object, the String object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the JSON object, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError and URIError.

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

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

4.2.1 Objects

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

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

An image of lots of boxes and arrows.
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, and structure, behaviour, and state are all inherited.

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

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example, cf3’s prototype is 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 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.

4.2.2 The Strict Variant of ECMAScript

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

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

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

4.3 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 8 of this specification

4.3.2 primitive value

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

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 initialises 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 native object

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

NOTE Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.

4.3.7 built-in object

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

NOTE Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify and define others. Every built-in object is a native object. A built-in constructor is a built-in object that is also a constructor.

4.3.8 host object

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

NOTE Any object that is not native is a host object.

4.3.9 undefined value

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

4.3.10 Undefined type

type whose sole value is the undefined value

4.3.11 null value

primitive value that represents the intentional absence of any object value

4.3.12 Null type

type whose sole value is the null value

4.3.13 Boolean value

member of the Boolean type

NOTE There are only two Boolean values, true and false.

4.3.14 Boolean type

type consisting of the primitive values true and false

4.3.15 Boolean object

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

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 property whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.3.16 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.

4.3.17 String type

set of all possible String values

4.3.18 String object

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

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 property whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (15.5.1).

4.3.19 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.

4.3.20 Number type

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

4.3.21 Number object

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

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 property whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (15.7.1).

4.3.22 Infinity

number value that is the positive infinite Number value

4.3.23 NaN

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

4.3.24 function

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

NOTE In addition to its named 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.

4.3.25 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.

4.3.26 property

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

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.

4.3.27 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.

4.3.28 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.

4.3.29 attribute

internal value that defines some characteristic of a property

4.3.30 own property

property that is directly contained by its object

4.3.31 inherited property

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

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

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

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

5.1.2 The Lexical and RegExp Grammars

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

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

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

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

5.1.3 The Numeric String Grammar

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

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

5.1.4 The Syntactic Grammar

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

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

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

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

5.1.5 The JSON Grammar

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

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

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

5.1.6 Grammar Notation

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

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

WhileStatement :
while ( Expression ) Statement

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

ArgumentList :
AssignmentExpression
ArgumentList , AssignmentExpression

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

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

VariableDeclaration :
Identifier Initialiseropt

is a convenient abbreviation for:

VariableDeclaration :
Identifier
Identifier Initialiser

and that:

IterationStatement :
for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement

is a convenient abbreviation for:

IterationStatement :
for ( ; Expressionopt ; Expressionopt ) Statement
for ( ExpressionNoIn ; Expressionopt ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :
for ( ; ; Expressionopt ) Statement
for ( ; Expression ; Expressionopt ) Statement
for ( ExpressionNoIn ; ; Expressionopt ) Statement
for ( ExpressionNoIn ; Expression ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :
for ( ; ; ) Statement
for ( ; ; Expression ) Statement
for ( ; Expression ; ) Statement
for ( ; Expression ; Expression ) Statement
for ( ExpressionNoIn ; ; ) Statement
for ( ExpressionNoIn ; ; Expression ) Statement
for ( ExpressionNoIn ; Expression ; ) Statement
for ( ExpressionNoIn ; Expression ; Expression ) Statement

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

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

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

which is merely a convenient abbreviation for:

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

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

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

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

the definition

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

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

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

ThrowStatement :
throw [no LineTerminator here] Expression ;

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

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

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

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

Identifier ::
IdentifierName but not ReservedWord

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

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

SourceCharacter ::
any Unicode code unit

5.2 Algorithm Conventions

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

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

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

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

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

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

A step may specify the iterative application of its substeps.

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

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

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

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

The 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 xk = q × y for some integer q.

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

NOTE floor(x) = x−(x modulo 1).

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

6 Source Text

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later. The text is expected to have been normalised to Unicode Normalization Form C (canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first converted to UTF-16.

Syntax

SourceCharacter ::
any Unicode code unit

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

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

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

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

7 Lexical Conventions

The source text of an ECMAScript program is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of characters as the next input element.

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

NOTE There are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 7.9); in examples such as the following:

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

where the first non-whitespace, non-comment character after a LineTerminator is slash (/) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

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

Syntax

InputElementDiv ::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
InputElementRegExp ::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

7.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as left-to-right mark or right-to-left mark) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals and regular expression literals.

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

<BOM> is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).

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

Table 1 — Format-Control Character Usage
Code Unit Value Name Formal Name Usage
\u200C Zero width non-joiner <ZWNJ> IdentifierPart
\u200D Zero width joiner <ZWJ> IdentifierPart
\uFEFF Byte Order Mark <BOM> Whitespace

7.2 White Space

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end of input. White space characters may also occur within a StringLiteral or a RegularExpressionLiteral (where they are considered significant characters forming part of the literal value) or within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space characters are listed in Table 2.

Table 2 — Whitespace Characters
Code Unit Value Name Formal Name
\u0009 Tab <TAB>
\u000B Vertical Tab <VT>
\u000C Form Feed <FF>
\u0020 Space <SP>
\u00A0 No-break space <NBSP>
\uFEFF

Other category “Zs”

Byte Order Mark

Any other Unicode “space separator”

<BOM>

<USP>

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

Syntax

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<BOM>
<USP>

7.3 Line Terminators

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (7.9). A line terminator cannot occur within any token except a StringLiteral. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment (7.4) but cannot occur within a SingleLineComment.

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

The ECMAScript line terminator characters are listed in Table 3.

Table 3 — Line Terminator Characters
Code Unit Value Name Formal Name
\u000A Line Feed <LF>
\u000D Carriage Return <CR>
\u2028 Line separator <LS>
\u2029 Paragraph separator <PS>

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

Syntax

LineTerminator ::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence ::
<LF>
<CR> [lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

7.4 Comments

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

Because a single-line comment can contain any character except a LineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognised separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 7.9).

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

Syntax

Comment ::
MultiLineComment
SingleLineComment
MultiLineComment ::
/* 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

7.5 Tokens

Syntax

Token ::
IdentifierName
Punctuator
NumericLiteral
StringLiteral

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

7.6 Identifier Names and Identifiers

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

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

Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single character to the IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by its UnicodeEscapeSequence's CV, the result must still be a valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. All interpretations of identifiers within this specification are based upon their actual characters regardless of whether or not an escape sequence was used to contribute any particular characters.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values). The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler.

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

Syntax

Identifier ::
IdentifierName but not ReservedWord
IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::
UnicodeLetter
$
_
\ UnicodeEscapeSequence
IdentifierPart ::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
<ZWNJ>
<ZWJ>
UnicodeLetter ::
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
UnicodeCombiningMark ::
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”
UnicodeDigit ::
any character in the Unicode category “Decimal number (Nd)”
UnicodeConnectorPunctuation ::
any character in the Unicode category “Connector punctuation (Pc)”

The definitions of the nonterminal UnicodeEscapeSequence is given in 7.8.4

7.6.1 Reserved Words

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

Syntax

ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

7.6.1.1 Keywords

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

Syntax

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

7.6.1.2 Future Reserved Words

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

Syntax

FutureReservedWord :: one of
class enum extends super
const export import

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

implements let private public yield
interface package protected static

7.7 Punctuators

Syntax

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

7.8 Literals

Syntax

Literal ::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral

7.8.1 Null Literals

Syntax

NullLiteral ::
null

Semantics

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

7.8.2 Boolean Literals

Syntax

BooleanLiteral ::
true
false

Semantics

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

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

7.8.3 Numeric Literals

Syntax

NumericLiteral ::
DecimalLiteral
HexIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral . 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
HexIntegerLiteral ::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit :: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

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

NOTE For example:

3in

is an error and not the two input elements 3 and in.

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

  • The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.

  • The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10n), where n is the number of characters in DecimalDigits.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral times 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 times 10n)) times 10e, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

  • The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits times 10n, where n is the number of characters in DecimalDigits.

  • The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10en, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral times 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 times 10n) plus the MV of DecimalDigits, where n is the number of characters in DecimalDigits.

  • The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.

  • The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.

  • The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.

  • The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.

  • The MV of DecimalDigit :: 0 or of HexDigit :: 0 is 0.

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

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

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

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

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

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

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

  • The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.

  • The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9.

  • The MV of HexDigit :: a or of HexDigit :: A is 10.

  • The MV of HexDigit :: b or of HexDigit :: B is 11.

  • The MV of HexDigit :: c or of HexDigit :: C is 12.

  • The MV of HexDigit :: d or of HexDigit :: D is 13.

  • The MV of HexDigit :: e or of HexDigit :: E is 14.

  • The MV of HexDigit :: f or of HexDigit :: F is 15.

  • The MV of HexIntegerLiteral :: 0x HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral :: 0X HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral :: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 8.5), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

7.8.4 String Literals

A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash, carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape sequence.

Syntax

StringLiteral ::
" 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
CharacterEscapeSequence ::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of
' " \ b f n r t v
NonEscapeCharacter ::
SourceCharacter but not one of EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
DecimalDigit
x
u
HexEscapeSequence ::
x HexDigit HexDigit
UnicodeEscapeSequence ::
u HexDigit HexDigit HexDigit HexDigit

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

Semantics

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of character values (CV) contributed by the various parts of the string literal. As part of this process, some characters within the string literal are interpreted as having a mathematical value (MV), as described below or in 7.8.3.

  • The SV of StringLiteral :: "" is the empty character sequence.

  • The SV of StringLiteral :: '' is the empty character sequence.

  • The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.

  • The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.

  • The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one character, the CV of DoubleStringCharacter.

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

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

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

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

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

  • The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

  • The CV of DoubleStringCharacter :: LineContinuation is the empty character sequence.

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

  • The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

  • The CV of SingleStringCharacter :: LineContinuation is the empty character sequence.

  • The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

  • The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is a <NUL> character (Unicode value 0000).

  • The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.

  • The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.

  • The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the character whose code unit value is determined by the SingleEscapeCharacter according to Table 4:

Table 4 — String Single Character Escape Sequences
Escape Sequence Code Unit Value Name Symbol
\b \u0008 backspace <BS>
\t \u0009 horizontal tab <HT>
\n \u000A line feed (new line) <LF>
\v \u000B vertical tab <VT>
\f \u000C form feed <FF>
\r \u000D carriage return <CR>
\" \u0022 double quote "
\' \u0027 single quote '
\\ \u005C backslash \
  • The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.

  • The CV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the SourceCharacter character itself.

  • The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code unit value is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

  • The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the character whose code unit value is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

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

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

7.8.5 Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp (see 15.10.4) or calling the RegExp constructor as a function (15.10.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The Strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions.

Syntax

RegularExpressionLiteral ::
/ RegularExpressionBody / RegularExpressionFlags
RegularExpressionBody ::
RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::
[empty]
RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::
RegularExpressionNonTerminator but not one of * or \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar ::
RegularExpressionNonTerminator but not one of \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence ::
\ RegularExpressionNonTerminator
RegularExpressionNonTerminator ::
SourceCharacter but not LineTerminator
RegularExpressionClass ::
[ RegularExpressionClassChars ]
RegularExpressionClassChars ::
[empty]
RegularExpressionClassChars RegularExpressionClassChar
RegularExpressionClassChar ::
RegularExpressionNonTerminator but not one of ] or \
RegularExpressionBackslashSequence
RegularExpressionFlags ::
[empty]
RegularExpressionFlags IdentifierPart

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

Semantics

A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new object is created as if by the expression new RegExp(Pattern, Flags) where RegExp is the standard built-in constructor with that name. The newly constructed object becomes the value of the RegularExpressionLiteral. If the call to new RegExp would generate an error as specified in 15.10.4.1, the error must be treated as an early error (Clause 16).

7.9 Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-while statement, continue statement, break statement, return statement, and throw statement) must be terminated with semicolons. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

7.9.1 Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

  1. When, as the program is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
    • The offending token is separated from the previous token by at least one LineTerminator.
    • The offending token is }.
  2. When, as the program is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript Program, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the program is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation [no LineTerminator here] within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 12.6.3).

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

PostfixExpression :
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --
ContinueStatement :
continue [no LineTerminator here] Identifier ;
BreakStatement :
break [no LineTerminator here] Identifier ;
ReturnStatement :
return [no LineTerminator here] Expression ;
ThrowStatement :
throw [no LineTerminator here] Expression ;

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.

When a continue, break, return, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.

An Expression in a return or throw statement should start on the same line as the return or throw token.

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

7.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

NOTE The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

NOTE The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesised expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

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

8 Types

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

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

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

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

8.1 The Undefined Type

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

8.2 The Null Type

The Null type has exactly one value, called null.

8.3 The Boolean Type

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

8.4 The String Type

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values (“elements”). The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a code unit value (see Clause 6). Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if any) at position 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

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

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

8.5 The Number Type

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

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

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

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

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

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

s × 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 denormalised, 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” mode.)

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

8.6 The Object Type

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

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

8.6.1 Property Attributes

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

Table 5 — Attributes of a Named Data Property
Attribute Name Value Domain Description
[[Value]] Any ECMAScript language type The value retrieved by reading the property.
[[Writable]] Boolean If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.
[[Enumerable]] Boolean If true, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]] Boolean If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.

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

Table 6 — Attributes of a Named Accessor Property
Attribute Name Value Domain Description
[[Get]] Object or Undefined If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to return the property value each time a get access of the property is performed.
[[Set]] Object or Undefined If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]] Boolean If true, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]] Boolean If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

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

Table 7 — Default Attribute Values
Attribute Name Default Value
[[Value]] undefined
[[Get]] undefined
[[Set]] undefined
[[Writable]] false
[[Enumerable]] false
[[Configurable]] false

8.6.2 Object Internal Properties and Methods

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

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

The “Value Type Domain” columns of the following tables define the types of values associated with internal properties. The type names refer to the types defined in Clause 8 augmented by the following additional names. “any” means the value may be any ECMAScript language type. “primitive” means Undefined, Null, Boolean, String, or Number. “SpecOp” means the internal property is an internal method, an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and the type of the returned value.

Table 8 — Internal Properties Common to All Objects
Internal Property Value Type Domain Description
[[Prototype]] Object or Null The prototype of this object.
[[Class]] String A String value indicating a specification defined classification of objects.
[[Extensible]] Boolean If true, own properties may be added to the object.
[[Get]] SpecOp(propertyName) any Returns the value of the named property.
[[GetOwnProperty]]

SpecOp (propertyName)

Undefined or Property Descriptor

Returns the Property Descriptor of the named own property of this object, or undefined if absent.
[[GetProperty]]

SpecOp (propertyName)

Undefined or Property Descriptor

Returns the fully populated Property Descriptor of the named property of this object, or undefined if absent.
[[Put]] SpecOp (propertyName, any, Boolean) Sets the specified named property to the value of the second parameter. The flag controls failure handling.
[[CanPut]] SpecOp (propertyName) Boolean Returns a Boolean value indicating whether a [[Put]] operation with PropertyName can be performed.
[[HasProperty]] SpecOp (propertyName) Boolean Returns a Boolean value indicating whether the object already has a property with the given name.
[[Delete]] SpecOp (propertyName, Boolean) Boolean Removes the specified named own property from the object. The flag controls failure handling.
[[DefaultValue]] SpecOp (Hint) primitive Hint is a String. Returns a default value for the object.
[[DefineOwnProperty]] SpecOp (propertyName, PropertyDescriptor, Boolean) Boolean Creates or alters the named own property to have the state described by a Property Descriptor. The flag controls failure handling.

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

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

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

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

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

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

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

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

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

  • If a property is described as a data property and its [[Writable]] and [[Configurable]] are both false, then the SameValue (according to 9.12) must be returned for the [[Value]] attribute of the property on all calls to [[GetOwnProperty]].

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

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

  • If the value of the host object’s [[Extensible]] internal property has been observed by ECMAScript code to be false, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also describe that property as non-existent.

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

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

Table 9 — Internal Properties Only Defined for Some Objects
Internal Property Value Type Domain Description
[[PrimitiveValue]] primitive Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].
[[Construct]] SpecOp(a List of any) Object Creates an object. Invoked via the new operator. The arguments to the SpecOp are the arguments passed to the new operator. Objects that implement this internal method are called constructors.
[[Call]] SpecOp(any, a List of any) any or Reference Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values.
[[HasInstance]] SpecOp(any) Boolean Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].
[[Scope]] Lexical Environment A lexical environment that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]].
[[FormalParameters]] List of Strings A possibly empty List containing the identifier Strings of a Function’s FormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].
[[Code]] ECMAScript code The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].
[[TargetFunction]] Object The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.
[[BoundThis]] any The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.
[[BoundArguments]] List of any The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.
[[Match]] SpecOp(String, index) MatchResult Tests for a regular expression match and returns a MatchResult value (see 15.10.2.1). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]].
[[ParameterMap]] Object Provides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.

8.7 The Reference Specification Type

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

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

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

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

8.7.1 GetValue (V)

  1. If Type(V) is not Reference, return V.
  2. Let base be the result of calling GetBase(V).
  3. If IsUnresolvableReference(V), throw a ReferenceError exception.
  4. If IsPropertyReference(V), then
    1. If HasPrimitiveBase(V) is false, then let get be the [[Get]] internal method of base, otherwise let get be the special [[Get]] internal method defined below.
    2. Return the result of calling the get internal method using base as its this value, and passing GetReferencedName(V) for the argument.
  5. Else, base must be an environment record.
    1. Return the result of calling the GetBindingValue (see 10.2.1) concrete method of base passing GetReferencedName(V) and IsStrictReference(V) as arguments.

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

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

NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of the object. The only situation where such an actual property access that uses this internal method can have visible effect is when it invokes an accessor function.

8.7.2 PutValue (V, W)

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

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

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

NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of that transient object. The only situations where such an actual property assignment that uses this internal method can have visible effect are when it either invokes an accessor function or is in violation of a Throw predicated error check. When Throw is true any property assignment that would create a new property on the transient object throws an error.

8.8 The List Specification Type

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

8.9 The Completion Specification Type

The Completion type is used to explain the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language value or empty, and target is any ECMAScript identifier or empty. If cv is a completion value then cv.type, cv.value, and cv.target may be used to directly refer to its constituent values.

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

8.10 The Property Descriptor and Property Identifier Specification Types

The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.

Property Descriptor values may be further classified as data property descriptors and accessor property descriptors based upon the existence or use of certain fields. A data property descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor property descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data property descriptor and an accessor property descriptor; however, it may be neither. A generic property descriptor is a Property Descriptor value that is neither a data property descriptor nor an accessor property descriptor. A fully populated property descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of the fields that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.

For notational convenience within this specification, an object literal-like syntax can be used to define a property descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true} defines a data property descriptor. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Property Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named [[Value]]”.

The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor value.

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

8.10.1 IsAccessorDescriptor ( Desc )

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

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

8.10.2 IsDataDescriptor ( Desc )

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

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

8.10.3 IsGenericDescriptor ( Desc )

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

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

8.10.4 FromPropertyDescriptor ( Desc )

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

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

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

8.10.5 ToPropertyDescriptor ( Obj )

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

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

8.11 The Lexical Environment and Environment Record Specification Types

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

8.12 Algorithms for Object Internal Methods

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

8.12.1 [[GetOwnProperty]] (P)

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

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

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

8.12.2 [[GetProperty]] (P)

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

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

8.12.3 [[Get]] (P)

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

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

8.12.4 [[CanPut]] (P)

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

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

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

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

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

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

8.12.6 [[HasProperty]] (P)

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

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

8.12.7 [[Delete]] (P, Throw)

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

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

8.12.8 [[DefaultValue]] (hint)

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

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

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

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

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

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

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

In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError exception, otherwise return false”. The algorithm contains steps that test various fields of the Property Descriptor Desc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.

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

  1. Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
  2. Let extensible be the value of the [[Extensible]] internal property of O.
  3. If current is undefined and extensible is false, then Reject.
  4. If current is undefined and extensible is true, then
    1. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
      1. Create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
    2. Else, Desc must be an accessor Property Descriptor so,
      1. Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
    3. Return true.
  5. Return true, if every field in Desc is absent.
  6. Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm (9.12).
  7. If the [[Configurable]] field of current is false then
    1. Reject, if the [[Configurable]] field of Desc is true.
    2. Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
  8. If IsGenericDescriptor(Desc) is true, then no further validation is required.
  9. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
    1. Reject, if the [[Configurable]] field of current is false.
    2. If IsDataDescriptor(current) is true, then
      1. Convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
    3. Else,
      1. Convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
  10. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
    1. If the [[Configurable]] field of current is false, then
      1. Reject, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
      2. If the [[Writable]] field of current is false, then
        1. Reject, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]], current.[[Value]]) is false.
    2. else, the [[Configurable]] field of current is true, so any change is acceptable.
  11. Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so,
    1. If the [[Configurable]] field of current is false, then
      1. Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
      2. Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
  12. For each attribute field of Desc that is present, set the correspondingly named attribute of the property named P of object O to the value of the field.
  13. Return true.

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

NOTE Step 10.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 Type Conversion and Testing

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

9.1 ToPrimitive

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 10:

Table 10 — ToPrimitive Conversions
Input Type Result
Undefined The result equals the input argument (no conversion).
Null The result equals the input argument (no conversion).
Boolean The result equals the input argument (no conversion).
Number The result equals the input argument (no conversion).
String The result equals the input argument (no conversion).
Object Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

9.2 ToBoolean

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

Table 11 — ToBoolean Conversions
Argument Type Result
Undefined false
Null false
Boolean The result equals the input argument (no conversion).
Number The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
String The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object true

9.3 ToNumber

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

Table 12 — To Number Conversions
Argument Type Result
Undefined NaN
Null +0
Boolean The result is 1 if the argument is true. The result is +0 if the argument is false.
Number The result equals the input argument (no conversion).
String See grammar and note below.
Object

Apply the following steps:

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

9.3.1 ToNumber Applied to the String Type

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

Syntax

StringNumericLiteral :::
StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::
WhiteSpace
LineTerminator
StrNumericLiteral :::
StrDecimalLiteral
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
HexIntegerLiteral :::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

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

  • A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.

  • A StringNumericLiteral that is decimal may have any number of leading 0 digits.

  • A StringNumericLiteral that is decimal may be preceded by + or - to indicate its sign.

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

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

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

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

  • The MV of StringNumericLiteral ::: 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 ::: 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 10n), where n is the number of characters 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 10n)) times 10e, where n is the number of characters in the second DecimalDigits and e is the MV of ExponentPart.

  • The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits is the MV of DecimalDigits times 10n, where n is the number of characters in DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10en, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits is the MV of DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10e, where e is the MV of ExponentPart.

  • The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.

  • The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.

  • The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.

  • The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger ::: + DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger ::: - DecimalDigits is the negative of the MV of DecimalDigits.

  • The MV of DecimalDigit ::: 0 or of HexDigit ::: 0 is 0.

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

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

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

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

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

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

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

  • The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.

  • The MV of DecimalDigit ::: 9 or of HexDigit ::: 9 is 9.

  • The MV of HexDigit ::: a or of HexDigit ::: A is 10.

  • The MV of HexDigit ::: b or of HexDigit ::: B is 11.

  • The MV of HexDigit ::: c or of HexDigit ::: C is 12.

  • The MV of HexDigit ::: d or of HexDigit ::: D is 13.

  • The MV of HexDigit ::: e or of HexDigit ::: E is 14.

  • The MV of HexDigit ::: f or of HexDigit ::: F is 15.

  • The MV of HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space character in the String numeric literal is ‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

9.4 ToInteger

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

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

9.5 ToInt32: (Signed 32 Bit Integer)

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

  1. Let number be the result of calling ToNumber on the input argument.
  2. If number is NaN, +0, −0, +∞, or −∞, return +0.
  3. Let posInt be sign(number) * floor(abs(number)).
  4. Let int32bit be posInt modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 232.
  5. If int32bit is greater than or equal to 231, return int32bit − 232, otherwise return int32bit.

NOTE Given the above definition of ToInt32:

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

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

  • ToInt32 maps −0 to +0.

9.6 ToUint32: (Unsigned 32 Bit Integer)

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

  1. Let number be the result of calling ToNumber on the input argument.
  2. If number is NaN, +0, −0, +, or −, return +0.
  3. Let posInt be sign(number) × floor(abs(number)).
  4. Let int32bit be posInt modulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 232.
  5. Return int32bit.

NOTE Given the above definition of ToUInt32:

  • Step 5 is the only difference between ToUint32 and ToInt32.

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

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

  • ToUint32 maps −0 to +0.

9.7 ToUint16: (Unsigned 16 Bit Integer)

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

  1. Let number be the result of calling ToNumber on the input argument.
  2. If number is NaN, +0, −0, +, or −, return +0.
  3. Let posInt be sign(number) × floor(abs(number)).
  4. Let int16bit be posInt modulo 216; that is, a finite integer value k of Number type with positive sign and less than 216 in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 216.
  5. Return int16bit.

NOTE Given the above definition of ToUint16:

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

9.8 ToString

The abstract operation ToString converts its argument to a value of type String according to Table 13:

Table 13 — ToString Conversions
Argument Type Result
Undefined "undefined"
Null "null"
Boolean

If the argument is true, then the result is "true".

If the argument is false, then the result is "false".

Number See 9.8.1.
String Return the input argument (no conversion)
Object

Apply the following steps:

1. Let primValue be ToPrimitive(input argument, hint String).

2. Return ToString(primValue).

9.8.1 ToString Applied to the Number Type

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

  1. If m is NaN, return the String "NaN".
  2. If m is +0 or −0, return the String "0".
  3. If m is less than zero, return the String concatenation of the String "-" and ToString(−m).
  4. If m is infinity, return the String "Infinity".
  5. Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1s < 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 kn ≤ 21, return the String consisting of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n−k occurrences of the character ‘0’.
  7. If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−n digits of the decimal representation of s.
  8. If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point ‘.’, followed by −n occurrences of the character ‘0’, followed by the k digits of the decimal representation of s.
  9. Otherwise, if k = 1, return the String consisting of the single digit of s, followed by lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).
  10. Return the String consisting of the most significant digit of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−1 digits of the decimal representation of s, followed by the lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).

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−1s < 10k, the Number value for s × 10nk is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10nk 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://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as
http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the various netlib mirror sites.

9.9 ToObject

The abstract operation ToObject converts its argument to a value of type Object according to Table 14:

Table 14 — ToObject
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Create a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.6 for a description of Boolean objects.
Number Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.7 for a description of Number objects.
String Create a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.5 for a description of String objects.
Object The result is the input argument (no conversion).

9.10 CheckObjectCoercible

The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 15:

Table 15 — CheckObjectCoercible Results
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return
Number Return
String Return
Object Return

9.11 IsCallable

The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable function Object according to Table 16:

Table 16 — IsCallable Results
Argument Type Result
Undefined Return false.
Null Return false.
Boolean Return false.
Number Return false.
String Return false.
Object If the argument object has a [[Call]] internal method, then return true, otherwise return false.

9.12 The SameValue Algorithm

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. 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 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.
  5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
  6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  7. Return true if x and y refer to the same object. Otherwise, return false.

10 Executable Code and Execution Contexts

10.1 Types of Executable Code

There are three types of ECMAScript executable code:

10.1.1 Strict Mode Code

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

10.2 Lexical Environments

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

An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment.

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

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

10.2.1 Environment Records

There are two kinds of Environment Record values used in this specification: declarative environment records and object environment records. Declarative environment records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object environment records are used to define the effect of ECMAScript elements such as Program and WithStatement that associate identifier bindings with the properties of some object.

For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with two concrete subclasses, declarative environment record and object environment record. The abstract class includes the abstract specification methods defined in Table 17. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 17 — Abstract Methods of Environment Records
Method Purpose
HasBinding(N) Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value N is the text of the identifier.
CreateMutableBinding(N, D) Create a new 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.
SetMutableBinding(N,V, S) Set the value of an already existing mutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S is used to identify strict mode references.
GetBindingValue(N,S) Returns the value of an already existing binding from an environment record. The String value N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist or is uninitialised throw a ReferenceError exception.
DeleteBinding(N) Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
ImplicitThisValue() Returns the value to use as the this value on calls to function objects that are obtained as binding values from this environment record.

10.2.1.1 Declarative Environment Records

Each declarative environment record is associated with an ECMAScript program scope containing variable and/or function declarations. A declarative environment record binds the set of identifiers defined by the declarations contained within its scope.

In addition to the mutable bindings supported by all Environment Records, declarative environment records also provide for immutable bindings. An immutable binding is one where the association between an identifier and a value may not be modified once it has been established. Creation and initialisation of immutable binding are distinct steps so it is possible for such bindings to exist in either an initialised or uninitialised state. Declarative environment records support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:

Table 18 — Additional Methods of Declarative Environment Records
Method Purpose
CreateImmutableBinding(N) Create a new but uninitialised immutable binding in an environment record. The String value N is the text of the bound name.
InitializeImmutableBinding(N,V) Set the value of an already existing but uninitialised immutable 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.

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

10.2.1.1.1 HasBinding(N)

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

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

10.2.1.1.2 CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the name N that is initialised to the value undefined. 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 set its bound value to undefined. If D is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.

10.2.1.1.3 SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for declarative environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N must already exist. If the binding is an immutable binding, a TypeError is thrown if S is true.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec must have a binding for N.
  3. If the binding for N in envRec is a mutable binding, change its bound value to V.
  4. Else this must be an attempt to change the value of an immutable binding so if S if true throw a TypeError exception.

10.2.1.1.4 GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for declarative environment records simply returns the value of its bound identifier whose name is the value of the argument N. The binding must already exist. If S is true and the binding is an uninitialised immutable binding throw a ReferenceError exception.

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

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

10.2.1.1.6 ImplicitThisValue()

Declarative Environment Records always return undefined as their ImplicitThisValue.

  1. Return undefined.

10.2.1.1.7 CreateImmutableBinding (N)

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

  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 uninitialised.

10.2.1.1.8 InitializeImmutableBinding (N,V)

The concrete Environment Record method InitializeImmutableBinding for declarative environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialised immutable 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 uninitialised immutable binding for N.
  3. Set the bound value for N in envRec to V.
  4. Record that the immutable binding for N in envRec has been initialised.

10.2.1.2 Object Environment Records

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

Object environment records can be configured to provide their binding object as an implicit this value for use in function calls. This capability is used to specify the behaviour of With Statement (12.10) induced bindings. The capability is controlled by a provideThis Boolean value that is associated with each object environment record. By default, the value of provideThis is false for any object environment record.

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

10.2.1.2.1 HasBinding(N)

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

  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 the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.

10.2.1.2.2 CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for object environment records creates in an environment record’s associated binding object a property whose name is the String value and initialises it to the value undefined. A property named N must not already exist in the binding object. If Boolean argument D is provided and has the value true the new property’s [[Configurable]] attribute is set to true, otherwise it is set to false.

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindings be the binding object for envRec.
  3. Assert: The result of calling the [[HasProperty]] internal method of bindings, passing N as the property name, is false.
  4. If D is true then let configValue be true otherwise let configValue be false.
  5. Call the [[DefineOwnProperty]] internal method of bindings, passing N, Property Descriptor {[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}, and true as arguments.

10.2.1.2.3 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 should already exist 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. Call the [[Put]] internal method of bindings with arguments N, V, and S.

10.2.1.2.4 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 the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.
  4. If value is false, then
    1. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
  5. Return the result of calling the [[Get]] internal method of bindings, passing N for the argument.

10.2.1.2.5 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 the result of calling the [[Delete]] internal method of bindings, passing N and false as arguments.

10.2.1.2.6 ImplicitThisValue()

Object Environment Records return undefined as their ImplicitThisValue unless their provideThis flag is true.

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

10.2.2 Lexical Environment Operations

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

10.2.2.1 GetIdentifierReference (lex, name, strict)

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, an identifier 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 mode flag is strict.
  2. Let envRec be lex’s environment record.
  3. Let exists be the result of calling the HasBinding(N) concrete method of envRec passing name as the argument N.
  4. 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 mode flag is strict.
  5. Else
    1. Let outer be the value of lex’s outer environment reference.
    2. Return the result of calling GetIdentifierReference passing outer, name, and strict as arguments.

10.2.2.2 NewDeclarativeEnvironment (E)

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

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

10.2.2.3 NewObjectEnvironment (O, E)

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

  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 environment record to be envRec.
  4. Set the outer lexical environment reference of env to E.
  5. Return env.

10.2.3 The Global Environment

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

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

10.3 Execution Contexts

When control is transferred to ECMAScript executable code, control is entering an execution context. Active execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. 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 state is necessary to track the execution progress of its associated code. In addition, each execution context has the state components listed in Table 19.

Table 19 —Execution Context State Components
Component Purpose
LexicalEnvironment Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.
VariableEnvironment Identifies the Lexical Environment whose environment record holds bindings created by VariableStatements and FunctionDeclarations within this execution context.
ThisBinding The value associated with the this keyword within ECMAScript code associated with this execution context.

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

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

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

10.3.1 Identifier Resolution

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

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

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

10.4 Establishing an Execution Context

Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a new execution context. Every invocation of an ECMAScript code function (13.2.1) also establishes and enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A thrown exception may also exit one or more execution contexts.

When control enters an execution context, the execution context’s ThisBinding is set, its VariableEnvironment and initial LexicalEnvironment are defined, and declaration binding instantiation (10.5) is performed. The exact manner in which these actions occur depend on the type of code being entered.

10.4.1 Entering Global Code

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

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

10.4.1.1 Initial Global Execution Context

The following steps are performed to initialise a global execution context for ECMAScript code C:

  1. Set the VariableEnvironment to the Global Environment.
  2. Set the LexicalEnvironment to the Global Environment.
  3. Set the ThisBinding to the global object.

10.4.2 Entering Eval Code

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

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

10.4.2.1 Strict Mode Restrictions

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code is strict code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code.

10.4.3 Entering Function Code

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

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

10.5 Declaration Binding Instantiation

Every execution context has an associated VariableEnvironment. Variables and functions declared in ECMAScript code evaluated in an execution context are added as bindings in that VariableEnvironment’s Environment Record. For function code, parameters are also added as bindings to that Environment Record.

Which Environment Record is used to bind a declaration and its kind depends upon the type of ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On entering an execution context, bindings are created in the VariableEnvironment as follows using the caller provided code and, if it is function code, argument List args:

  1. Let env be the environment record component of the running execution context’s VariableEnvironment.
  2. If code is eval code, then let configurableBindings be true else let configurableBindings be false.
  3. If code is strict mode code, then let strict be true else let strict be false.
  4. If code is function code, then
    1. Let func be the function whose [[Call]] internal method initiated execution of code. Let names be the value of func’s [[FormalParameters]] internal property.
    2. Let argCount be the number of elements in args.
    3. Let n be the number 0.
    4. For each String argName in names, in list order do
      1. Let n be the current value of n plus 1.
      2. If n is greater than argCount, let v be undefined otherwise let v be the value of the n’th element of args.
      3. Let argAlreadyDeclared be the result of calling env’s HasBinding concrete method passing argName as the argument.
      4. If argAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing argName as the argument.
      5. Call env’s SetMutableBinding concrete method passing argName, v, and strict as the arguments.
  5. For each FunctionDeclaration f in code, in source text order do
    1. Let fn be the Identifier in FunctionDeclaration f.
    2. Let fo be the result of instantiating FunctionDeclaration f as described in Clause 13.
    3. Let funcAlreadyDeclared be the result of calling env’s HasBinding concrete method passing fn as the argument.
    4. If funcAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing fn and configurableBindings as the arguments.
    5. Else if env is the environment record component of the global environment then
      1. Let go be the global object.
      2. Let existingProp be the resulting of calling the [[GetProperty]] internal method of go with argument fn.
      3. If existingProp .[[Configurable]] is true, then
        1. Call the [[DefineOwnProperty]] internal method of go, passing fn, Property Descriptor {[[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configurableBindings }, and true as arguments.
      4. Else if IsAccessorDescriptor(existingProp) or existingProp does not have attribute values {[[Writable]]: true, [[Enumerable]]: true}, then
        1. Throw a TypeError exception.
    6. Call env’s SetMutableBinding concrete method passing fn, fo, and strict as the arguments.
  6. Let argumentsAlreadyDeclared be the result of calling env’s HasBinding concrete method passing "arguments" as the argument.
  7. If code is function code and argumentsAlreadyDeclared is false, then
    1. Let argsObj be the result of calling the abstract operation CreateArgumentsObject (10.6) passing func, names, args, env and strict as arguments.
    2. If strict is true, then
      1. Call env’s CreateImmutableBinding concrete method passing the String "arguments" as the argument.
      2. Call env’s InitializeImmutableBinding concrete method passing "arguments" and argsObj as arguments.
    3. Else,
      1. Call env’s CreateMutableBinding concrete method passing the String "arguments" as the argument.
      2. Call env’s SetMutableBinding concrete method passing "arguments", argsObj, and false as arguments.
  8. For each VariableDeclaration and VariableDeclarationNoIn d in code, in source text order do
    1. Let dn be the Identifier in d.
    2. Let varAlreadyDeclared be the result of calling env’s HasBinding concrete method passing dn as the argument.
    3. If varAlreadyDeclared is false, then
      1. Call env’s CreateMutableBinding concrete method passing dn and configurableBindings as the arguments.
      2. Call env’s SetMutableBinding concrete method passing dn, undefined, and strict as the arguments.

10.6 Arguments Object

When control enters an execution context for function code, an arguments object is created unless (as specified in 10.5) the identifier arguments occurs as an Identifier in the function’s FormalParameterList or occurs as the Identifier of a VariableDeclaration or FunctionDeclaration contained in the function code.

The arguments object is created by calling the abstract operation CreateArgumentsObject with arguments func the function object whose code is to be evaluated, names a List containing the function’s formal parameter names, args the actual arguments passed to the [[Call]] internal method, env the variable environment for the function code, and strict a Boolean that indicates whether or not the function code is strict code. When CreateArgumentsObject is called the following steps are performed:

  1. Let len be the number of elements in args.
  2. Let obj be the result of creating a new ECMAScript object.
  3. Set all the internal methods of obj as specified in 8.12.
  4. Set the [[Class]] internal property of obj to "Arguments".
  5. Let Object be the standard built-in Object constructor (15.2.2).
  6. Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object (15.2.4).
  7. Call the [[DefineOwnProperty]] internal method on obj passing "length", the Property Descriptor {[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false as arguments.
  8. Let map be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name
  9. Let mappedNames be an empty List.
  10. Let indx = len - 1.
  11. Repeat while indx >= 0,
    1. Let val be the element of args at 0-origined list position indx.
    2. Call the [[DefineOwnProperty]] internal method on obj passing ToString(indx), the property descriptor {[[Value]]: val, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false as arguments.
    3. If indx is less than the number of elements in names, then
      1. Let name be the element of names at 0-origined list position indx.
      2. If strict is false and name is not an element of mappedNames, then
        1. Add name as an element of the list mappedNames.
        2. Let g be the result of calling the MakeArgGetter abstract operation with arguments name and env.
        3. Let p be the result of calling the MakeArgSetter abstract operation with arguments name and env.
        4. Call the [[DefineOwnProperty]] internal method of map passing ToString(indx), the Property Descriptor {[[Set]]: p, [[Get]]: g, [[Configurable]]: true}, and false as arguments.
    4. Let indx = indx - 1
  12. If mappedNames is not empty, then
    1. Set the [[ParameterMap]] internal property of obj to map.
    2. Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods of obj to the definitions provided below.
  13. If strict is false, then
    1. Call the [[DefineOwnProperty]] internal method on obj passing "callee", the property descriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false as arguments.
  14. Else, strict is true so
    1. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
    2. Call the [[DefineOwnProperty]] internal method of obj with arguments "caller", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
    3. Call the [[DefineOwnProperty]] internal method of obj with arguments "callee", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  15. Return obj

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

  1. Let body be the result of concatenating the Strings "return ", name, and ";".
  2. Return the result of creating a function object as described in 13.2 using no FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

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

  1. Let param be the String name concatenated with the String "_arg".
  2. Let body be the String "<name> = <param>;" with <name> replaced by the value of name and <param> replaced by the value of param.
  3. Return the result of creating a function object as described in 13.2 using a List containing the single String param as FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

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

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

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

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

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

  1. Let map be the value of the [[ParameterMap]] internal property of the arguments object.
  2. Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
  3. Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on the arguments object passing P, Desc, and false as the arguments.
  4. If allowed is false, then
    1. If Throw is true then throw a TypeError exception, otherwise return false.
  5. If the value of isMapped is not undefined, then
    1. If IsAccessorDescriptor(Desc) is true, then
      1. Call the [[Delete]] internal method of map passing P, and false as the arguments.
    2. Else
      1. If Desc.[[Value]] is present, then
        1. Call the [[Put]] internal method of map passing P, Desc.[[Value]], and Throw as the arguments.
      2. If Desc.[[Writable]] is present and its value is false, then
        1. Call the [[Delete]] internal method of map passing P and false as arguments.
  6. Return true.

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

  1. Let map be the value of the [[ParameterMap]] internal property of the arguments object.
  2. Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
  3. Let result be the result of calling the default [[Delete]] internal method (8.12.7) on the arguments object passing P and Throw as the arguments.
  4. If result is true and the value of isMapped is not undefined, then
    1. Call the [[Delete]] internal method of map passing P, and false as the arguments.
  5. Return result.

NOTE 1 For non-strict mode functions the array index (defined in 15.4) named data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

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

11 Expressions

11.1 Primary Expressions

Syntax

PrimaryExpression :
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
( Expression )

11.1.1 The this Keyword

The this keyword evaluates to the value of the ThisBinding of the current execution context.

11.1.2 Identifier Reference

An Identifier is evaluated by performing Identifier Resolution as specified in 10.3.1. The result of evaluating an Identifier is always a value of type Reference.

11.1.3 Literal Reference

A Literal is evaluated as described in 7.8.

11.1.4 Array Initialiser

An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal. It is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initialiser is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral :
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elisionopt ]
ElementList :
Elisionopt AssignmentExpression
ElementList , Elisionopt AssignmentExpression
Elision :
,
Elision ,

Semantics

The production ArrayLiteral : [ Elisionopt ] is evaluated as follows:

  1. Let array be the result of creating a new object as if by the expression new Array() where Array is the standard built-in constructor with that name.
  2. Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
  3. Call the [[Put]] internal method of array with arguments "length", pad, and false.
  4. Return array.

The production ArrayLiteral : [ ElementList ] is evaluated as follows:

  1. Return the result of evaluating ElementList.

The production ArrayLiteral : [ ElementList , Elisionopt ] is evaluated as follows:

  1. Let array be the result of evaluating ElementList.
  2. Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
  3. Let len be the result of calling the [[Get]] internal method of array with argument "length".
  4. Call the [[Put]] internal method of array with arguments "length", ToUint32(pad+len), and false.
  5. Return array.

The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:

  1. Let array be the result of creating a new object as if by the expression new Array() where Array is the standard built-in constructor with that name.
  2. Let firstIndex be the result of evaluating Elision; if not present, use the numeric value zero.
  3. Let initResult be the result of evaluating AssignmentExpression.
  4. Let initValue be GetValue(initResult).
  5. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(firstIndex), the Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
  6. Return array.

The production ElementList : ElementList , Elisionopt AssignmentExpression is evaluated as follows:

  1. Let array be the result of evaluating ElementList.
  2. Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
  3. Let initResult be the result of evaluating AssignmentExpression.
  4. Let initValue be GetValue(initResult).
  5. Let len be the result of calling the [[Get]] internal method of array with argument "length".
  6. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32((pad+len)) and the Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
  7. Return array.

The production Elision : , is evaluated as follows:

  1. Return the numeric value 1.

The production Elision : Elision , is evaluated as follows:

  1. Let preceding be the result of evaluating Elision.
  2. Return preceding+1.

NOTE [[DefineOwnProperty]] is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Put]].

11.1.5 Object Initialiser

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

Syntax

ObjectLiteral :
{ }
{ PropertyNameAndValueList }
{ PropertyNameAndValueList , }
PropertyNameAndValueList :
PropertyAssignment
PropertyNameAndValueList , PropertyAssignment
PropertyAssignment :
PropertyName : AssignmentExpression
get PropertyName ( ) { FunctionBody }
set PropertyName ( PropertySetParameterList ) { FunctionBody }
PropertyName :
IdentifierName
StringLiteral
NumericLiteral
PropertySetParameterList :
Identifier

Semantics

The production ObjectLiteral : { } is evaluated as follows:

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

The productions ObjectLiteral : { PropertyNameAndValueList } and
ObjectLiteral : { PropertyNameAndValueList ,} are evaluated as follows:

  1. Return the result of evaluating PropertyNameAndValueList.

The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:

  1. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
  2. Let propId be the result of evaluating PropertyAssignment.
  3. Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name, propId.descriptor, and false.
  4. Return obj.

The production
PropertyNameAndValueList : PropertyNameAndValueList , PropertyAssignment
is evaluated as follows:

  1. Let obj be the result of evaluating PropertyNameAndValueList.
  2. Let propId be the result of evaluating PropertyAssignment.
  3. Let previous be the result of calling the [[GetOwnProperty]] internal method of obj with argument propId.name.
  4. If previous is not undefined then throw a SyntaxError exception if any of the following conditions are true
    1. This production is contained in strict code and IsDataDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
    2. IsDataDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true.
    3. IsAccessorDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
    4. IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true and either both previous and propId.descriptor have [[Get]] fields or both previous and propId.descriptor have [[Set]] fields
  5. Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name, propId.descriptor, and false.
  6. Return obj.

If the above steps would throw a SyntaxError then an implementation must treat the error as an early error (Clause 16).

The production PropertyAssignment : PropertyName : AssignmentExpression is evaluated as follows:

  1. Let propName be the result of evaluating PropertyName.
  2. Let exprValue be the result of evaluating AssignmentExpression.
  3. Let propValue be GetValue(exprValue).
  4. Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}
  5. Return Property Identifier (propName, desc).

The production PropertyAssignment : get PropertyName ( ) { FunctionBody } is evaluated as follows:

  1. Let propName be the result of evaluating PropertyName.
  2. Let closure be the result of creating a new Function object as specified in 13.2 with an empty parameter list and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as the Strict flag if the PropertyAssignment is contained in strict code or if its FunctionBody is strict code.
  3. Let desc be the Property Descriptor{[[Get]]: closure, [[Enumerable]]: true, [[Configurable]]: true}
  4. Return Property Identifier (propName, desc).

The production PropertyAssignment : set PropertyName ( PropertySetParameterList ) { FunctionBody } is evaluated as follows:

  1. Let propName be the result of evaluating PropertyName.
  2. Let closure be the result of creating a new Function object as specified in 13.2 with parameters specified by PropertySetParameterList and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as the Strict flag if the PropertyAssignment is contained in strict code or if its FunctionBody is strict code.
  3. Let desc be the Property Descriptor{[[Set]]: closure, [[Enumerable]]: true, [[Configurable]]: true}
  4. Return Property Identifier (propName, desc).

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code or if its FunctionBody is strict code.

The production PropertyName : IdentifierName is evaluated as follows:

  1. Return the String value containing the same sequence of characters as the IdentifierName.

The production PropertyName : StringLiteral is evaluated as follows:

  1. Return the SV of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:

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

11.1.6 The Grouping Operator

The production PrimaryExpression : ( Expression ) is evaluated as follows:

  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 parenthesised expressions.

11.2 Left-Hand-Side Expressions

Syntax

MemberExpression :
PrimaryExpression
FunctionExpression
MemberExpression [ Expression ]
MemberExpression . IdentifierName
new MemberExpression Arguments
NewExpression :
MemberExpression
new NewExpression
CallExpression :
MemberExpression Arguments
CallExpression Arguments
CallExpression [ Expression ]
CallExpression . IdentifierName
Arguments :
( )
( ArgumentList )
ArgumentList :
AssignmentExpression
ArgumentList , AssignmentExpression
LeftHandSideExpression :
NewExpression
CallExpression

11.2.1 Property Accessors

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

MemberExpression . IdentifierName
CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]

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

The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:

  1. Let baseReference be the result of evaluating MemberExpression.
  2. Let baseValue be GetValue(baseReference).
  3. Let propertyNameReference be the result of evaluating Expression.
  4. Let propertyNameValue be GetValue(propertyNameReference).
  5. Call CheckObjectCoercible(baseValue).
  6. Let propertyNameString be ToString(propertyNameValue).
  7. If the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let strict be false.
  8. Return a value of type Reference whose base value is baseValue and whose referenced name is propertyNameString, and whose strict mode flag is strict.

The production CallExpression : CallExpression [ Expression ] is evaluated in exactly the same manner, except that the contained CallExpression is evaluated in step 1.

11.2.2 The new Operator

The production NewExpression : new NewExpression is evaluated as follows:

  1. Let ref be the result of evaluating NewExpression.
  2. Let constructor be GetValue(ref).
  3. If Type(constructor) is not Object, throw a TypeError exception.
  4. If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
  5. Return the result of calling the [[Construct]] internal method on constructor, providing no arguments (that is, an empty list of arguments).

The production MemberExpression : new MemberExpression Arguments is evaluated as follows:

  1. Let ref be the result of evaluating MemberExpression.
  2. Let constructor be GetValue(ref).
  3. Let argList be the result of evaluating Arguments, producing an internal list of argument values (11.2.4).
  4. If Type(constructor) is not Object, throw a TypeError exception.
  5. If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
  6. Return the result of calling the [[Construct]] internal method on constructor, providing the list argList as the argument values.

11.2.3 Function Calls

The production CallExpression : MemberExpression Arguments is evaluated as follows:

  1. Let ref be the result of evaluating MemberExpression.
  2. Let func be GetValue(ref).
  3. Let argList be the result of evaluating Arguments, producing an internal list of argument values (see 11.2.4).
  4. If Type(func) is not Object, throw a TypeError exception.
  5. If IsCallable(func) is false, throw a TypeError exception.
  6. If Type(ref) is Reference, then
    1. If IsPropertyReference(ref) is true, then
      1. Let thisValue be GetBase(ref).
    2. Else, the base of ref is an Environment Record
      1. Let thisValue be the result of calling the ImplicitThisValue concrete method of GetBase(ref).
  7. Else, Type(ref) is not Reference.
    1. Let thisValue be undefined.
  8. Return the result of calling the [[Call]] internal method on func, providing thisValue as the this value and providing the list argList as the argument values.

The production CallExpression : CallExpression Arguments is evaluated in exactly the same manner, except that the contained CallExpression is evaluated in step 1.

NOTE The returned result will never be of type Reference if func is a native ECMAScript object. Whether calling a host object can return a value of type Reference is implementation-dependent. If a value of type Reference is returned, it must be a non-strict Property Reference.

11.2.4 Argument Lists

The evaluation of an argument list produces a List of values (see 8.8).

The production Arguments : ( ) is evaluated as follows:

  1. Return an empty List.

The production Arguments : ( ArgumentList ) is evaluated as follows:

  1. Return the result of evaluating ArgumentList.

The production ArgumentList : AssignmentExpression is evaluated as follows:

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

The production ArgumentList : ArgumentList , AssignmentExpression is evaluated as follows:

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

11.2.5 Function Expressions

The production MemberExpression : FunctionExpression is evaluated as follows:

  1. Return the result of evaluating FunctionExpression.

11.3 Postfix Expressions

Syntax

PostfixExpression :
LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --

11.3.1 Postfix Increment Operator

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] ++ is evaluated as follows:

  1. Let lhs be the result of evaluating LeftHandSideExpression.
  2. Throw a SyntaxError exception if the following conditions are all true:
  1. Let oldValue be ToNumber(GetValue(lhs)).
  2. Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 11.6.3).
  3. Call PutValue(lhs, newValue).
  4. Return oldValue.

11.3.2 Postfix Decrement Operator

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] -- is evaluated as follows:

  1. Let lhs be the result of evaluating LeftHandSideExpression.
  2. Throw a SyntaxError exception if the following conditions are all true:
  1. Let oldValue be ToNumber(GetValue(lhs)).
  2. Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the - operator (11.6.3).
  3. Call PutValue(lhs, newValue).
  4. Return oldValue.

11.4 Unary Operators

Syntax

UnaryExpression :
PostfixExpression
delete UnaryExpression
void UnaryExpression
typeof UnaryExpression
++ UnaryExpression
-- UnaryExpression
+ UnaryExpression
- UnaryExpression
~ UnaryExpression
! UnaryExpression

11.4.1 The delete Operator

The production UnaryExpression : delete UnaryExpression is evaluated as follows:

  1. Let ref be the result of evaluating UnaryExpression.
  2. If Type(ref) is not Reference, return true.
  3. If IsUnresolvableReference(ref) then,
    1. If IsStrictReference(ref) is true, throw a SyntaxError exception.
    2. Else, return true.
  4. If IsPropertyReference(ref) is true, then
    1. Return the result of calling the [[Delete]] internal method on ToObject(GetBase(ref)) providing GetReferencedName(ref) and IsStrictReference(ref) as the arguments.
  5. Else, ref is a Reference to an Environment Record binding, so
    1. If IsStrictReference(ref) is true, throw a SyntaxError exception.
    2. Let bindings be GetBase(ref).
    3. Return the result of calling the DeleteBinding concrete method of bindings, providing GetReferencedName(ref) as the argument.

NOTE When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError exception is thrown.

11.4.2 The void Operator

The production UnaryExpression : void UnaryExpression is evaluated as follows:

  1. Let expr be the result of evaluating UnaryExpression.
  2. Call GetValue(expr).
  3. Return undefined.

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

11.4.3 The typeof Operator

The production UnaryExpression : typeof UnaryExpression is evaluated as follows:

  1. Let val be the result of evaluating UnaryExpression.
  2. If Type(val) is Reference, then
    1. If IsUnresolvableReference(val) is true, return "undefined".
    2. Let val be GetValue(val).
  3. Return a String determined by Type(val) according to Table 20.
Table 20 — typeof Operator Results
Type of val Result
Undefined "undefined"
Null "object"
Boolean "boolean"
Number "number"
String "string"
Object (native and does not implement [[Call]]) "object"
Object (native or host and does implement [[Call]]) "function"
Object (host and does not implement [[Call]]) Implementation-defined except may not be "undefined", "boolean", "number", or "string".

11.4.4 Prefix Increment Operator

The production UnaryExpression : ++ UnaryExpression is evaluated as follows:

  1. Let expr be the result of evaluating UnaryExpression.
  2. Throw a SyntaxError exception if the following conditions are all true:
  1. Let oldValue be ToNumber(GetValue(expr)).
  2. Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 11.6.3).
  3. Call PutValue(expr, newValue).
  4. Return newValue.

11.4.5 Prefix Decrement Operator

The production UnaryExpression : -- UnaryExpression is evaluated as follows:

  1. Let expr be the result of evaluating UnaryExpression.
  2. Throw a SyntaxError exception if the following conditions are all true:
  1. Let oldValue be ToNumber(GetValue(expr)).
  2. Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the - operator (see 11.6.3).
  3. Call PutValue(expr, newValue).
  4. Return newValue.

11.4.6 Unary + Operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

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

11.4.7 Unary - Operator

The unary - operator converts its operand to Number type and then negates it. Note that negating +0 produces −0, and negating −0 produces +0.

The production UnaryExpression : - UnaryExpression is evaluated as follows:

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

11.4.8 Bitwise NOT Operator ( ~ )

The production UnaryExpression : ~ UnaryExpression is evaluated as follows:

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

11.4.9 Logical NOT Operator ( ! )

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

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

11.5 Multiplicative Operators

Syntax

MultiplicativeExpression :
UnaryExpression
MultiplicativeExpression * UnaryExpression
MultiplicativeExpression / UnaryExpression
MultiplicativeExpression % UnaryExpression

Semantics

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

  1. Let left be the result of evaluating MultiplicativeExpression.
  2. Let leftValue be GetValue(left).
  3. Let right be the result of evaluating UnaryExpression.
  4. Let rightValue be GetValue(right).
  5. Let leftNum be ToNumber(leftValue).
  6. Let rightNum be ToNumber(rightValue).
  7. Return the result of applying the specified operation (*, /, or %) to leftNum and rightNum. See the Notes below 11.5.1, 11.5.2, 11.5.3.

11.5.1 Applying the * Operator

The * operator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision arithmetic:

  • If either operand is NaN, the result is NaN.

  • The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

  • Multiplication of an infinity by a zero results in NaN.

  • Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.

  • Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.

  • In the remaining cases, where neither an infinity or NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

11.5.2 Applying the / Operator

The / operator performs division, producing the quotient of its operands. The left operand is the dividend and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754 arithmetic:

  • If either operand is NaN, the result is NaN.

  • The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

  • Division of an infinity by an infinity results in NaN.

  • Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.

  • Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.

  • Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.

  • Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.

  • Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.

  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

11.5.3 Applying the % Operator

The % operator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.

NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

  • If either operand is NaN, the result is NaN.

    • The sign of the result equals the sign of the dividend.

    • If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.

    • If the dividend is finite and the divisor is an infinity, the result equals the dividend.

    • If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.

    • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n − (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode.

11.6 Additive Operators

Syntax

AdditiveExpression :
MultiplicativeExpression
AdditiveExpression + MultiplicativeExpression
AdditiveExpression - MultiplicativeExpression

11.6.1 The Addition operator ( + )

The addition operator either performs string concatenation or numeric addition.

The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows:

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

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.

11.6.2 The Subtraction Operator ( - )

The production AdditiveExpression : AdditiveExpression - MultiplicativeExpression is evaluated as follows:

  1. Let lref be the result of evaluating AdditiveExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating MultiplicativeExpression.
  4. Let rval be GetValue(rref).
  5. Let lnum be ToNumber(lval).
  6. Let rnum be ToNumber(rval).
  7. Return the result of applying the subtraction operation to lnum and rnum. See the note below 11.6.3.

11.6.3 Applying the Additive Operators to Numbers

The + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The - operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754 binary double-precision arithmetic:

  • If either operand is NaN, the result is NaN.

  • The sum of two infinities of opposite sign is NaN.

  • The sum of two infinities of the same sign is the infinity of that sign.

  • The sum of an infinity and a finite value is equal to the infinite operand.

  • The sum of two negative zeroes is −0. The sum of two positive zeroes, or of two zeroes of opposite sign, is +0.

  • The sum of a zero and a nonzero finite value is equal to the nonzero operand.

  • The sum of two nonzero finite values of the same magnitude and opposite sign is +0.

  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that ab produces the same result as a +(–b).

11.7 Bitwise Shift Operators

Syntax

ShiftExpression :
AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression

11.7.1 The Left Shift Operator ( << )

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

The production ShiftExpression : ShiftExpression << AdditiveExpression is evaluated as follows:

  1. Let lref be the result of evaluating ShiftExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating AdditiveExpression.
  4. Let rval be GetValue(rref).
  5. Let lnum be ToInt32(lval).
  6. Let rnum be ToUint32(rval).
  7. Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
  8. Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit integer.

11.7.2 The Signed Right Shift Operator ( >> )

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The production ShiftExpression : ShiftExpression >> AdditiveExpression is evaluated as follows:

  1. Let lref be the result of evaluating ShiftExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating AdditiveExpression.
  4. Let rval be GetValue(rref).
  5. Let lnum be ToInt32(lval).
  6. Let rnum be ToUint32(rval).
  7. Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
  8. Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.

11.7.3 The Unsigned Right Shift Operator ( >>> )

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The production ShiftExpression : ShiftExpression >>> AdditiveExpression is evaluated as follows:

  1. Let lref be the result of evaluating ShiftExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating AdditiveExpression.
  4. Let rval be GetValue(rref).
  5. Let lnum be ToUint32(lval).
  6. Let rnum be ToUint32(rval).
  7. Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
  8. Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.

11.8 Relational Operators

Syntax

RelationalExpression :
ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression
RelationalExpressionNoIn :
ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression

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

Semantics

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

The RelationalExpressionNoIn productions are evaluated in the same manner as the RelationalExpression productions except that the contained RelationalExpressionNoIn is evaluated instead of the contained RelationalExpression.

11.8.1 The Less-than Operator ( < )

The production RelationalExpression : RelationalExpression < ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
  6. If r is undefined, return false. Otherwise, return r.

11.8.2 The Greater-than Operator ( > )

The production RelationalExpression : RelationalExpression > ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see 11.8.5).
  6. If r is undefined, return false. Otherwise, return r.

11.8.3 The Less-than-or-equal Operator ( <= )

The production RelationalExpression : RelationalExpression <= ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see 11.8.5).
  6. If r is true or undefined, return false. Otherwise, return true.

11.8.4 The Greater-than-or-equal Operator ( >= )

The production RelationalExpression : RelationalExpression >= ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
  6. If r is true or undefined, return false. Otherwise, return true.

11.8.5 The Abstract Relational Comparison Algorithm

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

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

NOTE 1 Step 3 differs from step 7 in the algorithm for the addition operator + (11.6.1) in using and instead of or.

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

11.8.6 The instanceof operator

The production RelationalExpression : RelationalExpression instanceof ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. If Type(rval) is not Object, throw a TypeError exception.
  6. If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
  7. Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

11.8.7 The in operator

The production RelationalExpression : RelationalExpression in ShiftExpression is evaluated as follows:

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be GetValue(rref).
  5. If Type(rval) is not Object, throw a TypeError exception.
  6. Return the result of calling the [[HasProperty]] internal method of rval with argument ToString(lval).

11.9 Equality Operators

Syntax

EqualityExpression :
RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression
EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression
EqualityExpressionNoIn :
RelationalExpressionNoIn
EqualityExpressionNoIn == RelationalExpressionNoIn
EqualityExpressionNoIn != RelationalExpressionNoIn
EqualityExpressionNoIn === RelationalExpressionNoIn
EqualityExpressionNoIn !== RelationalExpressionNoIn

Semantics

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are evaluated instead of the contained EqualityExpression and RelationalExpression, respectively.

11.9.1 The Equals Operator ( == )

The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Return the result of performing abstract equality comparison rval == lval. (see 11.9.3).

11.9.2 The Does-not-equals Operator ( != )

The production EqualityExpression : EqualityExpression != RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing abstract equality comparison rval == lval. (see 11.9.3).
  6. If r is true, return false. Otherwise, return true.

11.9.3 The Abstract Equality Comparison Algorithm

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

  1. If Type(x) is the same as Type(y), then
    1. If Type(x) is Undefined, return true.
    2. If Type(x) is Null, return true.
    3. 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.
    4. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
    5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
    6. Return true if x and y refer to the same object. Otherwise, return false.
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. If Type(x) is Number and Type(y) is String,
    return the result of the comparison x == ToNumber(y).
  5. If Type(x) is String and Type(y) is Number,
    return the result of the comparison ToNumber(x) == y.
  6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  8. If Type(x) is either String or Number and Type(y) is Object,
    return the result of the comparison x == ToPrimitive(y).
  9. If Type(x) is Object and Type(y) is either String or Number,
    return the result of the comparison ToPrimitive(x) == y.
  10. Return false.

NOTE 1 Given the above definition of equality:

  • String comparison can be forced by: "" + a == "" + b.
  • Numeric comparison can be forced by: +a == +b.
  • Boolean comparison can be forced by: !a == !b.

NOTE 2 The equality operators maintain the following invariants:

  • A != B is equivalent to !(A == B).
  • A == B is equivalent to B == A, except in the order of evaluation of A and B.

NOTE 3 The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For Example:

  • new String("a") == "a" and "a" == new String("a")are both true.
  • new String("a") == new String("a") is false.

NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.

11.9.4 The Strict Equals Operator ( === )

The production EqualityExpression : EqualityExpression === RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Return the result of performing the strict equality comparison rval === lval. (See 11.9.6)

11.9.5 The Strict Does-not-equal Operator ( !== )

The production EqualityExpression : EqualityExpression !== RelationalExpression is evaluated as follows:

  1. Let lref be the result of evaluating EqualityExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating RelationalExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of performing strict equality comparison rval === lval. (See 11.9.6)
  6. If r is true, return false. Otherwise, return true.

11.9.6 The Strict Equality Comparison Algorithm

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

  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 return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
  6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  7. Return true if x and y refer to the same object. Otherwise, return false.

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

11.10 Binary Bitwise Operators

Syntax

BitwiseANDExpression :
EqualityExpression
BitwiseANDExpression & EqualityExpression
BitwiseANDExpressionNoIn :
EqualityExpressionNoIn
BitwiseANDExpressionNoIn & EqualityExpressionNoIn
BitwiseXORExpression :
BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression
BitwiseXORExpressionNoIn :
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn
BitwiseORExpression :
BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression
BitwiseORExpressionNoIn :
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

Semantics

The production A : A @ B, where @ is one of the bitwise operators in the productions above, is evaluated as follows:

  1. Let lref be the result of evaluating A.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating B.
  4. Let rval be GetValue(rref).
  5. Let lnum be ToInt32(lval).
  6. Let rnum be ToInt32(rval).
  7. Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32 bit integer.

11.11 Binary Logical Operators

Syntax

LogicalANDExpression :
BitwiseORExpression
LogicalANDExpression && BitwiseORExpression
LogicalANDExpressionNoIn :
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn && BitwiseORExpressionNoIn
LogicalORExpression :
LogicalANDExpression
LogicalORExpression || LogicalANDExpression
LogicalORExpressionNoIn :
LogicalANDExpressionNoIn
LogicalORExpressionNoIn || LogicalANDExpressionNoIn

Semantics

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:

  1. Let lref be the result of evaluating LogicalANDExpression.
  2. Let lval be GetValue(lref).
  3. If ToBoolean(lval) is false, return lval.
  4. Let rref be the result of evaluating BitwiseORExpression.
  5. Return GetValue(rref).

The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as follows:

  1. Let lref be the result of evaluating LogicalORExpression.
  2. Let lval be GetValue(lref).
  3. If ToBoolean(lval) is true, return lval.
  4. Let rref be the result of evaluating LogicalANDExpression.
  5. Return GetValue(rref).

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

NOTE The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

11.12 Conditional Operator ( ? : )

Syntax

ConditionalExpression :
LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression
ConditionalExpressionNoIn :
LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

Semantics

The production ConditionalExpression : LogicalORExpression ? AssignmentExpression : AssignmentExpression is evaluated as follows:

  1. Let lref be the result of evaluating LogicalORExpression.
  2. If ToBoolean(GetValue(lref)) is true, then
    1. Let trueRef be the result of evaluating the first AssignmentExpression.
    2. Return GetValue(trueRef).
  3. Else
    1. Let falseRef be the result of evaluating the second AssignmentExpression.
    2. Return GetValue(falseRef).

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

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

11.13 Assignment Operators

Syntax

AssignmentExpression :
ConditionalExpression
LeftHandSideExpression = AssignmentExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
AssignmentExpressionNoIn :
ConditionalExpressionNoIn
LeftHandSideExpression = AssignmentExpressionNoIn
LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn
AssignmentOperator : one of
*= /= %= += -= <<= >>= >>>= &= ^= |=

Semantics

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

11.13.1 Simple Assignment ( = )

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:

  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let rref be the result of evaluating AssignmentExpression.
  3. Let rval be GetValue(rref).
  4. Throw a SyntaxError exception if the following conditions are all true:
  1. Call PutValue(lref, rval).
  2. Return rval.

NOTE When an assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable reference. If it does a ReferenceError exception is thrown upon assignment. The LeftHandSide also may not be a reference to a data property with the attribute value {[[Writable]]:false}, to an accessor property with the attribute value {[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal property has the value false. In these cases a TypeError exception is thrown.

11.13.2 Compound Assignment ( op= )

The production AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression , where AssignmentOperator is @= and @ represents one of the operators indicated above, is evaluated as follows:

  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let lval be GetValue(lref).
  3. Let rref be the result of evaluating AssignmentExpression.
  4. Let rval be GetValue(rref).
  5. Let r be the result of applying operator @ to lval and rval.
  6. Throw a SyntaxError exception if the following conditions are all true:
  1. Call PutValue(lref, r).
  2. Return r.

NOTE See NOTE 11.13.1.

11.14 Comma Operator ( , )

Syntax

Expression :
AssignmentExpression
Expression , AssignmentExpression
ExpressionNoIn :
AssignmentExpressionNoIn
ExpressionNoIn , AssignmentExpressionNoIn

Semantics

The production Expression : Expression , AssignmentExpression is evaluated as follows:

  1. Let lref be the result of evaluating Expression.
  2. Call GetValue(lref).
  3. Let rref be the result of evaluating AssignmentExpression.
  4. Return GetValue(rref).

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

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

12 Statements

Syntax

Statement :
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement

Semantics

A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements. A LabelledStatement has no semantic meaning other than the introduction of a label to a label set. The label set of an IterationStatement or a SwitchStatement initially contains the single element empty. The label set of any other statement is initially empty.

The result of evaluating a Statement is always a Completion value.

NOTE Several widely used implementations of ECMAScript are known to support the use of FunctionDeclaration as a Statement. However there are significant and irreconcilable variations among the implementations in the semantics applied to such FunctionDeclarations. Because of these irreconcilable differences, the use of a FunctionDeclaration as a Statement results in code that is not reliably portable among implementations. It is recommended that ECMAScript implementations either disallow this usage of FunctionDeclaration or issue a warning when such a usage is encountered. Future editions of ECMAScript may define alternative portable means for declaring functions in a Statement context.

12.1 Block

Syntax

Block :
{ StatementListopt }
StatementList :
Statement
StatementList Statement

Semantics

The production Block : { } is evaluated as follows:

  1. Return (normal, empty, empty).

The production Block : { StatementList } is evaluated as follows:

  1. Return the result of evaluating StatementList.

The production StatementList : Statement is evaluated as follows:

  1. Let s be the result of evaluating Statement.
  2. If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)
  3. Return s.

The production StatementList : StatementList Statement is evaluated as follows:

  1. Let sl be the result of evaluating StatementList.
  2. If sl is an abrupt completion, return sl.
  3. Let s be the result of evaluating Statement.
  4. If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no exception were thrown.)
  5. If s.value is empty, let V = sl.value, otherwise let V = s.value.
  6. Return (s.type, V, s.target).

NOTE Steps 5 and 6 of the above algoritm ensure that the value of a StatementList is the value of the last value producing Statement in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

12.2 Variable Statement

Syntax

VariableStatement :
var VariableDeclarationList ;
VariableDeclarationList :
VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclarationListNoIn :
VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn
VariableDeclaration :
Identifier Initialiseropt
VariableDeclarationNoIn :
Identifier InitialiserNoInopt
Initialiser :
= AssignmentExpression
InitialiserNoIn :
= AssignmentExpressionNoIn

A variable statement declares variables that are created as defined in 10.5. Variables are initialised to undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

Semantics

The production VariableStatement : var VariableDeclarationList ; is evaluated as follows:

  1. Evaluate VariableDeclarationList.
  2. Return (normal, empty, empty).

The production VariableDeclarationList :VariableDeclaration is evaluated as follows:

  1. Evaluate VariableDeclaration.

The production VariableDeclarationList : VariableDeclarationList , VariableDeclaration is evaluated as follows:

  1. Evaluate VariableDeclarationList.
  2. Evaluate VariableDeclaration.

The production VariableDeclaration : Identifier is evaluated as follows:

  1. Return a String value containing the same sequence of characters as in the Identifier.

The production VariableDeclaration : Identifier Initialiser is evaluated as follows:

  1. Let lhs be the result of evaluating Identifier as described in 11.1.2.
  2. Let rhs be the result of evaluating Initialiser.
  3. Let value be GetValue(rhs).
  4. Call PutValue(lhs, value).
  5. Return a String value containing the same sequence of characters as in the Identifier.

NOTE The String value of a VariableDeclaration is used in the evaluation of for-in statements (12.6.4).

If a VariableDeclaration is nested within a with statement and the Identifier in the VariableDeclaration is the same as a property name of the binding object of the with statement’s object environment record, then step 4 will assign value to the property instead of to the VariableEnvironment binding of the Identifier.

The production Initialiser : = AssignmentExpression is evaluated as follows:

  1. Return the result of evaluating AssignmentExpression.

The VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are evaluated in the same manner as the VariableDeclarationList, VariableDeclaration and Initialiser productions except that the contained VariableDeclarationListNoIn, VariableDeclarationNoIn, InitialiserNoIn and AssignmentExpressionNoIn are evaluated instead of the contained VariableDeclarationList, VariableDeclaration, Initialiser and AssignmentExpression, respectively.

12.2.1 Strict Mode Restrictions

It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code and its Identifier is either "eval" or "arguments".

12.3 Empty Statement

Syntax

EmptyStatement :
;

Semantics

The production EmptyStatement : ; is evaluated as follows:

  1. Return (normal, empty, empty).

12.4 Expression Statement

Syntax

ExpressionStatement :
[lookahead ∉ {{, function}] Expression ;

NOTE An ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

Semantics

The production ExpressionStatement : [lookahead ∉ {{, function}] Expression ; is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. Return (normal, GetValue(exprRef), empty).

12.5 The if Statement

Syntax

IfStatement :
if ( Expression ) Statement else Statement
if ( Expression ) Statement

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

Semantics

The production IfStatement : if ( Expression ) Statement else Statement is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. If ToBoolean(GetValue(exprRef)) is true, then
    1. Return the result of evaluating the first Statement.
  3. Else,
    1. Return the result of evaluating the second Statement.

The production IfStatement : if ( Expression ) Statement is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. If ToBoolean(GetValue(exprRef)) is false, return (normal, empty, empty).
  3. Return the result of evaluating Statement.

12.6 Iteration Statements

Syntax

IterationStatement :
do Statement while ( Expression );
while ( Expression ) Statement
for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement
for ( LeftHandSideExpression in Expression ) Statement
for ( var VariableDeclarationNoIn in Expression ) Statement

12.6.1 The do-while Statement

The production do Statement while ( Expression ); is evaluated as follows:

  1. Let V = empty.
  2. Let iterating be true.
  3. Repeat, while iterating is true
    1. Let stmt be the result of evaluating Statement.
    2. If stmt.value is not empty, let V = stmt.value.
    3. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
      2. If stmt is an abrupt completion, return stmt.
    4. Let exprRef be the result of evaluating Expression.
    5. If ToBoolean(GetValue(exprRef)) is false, set iterating to false.
  4. Return (normal, V, empty);

12.6.2 The while Statement

The production IterationStatement : while ( Expression ) Statement is evaluated as follows:

  1. Let V = empty.
  2. Repeat
    1. Let exprRef be the result of evaluating Expression.
    2. If ToBoolean(GetValue(exprRef)) is false, return (normal, V, empty).
    3. Let stmt be the result of evaluating Statement.
    4. If stmt.value is not empty, let V = stmt.value.
    5. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt.type is break and stmt.target is in the current label set, then
        1. Return (normal, V, empty).
      2. If stmt is an abrupt completion, return stmt.

12.6.3 The for Statement

The production
IterationStatement : for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement
is evaluated as follows:

  1. If ExpressionNoIn is present, then.
    1. Let exprRef be the result of evaluating ExpressionNoIn.
    2. Call GetValue(exprRef). (This value is not used but the call may have side-effects.)
  2. Let V = empty.
  3. Repeat
    1. If the first Expression is present, then
      1. Let testExprRef be the result of evaluating the first Expression.
      2. If ToBoolean(GetValue(testExprRef)) is false, return (normal, V, empty).
    2. Let stmt be the result of evaluating Statement.
    3. If stmt.value is not empty, let V = stmt.value
    4. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
    5. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt is an abrupt completion, return stmt.
    6. If the second Expression is present, then
      1. Let incExprRef be the result of evaluating the second Expression.
      2. Call GetValue(incExprRef). (This value is not used.)

The production
IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement
is evaluated as follows:

  1. Evaluate VariableDeclarationListNoIn.
  2. Let V = empty.
  3. Repeat
    1. If the first Expression is present, then
      1. Let testExprRef be the result of evaluating the first Expression.
      2. If ToBoolean(GetValue(testExprRef)) is false, then return (normal, V, empty).
    2. Let stmt be the result of evaluating Statement.
    3. If stmt.value is not empty, let V = stmt.value.
    4. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
    5. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt is an abrupt completion, return stmt.
    6. If the second Expression is present, then.
      1. Let incExprRef be the result of evaluating the second Expression.
      2. Call GetValue(incExprRef). (This value is not used.)

12.6.4 The for-in Statement

The production IterationStatement : for ( LeftHandSideExpression in Expression ) Statement is evaluated as follows:

  1. Let exprRef be the result of evaluating the Expression.
  2. Let experValue be GetValue(exprRef).
  3. If experValue is null or undefined, return (normal, empty, empty).
  4. Let obj be ToObject(experValue).
  5. Let V = empty.
  6. Repeat
    1. Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is no such property, return (normal, V, empty).
    2. Let lhsRef be the result of evaluating the LeftHandSideExpression ( it may be evaluated repeatedly).
    3. Call PutValue(lhsRef, P).
    4. Let stmt be the result of evaluating Statement.
    5. If stmt.value is not empty, let V = stmt.value.
    6. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
    7. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt is an abrupt completion, return stmt.

The production
IterationStatement : for ( var VariableDeclarationNoIn in Expression ) Statement
is evaluated as follows:

  1. Let varName be the result of evaluating VariableDeclarationNoIn.
  2. Let exprRef be the result of evaluating the Expression.
  3. Let experValue be GetValue(exprRef).
  4. If experValue is null or undefined, return (normal, empty, empty).
  5. Let obj be ToObject(experValue).
  6. Let V = empty.
  7. Repeat
    1. Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is no such property, return (normal, V, empty).
    2. Let varRef be the result of evaluating varName as if it were an Identifier Reference (11.1.2); it may be evaluated repeatedly.
    3. Call PutValue(varRef, P).
    4. Let stmt be the result of evaluating Statement.
    5. If stmt.value is not empty, let V = stmt.value.
    6. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
    7. If stmt.type is not continue || stmt.target is not in the current label set, then
      1. If stmt is an abrupt completion, return stmt.

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second) is not specified. Properties of the object being enumerated may be deleted during enumeration. If a property that has not yet been visited during enumeration is deleted, then it will not be visited. If new properties are added to the object being enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A property name must not be visited more than once in any enumeration.

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

NOTE See NOTE 11.13.1.

12.7 The continue Statement

Syntax

ContinueStatement :
continue ;
continue [no LineTerminator here] Identifier ;

Semantics

A program is considered syntactically incorrect if either of the following is true:

A ContinueStatement without an Identifier is evaluated as follows:

  1. Return (continue, empty, empty).

A ContinueStatement with the optional Identifier is evaluated as follows:

  1. Return (continue, empty, Identifier).

12.8 The break Statement

Syntax

BreakStatement :
break ;
break [no LineTerminator here] Identifier ;

Semantics

A program is considered syntactically incorrect if either of the following is true:

A BreakStatement without an Identifier is evaluated as follows:

  1. Return (break, empty, empty).

A BreakStatement with an Identifier is evaluated as follows:

  1. Return (break, empty, Identifier).

12.9 The return Statement

Syntax

ReturnStatement :
return ;
return [no LineTerminator here] Expression ;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains a return statement that is not within a FunctionBody. A return statement causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression.

A ReturnStatement is evaluated as follows:

  1. If the Expression is not present, return (return, undefined, empty).
  2. Let exprRef be the result of evaluating Expression.
  3. Return (return, GetValue(exprRef), empty).

12.10 The with Statement

Syntax

WithStatement :
with ( Expression ) Statement

The with statement adds an object environment record for a computed object to the lexical environment of the current execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

Semantics

The production WithStatement : with ( Expression ) Statement is evaluated as follows:

  1. Let val be the result of evaluating Expression.
  2. Let obj be ToObject(GetValue(val)).
  3. Let oldEnv be the running execution context’s LexicalEnvironment.
  4. Let newEnv be the result of calling NewObjectEnvironment passing obj and oldEnv as the arguments.
  5. Set the provideThis flag of newEnv to true.
  6. Set the running execution context’s LexicalEnvironment to newEnv.
  7. Let C be the result of evaluating Statement but if an exception is thrown during the evaluation, let C be (throw, V, empty), where V is the exception. (Execution now proceeds as if no exception were thrown.)
  8. Set the running execution context’s Lexical Environment to oldEnv.
  9. Return C.

NOTE No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

12.10.1 Strict Mode Restrictions

Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a context is treated as a SyntaxError.

12.11 The switch Statement

Syntax

SwitchStatement :
switch ( Expression ) CaseBlock
CaseBlock :
{ CaseClausesopt }
{ CaseClausesopt DefaultClause CaseClausesopt }
CaseClauses :
CaseClause
CaseClauses CaseClause
CaseClause :
case Expression : StatementListopt
DefaultClause :
default : StatementListopt

Semantics

The production SwitchStatement : switch ( Expression ) CaseBlock is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. Let R be the result of evaluating CaseBlock, passing it GetValue(exprRef) as a parameter.
  3. If R.type is break and R.target is in the current label set, return (normal, R.value, empty).
  4. Return R.

The production CaseBlock : { CaseClausesopt } is given an input parameter, input, and is evaluated as follows:

  1. Let V = empty.
  2. Let A be the list of CaseClause items in source text order.
  3. Let searching be true.
  4. Repeat, while searching is true
    1. Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V, empty).
    2. Let clauseSelector be the result of evaluating C.
    3. If input is equal to clauseSelector as defined by the === operator, then
      1. Set searching to false.
      2. If C has a StatementList, then
        1. Evaluate C’s StatementList and let R be the result.
        2. If R is an abrupt completion, then return R.
        3. Let V = R.value.
  5. Repeat
    1. Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V, empty).
    2. If C has a StatementList, then
      1. Evaluate C’s StatementList and let R be the result.
      2. If R.value is not empty, then let V = R.value.
      3. If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt } is given an input parameter, input, and is evaluated as follows:

  1. Let V = empty.
  2. Let A be the list of CaseClause items in the first CaseClauses, in source text order.
  3. Let B be the list of CaseClause items in the second CaseClauses, in source text order.
  4. Let found be false.
  5. Repeat letting C be in order each CaseClause in A
    1. If found is false, then
      1. Let clauseSelector be the result of evaluating C.
      2. If input is equal to clauseSelector as defined by the === operator, then set found to true.
    2. If found is true, then
      1. If C has a StatementList, then
        1. Evaluate C’s StatementList and let R be the result.
        2. If R.value is not empty, then let V = R.value.
        3. R is an abrupt completion, then return (R.type, V, R.target).
  6. Let foundInB be false.
  7. If found is false, then
    1. Repeat, while foundInB is false and all elements of B have not been processed
      1. Let C be the next CaseClause in B.
      2. Let clauseSelector be the result of evaluating C.
      3. If input is equal to clauseSelector as defined by the === operator, then
        1. Set foundInB to true.
        2. If C has a StatementList, then
          1. Evaluate C’s StatementList and let R be the result.
          2. If R.value is not empty, then let V = R.value.
          3. R is an abrupt completion, then return (R.type, V, R.target).
  8. If foundInB is false and the DefaultClause has a StatementList, then
    1. Evaluate the DefaultClause’s StatementList and let R be the result.
    2. If R.value is not empty, then let V = R.value.
    3. If R is an abrupt completion, then return (R.type, V, R.target).
  9. Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning of B)
    1. Let C be the next CaseClause in B. If there is no such CaseClause, return (normal, V, empty).
    2. If C has a StatementList, then
      1. Evaluate C’s StatementList and let R be the result.
      2. If R.value is not empty, then let V = R.value.
      3. If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseClause : case Expression : StatementListopt is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. Return GetValue(exprRef).

NOTE Evaluating CaseClause does not execute the associated StatementList. It simply evaluates the Expression and returns the value, which the CaseBlock algorithm uses to determine which StatementList to start executing.

12.12 Labelled Statements

Syntax

LabelledStatement :
Identifier : Statement

Semantics

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement.

An ECMAScript program is considered syntactically incorrect if it contains a LabelledStatement that is enclosed by a LabelledStatement with the same Identifier as label. This does not apply to labels appearing within the body of a FunctionDeclaration that is nested, directly or indirectly, within a labelled statement.

The production Identifier : Statement is evaluated by adding Identifier to the label set of Statement and then evaluating Statement. If the LabelledStatement itself has a non-empty label set, these labels are also added to the label set of Statement before evaluating it. If the result of evaluating Statement is (break, V, L) where L is equal to Identifier, the production results in (normal, V, empty).

Prior to the evaluation of a LabelledStatement, the contained Statement is regarded as possessing an empty label set, unless it is an IterationStatement or a SwitchStatement, in which case it is regarded as possessing a label set consisting of the single element, empty.

12.13 The throw Statement

Syntax

ThrowStatement :
throw [no LineTerminator here] Expression ;

Semantics

The production ThrowStatement : throw [no LineTerminator here] Expression ; is evaluated as follows:

  1. Let exprRef be the result of evaluating Expression.
  2. Return (throw, GetValue(exprRef), empty).

12.14 The try Statement

Syntax

TryStatement :
try Block Catch
try Block Finally
try Block Catch Finally
Catch :
catch ( Identifier ) Block
Finally :
finally Block

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its Identifier is bound to that exception.

Semantics

The production TryStatement : try Block Catch is evaluated as follows:

  1. Let B be the result of evaluating Block.
  2. If B.type is not throw, return B.
  3. Return the result of evaluating Catch with parameter B.value.

The production TryStatement : try Block Finally is evaluated as follows:

  1. Let B be the result of evaluating Block.
  2. Let F be the result of evaluating Finally.
  3. If F.type is normal, return B.
  4. Return F.

The production TryStatement : try Block Catch Finally is evaluated as follows:

  1. Let B be the result of evaluating Block.
  2. If B.type is throw, then
    1. Let C be the result of evaluating Catch with parameter B.value.
  3. Else, B.type is not throw,
    1. Let C be B.
  4. Let F be the result of evaluating Finally.
  5. If F.type is normal, return C.
  6. Return F.

The production Catch : catch ( Identifier ) Block is evaluated as follows:

  1. Let C be the parameter that has been passed to this production.
  2. Let oldEnv be the running execution context’s LexicalEnvironment.
  3. Let catchEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
  4. Call the CreateMutableBinding concrete method of catchEnv passing the Identifier String value as the argument.
  5. Call the SetMutableBinding concrete method of catchEnv passing the Identifier, C, and false as arguments. Note that the last argument is immaterial in this situation.
  6. Set the running execution context’s LexicalEnvironment to catchEnv.
  7. Let B be the result of evaluating Block.
  8. Set the running execution context’s LexicalEnvironment to oldEnv.
  9. Return B.

NOTE No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

The production Finally : finally Block is evaluated as follows:

  1. Return the result of evaluating Block.

12.14.1 Strict Mode Restrictions

It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier of the Catch production is either "eval" or "arguments".

12.15 The debugger statement

Syntax

DebuggerStatement :
debugger ;

Semantics

Evaluating the DebuggerStatement production may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

The production DebuggerStatement : debugger ; is evaluated as follows:

  1. If an implementation defined debugging facility is available and enabled, then
    1. Perform an implementation defined debugging action.
    2. Let result be an implementation defined Completion value.
  2. Else
    1. Let result be (normal, empty, empty).
  3. Return result.

13 Function Definition

Syntax

FunctionDeclaration :
function Identifier ( FormalParameterListopt ) { FunctionBody }
FunctionExpression :
function Identifieropt ( FormalParameterListopt ) { FunctionBody }
FormalParameterList :
Identifier
FormalParameterList , Identifier
FunctionBody :
SourceElementsopt

Semantics

The production
FunctionDeclaration : function Identifier ( FormalParameterListopt ) { FunctionBody }
is instantiated as follows during Declaration Binding instantiation (10.5):

  1. Return the result of creating a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt, and body specified by FunctionBody. Pass in the VariableEnvironment of the running execution context as the Scope. Pass in true as the Strict flag if the FunctionDeclaration is contained in strict code or if its FunctionBody is strict code.

The production
FunctionExpression : function ( FormalParameterListopt ) { FunctionBody }
is evaluated as follows:

  1. Return the result of creating a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as the Strict flag if the FunctionExpression is contained in strict code or if its FunctionBody is strict code.

The production
FunctionExpression : function Identifier ( FormalParameterListopt ) { FunctionBody }
is evaluated as follows:

  1. Let funcEnv be the result of calling NewDeclarativeEnvironment passing the running execution context’s Lexical Environment as the argument
  2. Let envRec be funcEnv’s environment record.
  3. Call the CreateImmutableBinding concrete method of envRec passing the String value of Identifier as the argument.
  4. Let closure be the result of creating a new Function object as specified in 13.2 with parameters specified by FormalParameterListopt and body specified by FunctionBody. Pass in funcEnv as the Scope. Pass in true as the Strict flag if the FunctionExpression is contained in strict code or if its FunctionBody is strict code.
  5. Call the InitializeImmutableBinding concrete method of envRec passing the String value of Identifier and closure as the arguments.
  6. Return closure.

NOTE The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

The production FunctionBody : SourceElementsopt is evaluated as follows:

  1. The code of this FunctionBody is strict mode code if it is part of a FunctionDeclaration or FunctionExpression that is contained in strict mode code or if the Directive Prologue (14.1) of its SourceElements contains a Use Strict Directive or if any of the conditions in 10.1.1 apply. If the code of this FunctionBody is strict mode code, SourceElements is evaluated in the following steps as strict mode code. Otherwise, SourceElements is evaluated in the following steps as non-strict mode code.
  2. If SourceElements is present return the result of evaluating SourceElements.
  3. Else return (normal, undefined, empty).

13.1 Strict Mode Restrictions

It is a SyntaxError if any Identifier value occurs more than once within a FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression.

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs within a FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression.

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier of a strict mode FunctionDeclaration or FunctionExpression.

13.2 Creating Function Objects

Given an optional parameter list specified by FormalParameterList, a body specified by FunctionBody, a Lexical Environment specified by Scope, and a Boolean flag Strict, a Function object is constructed as follows:

  1. Create a new native ECMAScript object and let F be that object.
  2. Set all the internal methods, except for [[Get]], of F as described in 8.12.
  3. Set the [[Class]] internal property of F to "Function".
  4. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
  5. Set the [[Get]] internal property of F as described in 15.3.5.4.
  6. Set the [[Call]] internal property of F as described in 13.2.1.
  7. Set the [[Construct]] internal property of F as described in 13.2.2.
  8. Set the [[HasInstance]] internal property of F as described in 15.3.5.3.
  9. Set the [[Scope]] internal property of F to the value of Scope.
  10. Let names be a List containing, in left to right textual order, the Strings corresponding to the identifiers of FormalParameterList. If no parameters are specified, let names be the empty list.
  11. Set the [[FormalParameters]] internal property of F to names.
  12. Set the [[Code]] internal property of F to FunctionBody.
  13. Set the [[Extensible]] internal property of F to true.
  14. Let len be the number of formal parameters specified in FormalParameterList. If no parameters are specified, let len be 0.
  15. Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor {[[Value]]: len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  16. Let proto be the result of creating a new object as would be constructed by the expression new Object()where Object is the standard built-in constructor with that name.
  17. Call the [[DefineOwnProperty]] internal method of proto with arguments "constructor", Property Descriptor {[[Value]]: F, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false.
  18. Call the [[DefineOwnProperty]] internal method of F with arguments "prototype", Property Descriptor {[[Value]]: proto, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  19. If Strict is true, then
    1. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
    2. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
    3. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  20. Return F.

NOTE A prototype property is automatically created for every function, to allow for the possibility that the function will be used as a constructor.

13.2.1 [[Call]]

When the [[Call]] internal method for a Function object F is called with a this value and a list of arguments, the following steps are taken:

  1. Let funcCtx be the result of establishing a new execution context for function code using the value of F's [[FormalParameters]] internal property, the passed arguments List args, and the this value as described in 10.4.3.
  2. Let result be the result of evaluating the FunctionBody that is the value of F's [[Code]] internal property. If F does not have a [[Code]] internal property or if its value is an empty FunctionBody, then result is (normal, undefined, empty).
  3. Exit the execution context funcCtx, restoring the previous execution context.
  4. If result.type is throw then throw result.value.
  5. If result.type is return then return result.value.
  6. Otherwise result.type must be normal. Return undefined.

13.2.2 [[Construct]]

When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:

  1. Let obj be a newly created native ECMAScript object.
  2. Set all the internal methods of obj as specified in 8.12.
  3. Set the [[Class]] internal property of obj to "Object".
  4. Set the [[Extensible]] internal property of obj to true.
  5. Let proto be the value of calling the [[Get]] internal property of F with argument "prototype".
  6. If Type(proto) is Object, set the [[Prototype]] internal property of obj to proto.
  7. If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in Object prototype object as described in 15.2.4.
  8. Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
  9. If Type(result) is Object then return result.
  10. Return obj.

13.2.3 The [[ThrowTypeError]] Function Object

The [[ThrowTypeError]] object is a unique function object that is defined once as follows:

  1. Create a new native ECMAScript object and let F be that object.
  2. Set all the internal methods of F as described in 8.12.
  3. Set the [[Class]] internal property of F to "Function".
  4. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
  5. Set the [[Call]] internal property of F as described in 13.2.1.
  6. Set the [[Scope]] internal property of F to the Global Environment.
  7. Set the [[FormalParameters]] internal property of F to an empty List.
  8. Set the [[Code]] internal property of F to be a FunctionBody that unconditionally throws a TypeError exception and performs no other action.
  9. Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor {[[Value]]: 0, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  10. Set the [[Extensible]] internal property of F to false.
  11. Let [[ThrowTypeError]] be F.

14 Program

Syntax

Program :
SourceElementsopt
SourceElements :
SourceElement
SourceElements SourceElement
SourceElement :
Statement
FunctionDeclaration

Semantics

The production Program : SourceElementsopt is evaluated as follows:

  1. The code of this Program is strict mode code if the Directive Prologue (14.1) of its SourceElements contains a Use Strict Directive or if any of the conditions of 10.1.1 apply. If the code of this Program is strict mode code, SourceElements is evaluated in the following steps as strict mode code. Otherwise SourceElements is evaluated in the following steps as non-strict mode code.
  2. If SourceElements is not present, return (normal, empty, empty).
  3. Let progCxt be a new execution context for global code as described in 10.4.1.
  4. Let result be the result of evaluating SourceElements.
  5. Exit the execution context progCxt.
  6. Return result.

NOTE The processes for initiating the evaluation of a Program and for dealing with the result of such an evaluation are defined by an ECMAScript implementation and not by this specification.

The production SourceElements : SourceElements SourceElement is evaluated as follows:

  1. Let headResult be the result of evaluating SourceElements.
  2. If headResult is an abrupt completion, return headResult.
  3. Let tailResult be result of evaluating SourceElement.
  4. If tailResult.value is empty, let V = headResult.value, otherwise let V = tailResult.value.
  5. Return (tailResult.type, V, tailResult.target)

The production SourceElement : Statement is evaluated as follows:

  1. Return the result of evaluating Statement.

The production SourceElement : FunctionDeclaration is evaluated as follows:

  1. Return (normal, empty, empty).

14.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact character sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

NOTE The ExpressionStatement productions of a Directive Prologue are evaluated normally during evaluation of the containing SourceElements production. Implementations may define implementation specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive or which does not have a meaning defined by the implementation.

15 Standard Built-in ECMAScript Objects

There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the global object.

Unless specified otherwise, the [[Class]] internal property of a built-in object is "Function" if that built-in object has a [[Call]] internal property, or "Object" if that built-in object does not have a [[Call]] internal property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has the value true.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and properties of the Function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.

NOTE Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (15.3.4), as the value of its [[Prototype]] internal property.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (15.2.4), as the value of its [[Prototype]] internal property, except the Object prototype object itself.

None of the built-in functions described in this clause that are not constructors shall implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. None of the built-in functions described in this clause shall have a prototype property unless otherwise specified in the description of a particular function.

This clause generally describes distinct behaviours for when a constructor is “called as a function” and for when it is “called as part of a new expression”. The “called as a function” behaviour corresponds to the invocation of the constructor’s [[Call]] internal method and the “called as part of a new expression” behaviour corresponds to the invocation of the constructor’s [[Construct]] internal method.

Every built-in Function object described in this clause—whether as a constructor, an ordinary function, or both—has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description, including optional parameters.

NOTE For example, the Function object that is the initial value of the slice property of the String prototype object is described under the subclause heading “String.prototype.slice (start, end)” which shows the two named arguments start and end; therefore the value of the length property of that Function object is 2.

In every case, the length property of a built-in Function object described in this clause has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other property described in this clause has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.

15.1 The Global Object

The unique global object is created before control enters any execution context.

Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}.

The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a constructor with the new operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.

15.1.1 Value Properties of the Global Object

15.1.1.1 NaN

The value of NaN is NaN (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.2 Infinity

The value of Infinity is +∞ (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.3 undefined

The value of undefined is undefined (see 8.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.2 Function Properties of the Global Object

15.1.2.1 eval (x)

When the eval function is called with one argument x, the following steps are taken:

  1. If Type(x) is not String, return x.
  2. Let prog be the ECMAScript code that is the result of parsing x as a Program. If the parse fails, throw a SyntaxError exception (but see also clause 16).
  3. Let evalCtx be the result of establishing a new execution context (10.4.2) for the eval code prog.
  4. Let result be the result of evaluating the program prog.
  5. Exit the running execution context evalCtx, restoring the previous execution context.
  6. If result.type is normal and its completion value is a value V, then return the value V.
  7. If result.type is normal and its completion value is empty, then return the value undefined.
  8. Otherwise, result.type must be throw. Throw result.value as an exception.

15.1.2.1.1 Direct Call to Eval

A direct call to the eval function is one that is expressed as a CallExpression that meets the following two conditions:

The Reference that is the result of evaluating the MemberExpression in the CallExpression has an environment record as its base value and its reference name is "eval".

The result of calling the abstract operation GetValue with that Reference as the argument is the standard built-in function defined in 15.1.2.1.

15.1.2.2 parseInt (string , radix)

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the character pairs 0x or 0X.

When the parseInt function is called, the following steps are taken:

  1. Let inputString be ToString(string).
  2. Let S be a newly created substring of inputString consisting of the first character that is not a StrWhiteSpaceChar and all characters following that character. (In other words, remove leading white space.) If inputString does not contain any such characters, let S be the empty string.
  3. Let sign be 1.
  4. If S is not empty and the first character of S is a minus sign -, let sign be −1.
  5. If S is not empty and the first character of S is a plus sign + or a minus sign -, then remove the first character from S.
  6. Let R = ToInt32(radix).
  7. Let stripPrefix be true.
  8. If R ≠ 0, then
    1. If R < 2 or R > 36, then return NaN.
    2. If R ≠ 16, let stripPrefix be false.
  9. Else, R = 0
    1. Let R = 10.
  10. If stripPrefix is true, then
    1. If the length of S is at least 2 and the first two characters of S are either “0x” or “0X”, then remove the first two characters from S and let R = 16.
  11. If S contains any character that is not a radix-R digit, then let Z be the substring of S consisting of all characters before the first such character; otherwise, let Z be S.
  12. If Z is empty, return NaN.
  13. Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
  14. Let number be the Number value for mathInt.
  15. Return sign × number.

NOTE parseInt may interpret only a leading portion of string as an integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no indication is given that any such characters were ignored.

15.1.2.3 parseFloat (string)

The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.

When the parseFloat function is called, the following steps are taken:

  1. Let inputString be ToString(string).
  2. Let trimmedString be a substring of inputString consisting of the leftmost character that is not a StrWhiteSpaceChar and all characters to the right of that character. (In other words, remove leading white space.) If inputString does not contain any such characters, let trimmedString be the empty string.
  3. If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 9.3.1), return NaN.
  4. Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
  5. Return the Number value for the MV of numberString.

NOTE parseFloat may interpret only a leading portion of string as a Number value; it ignores any characters that cannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such characters were ignored.

15.1.2.4 isNaN (number)

Returns true if the argument coerces to NaN, and otherwise returns false.

  1. If ToNumber(number) is NaN, return true.
  2. Otherwise, return false.

NOTE A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.

15.1.2.5 isFinite (number)

Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.

  1. If ToNumber(number) is NaN, +∞, or −∞, return false.
  2. Otherwise, return true.

15.1.3 URI Handling Function Properties

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4.

NOTE Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme : First / Second ; Third ? Fourth

where the italicised names represent components and “:”, “/”, “;” and “?” are reserved characters used as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved characters in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved characters represent text and so must be encoded so that they are not interpreted as reserved characters when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

uri :::
uriCharactersopt
uriCharacters :::
uriCharacter uriCharactersopt
uriCharacter :::
uriReserved
uriUnescaped
uriEscaped
uriReserved ::: one of
; / ? : @ & = + $ ,
uriUnescaped :::
uriAlpha
DecimalDigit
uriMark
uriEscaped :::
% HexDigit HexDigit
uriAlpha ::: one of
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
uriMark ::: one of
- _ . ! ~ * ' ( )

NOTE The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved characters, that character must be encoded. The character is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form “%xx”.

The encoding and escaping process is described by the abstract operation Encode taking two String arguments string and unescapedSet.

  1. Let strLen be the number of characters in string.
  2. Let R be the empty String.
  3. Let k be 0.
  4. Repeat
    1. If k equals strLen, return R.
    2. Let C be the character at position k within string.
    3. If C is in unescapedSet, then
      1. Let S be a String containing only the character C.
      2. Let R be a new String value computed by concatenating the previous value of R and S.
    4. Else, C is not in unescapedSet
      1. If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a URIError exception.
      2. If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then
        1. Let V be the code unit value of C.
      3. Else,
        1. Increase k by 1.
        2. If k equals strLen, throw a URIError exception.
        3. Let kChar be the code unit value of the character at position k within string.
        4. If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.
        5. Let V be (((the code unit value of C) – 0xD800) × 0x400 + (kChar – 0xDC00) + 0x10000).
      4. Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, and let L be the array size.
      5. Let j be 0.
      6. Repeat, while j < L
        1. Let jOctet be the value at position j within Octets.
        2. Let S be a String containing three characters “%XY” where XY are two uppercase hexadecimal digits encoding the value of jOctet.
        3. Let R be a new String value computed by concatenating the previous value of R and S.
        4. Increase j by 1.
    5. Increase k by 1.

The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.

  1. Let strLen be the number of characters in string.
  2. Let R be the empty String.
  3. Let k be 0.
  4. Repeat
    1. If k equals strLen, return R.
    2. Let C be the character at position k within string.
    3. If C is not ‘%’, then
      1. Let S be the String containing only the character C.
    4. Else, C is ‘%
      1. Let start be k.
      2. If k + 2 is greater than or equal to strLen, throw a URIError exception.
      3. If the characters at position (k+1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
      4. Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
      5. Increment k by 2.
      6. If the most significant bit in B is 0, then
        1. Let C be the character with code unit value B.
        2. If C is not in reservedSet, then
          1. Let S be the String containing only the character C.
        3. Else, C is in reservedSet
          1. Let S be the substring of string from position start to position k included.
      7. Else, the most significant bit in B is 1
        1. Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.
        2. If n equals 1 or n is greater than 4, throw a URIError exception.
        3. Let Octets be an array of 8-bit integers of size n.
        4. Put B into Octets at position 0.
        5. If k + (3 × (n – 1)) is greater than or equal to strLen, throw a URIError exception.
        6. Let j be 1.
        7. Repeat, while j < n
          1. Increment k by 1.
          2. If the character at position k is not ‘%’, throw a URIError exception.
          3. If the characters at position (k +1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
          4. Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
          5. If the two most significant bits in B are not 10, throw a URIError exception.
          6. Increment k by 2.
          7. Put B into Octets at position j.
          8. Increment j by 1.
        8. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 21-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode code point throw an URIError exception.
        9. If V is less than 0x10000, then
          1. Let C be the character with code unit value V.
          2. If C is not in reservedSet, then
            1. Let S be the String containing only the character C.
          3. Else, C is in reservedSet
            1. Let S be the substring of string from position start to position k included.
        10. Else, V is ≥ 0x10000
          1. Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).
          2. Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
          3. Let S be the String containing the two characters with code unit values H and L.
    5. Let R be a new String value computed by concatenating the previous value of R and S.
    6. Increase k by 1.

NOTE This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 21.

Table 21 — UTF-8 Encodings
Code Unit Value Representation 1st Octet 2nd Octet 3rd Octet 4th Octet
0x0000 - 0x007F 00000000 0zzzzzzz 0zzzzzzz
0x0080 - 0x07FF 00000yyy yyzzzzzz 110yyyyy 10zzzzzz
0x0800 - 0xD7FF xxxxyyyy yyzzzzzz 1110xxxx 10yyyyyy 10zzzzzz

0xD800 - 0xDBFF

followed by

0xDC00 – 0xDFFF

110110vv vvwwwwxx

followed by

110111yy yyzzzzzz

11110uuu 10uuwwww 10xxyyyy 10zzzzzz

0xD800 - 0xDBFF

not followed by

0xDC00 – 0xDFFF

causes URIError
0xDC00 – 0xDFFF causes URIError
0xE000 - 0xFFFF xxxxyyyy yyzzzzzz 1110xxxx 10yyyyyy 10zzzzzz

Where

uuuuu = vvvv + 1

to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.

The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the character U+0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.

15.1.3.1 decodeURI (encodedURI)

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the character that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

When the decodeURI function is called with one argument encodedURI, the following steps are taken:

  1. Let uriString be ToString(encodedURI).
  2. Let reservedURISet be a String containing one instance of each character valid in uriReserved plus “#”.
  3. Return the result of calling Decode(uriString, reservedURISet)

NOTE The character “#” is not decoded from escape sequences even though it is not a reserved URI character.

15.1.3.2 decodeURIComponent (encodedURIComponent)

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the character that it represents.

When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

  1. Let componentString be ToString(encodedURIComponent).
  2. Let reservedURIComponentSet be the empty String.
  3. Return the result of calling Decode(componentString, reservedURIComponentSet)

15.1.3.3 encodeURI (uri)

The encodeURI function computes a new version of a URI in which each instance of certain characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When the encodeURI function is called with one argument uri, the following steps are taken:

  1. Let uriString be ToString(uri).
  2. Let unescapedURISet be a String containing one instance of each character valid in uriReserved and uriUnescaped plus “#”.
  3. Return the result of calling Encode(uriString, unescapedURISet)

NOTE The character “#” is not encoded to an escape sequence even though it is not a reserved or unescaped URI character.

15.1.3.4 encodeURIComponent (uriComponent)

The encodeURIComponent function computes a new version of a URI in which each instance of certain characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

  1. Let componentString be ToString(uriComponent).
  2. Let unescapedURIComponentSet be a String containing one instance of each character valid in uriUnescaped.
  3. Return the result of calling Encode(componentString, unescapedURIComponentSet)

15.1.4 Constructor Properties of the Global Object

15.1.4.1 Object ( . . . )

See 15.2.1 and 15.2.2.

15.1.4.2 Function ( . . . )

See 15.3.1 and 15.3.2.

15.1.4.3 Array ( . . . )

See 15.4.1 and 15.4.2.

15.1.4.4 String ( . . . )

See 15.5.1 and 15.5.2.

15.1.4.5 Boolean ( . . . )

See 15.6.1 and 15.6.2.

15.1.4.6 Number ( . . . )

See 15.7.1 and 15.7.2.

15.1.4.7 Date ( . . . )

See 15.9.2.

15.1.4.8 RegExp ( . . . )

See 15.10.3 and 15.10.4.

15.1.4.9 Error ( . . . )

See 15.11.1 and 15.11.2.

15.1.4.10 EvalError ( . . . )

See 15.11.6.1.

15.1.4.11 RangeError ( . . . )

See 15.11.6.2.

15.1.4.12 ReferenceError ( . . . )

See 15.11.6.3.

15.1.4.13 SyntaxError ( . . . )

See 15.11.6.4.

15.1.4.14 TypeError ( . . . )

See 15.11.6.5.

15.1.4.15 URIError ( . . . )

See 15.11.6.6.

15.1.5 Other Properties of the Global Object

15.1.5.1 Math

See 15.8.

15.1.5.2 JSON

See 15.12.

15.2 Object Objects

15.2.1 The Object Constructor Called as a Function

When Object is called as a function rather than as a constructor, it performs a type conversion.

15.2.1.1 Object ( [ value ] )

When the Object function is called with no arguments or with one argument value, the following steps are taken:

  1. If value is null, undefined or not supplied, create and return a new Object object exactly as if the standard built-in Object constructor had been called with the same arguments (15.2.2.1).
  2. Return ToObject(value).

15.2.2 The Object Constructor

When Object is called as part of a new expression, it is a constructor that may create an object.

15.2.2.1 new Object ( [ value ] )

When the Object constructor is called with no arguments or with one argument value, the following steps are taken:

  1. If value is supplied, then
    1. If Type(value) is Object, then
      1. If the value is a native ECMAScript object, do not create a new object but simply return value.
      2. If the value is a host object, then actions are taken and a result is returned in an implementation-dependent manner that may depend on the host object.
    2. If Type(value) is String, return ToObject(value).
    3. If Type(value) is Boolean, return ToObject(value).
    4. If Type(value) is Number, return ToObject(value).
  2. Assert: The argument value was not supplied or its type was Null or Undefined.
  3. Let obj be a newly created native ECMAScript object.
  4. Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object (15.2.4).
  5. Set the [[Class]] internal property of obj to "Object".
  6. Set the [[Extensible]] internal property of obj to true.
  7. Set all the internal methods of obj as specified in 8.12.
  8. Return obj.

15.2.3 Properties of the Object Constructor

The value of the [[Prototype]] internal property of the Object constructor is the standard built-in Function prototype object.

Besides the internal properties and the length property (whose value is 1), the Object constructor has the following properties:

15.2.3.1 Object.prototype

The initial value of Object.prototype is the standard built-in Object prototype object (15.2.4).

This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.2.3.2 Object.getPrototypeOf ( O )

When the getPrototypeOf function is called with argument O, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Return the value of the [[Prototype]] internal property of O.

15.2.3.3 Object.getOwnPropertyDescriptor ( O, P )

When the getOwnPropertyDescriptor function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Let name be ToString(P).
  3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
  4. Return the result of calling FromPropertyDescriptor(desc) (8.10.4).

15.2.3.4 Object.getOwnPropertyNames ( O )

When the getOwnPropertyNames function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Let array be the result of creating a new object as if by the expression new Array () where Array is the standard built-in constructor with that name.
  3. Let n be 0.
  4. For each named own property P of O
    1. Let name be the String value that is the name of P.
    2. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(n), the PropertyDescriptor {[[Value]]: name, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    3. Increment n by 1.
  5. Return array.

NOTE If O is a String instance, the set of own properties processed in step 4 includes the implicit properties defined in 15.5.5.2 that correspond to character positions within the object’s [[PrimitiveValue]] String.

15.2.3.5 Object.create ( O [, Properties] )

The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:

  1. If Type(O) is not Object or Null throw a TypeError exception.
  2. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name
  3. Set the [[Prototype]] internal property of obj to O.
  4. If the argument Properties is present and not undefined, add own properties to obj as if by calling the standard built-in function Object.defineProperties with arguments obj and Properties.
  5. Return obj.

15.2.3.6 Object.defineProperty ( O, P, Attributes )

The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Let name be ToString(P).
  3. Let desc be the result of calling ToPropertyDescriptor with Attributes as the argument.
  4. Call the [[DefineOwnProperty]] internal method of O with arguments name, desc, and true.
  5. Return O.

15.2.3.7 Object.defineProperties ( O, Properties )

The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Let props be ToObject(Properties).
  3. Let names be an internal list containing the names of each enumerable own property of props.
  4. Let descriptors be an empty internal List.
  5. For each element P of names in list order,
    1. Let descObj be the result of calling the [[Get]] internal method of props with P as the argument.
    2. Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
    3. Append the pair (a two element List) consisting of P and desc to the end of descriptors.
  6. For each pair from descriptors in list order,
    1. Let P be the first element of pair.
    2. Let desc be the second element of pair.
    3. Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.
  7. Return O.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used to order the list elements in step 3 of this algorithm.

15.2.3.8 Object.seal ( O )

When the seal function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. For each named own property name P of O,
    1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
    3. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
  3. Set the [[Extensible]] internal property of O to false.
  4. Return O.

15.2.3.9 Object.freeze ( O )

When the freeze function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. For each named own property name P of O,
    1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2. If IsDataDescriptor(desc) is true, then
      1. If desc.[[Writable]] is true, set desc.[[Writable]] to false.
    3. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
    4. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
  3. Set the [[Extensible]] internal property of O to false.
  4. Return O.

15.2.3.10 Object.preventExtensions ( O )

When the preventExtensions function is called, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Set the [[Extensible]] internal property of O to false.
  3. Return O.

15.2.3.11 Object.isSealed ( O )

When the isSealed function is called with argument O, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. For each named own property name P of O,
    1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2. If desc.[[Configurable]] is true, then return false.
  3. If the [[Extensible]] internal property of O is false, then return true.
  4. Otherwise, return false.

15.2.3.12 Object.isFrozen ( O )

When the isFrozen function is called with argument O, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. For each named own property name P of O,
    1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2. If IsDataDescriptor(desc) is true then
      1. If desc.[[Writable]] is true, return false.
    3. If desc.[[Configurable]] is true, then return false.
  3. If the [[Extensible]] internal property of O is false, then return true.
  4. Otherwise, return false.

15.2.3.13 Object.isExtensible ( O )

When the isExtensible function is called with argument O, the following steps are taken:

  1. If Type(O) is not Object throw a TypeError exception.
  2. Return the Boolean value of the [[Extensible]] internal property of O.

15.2.3.14 Object.keys ( O )

When the keys function is called with argument O, the following steps are taken:

  1. If the Type(O) is not Object, throw a TypeError exception.
  2. Let n be the number of own enumerable properties of O
  3. Let array be the result of creating a new Object as if by the expression new Array(n) where Array is the standard built-in constructor with that name.
  4. Let index be 0.
  5. For each own enumerable property of O whose name String is P
    1. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(index), the PropertyDescriptor {[[Value]]: P, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    2. Increment index by 1.
  6. Return array.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used in step 5 of this algorithm.

15.2.4 Properties of the Object Prototype Object

The value of the [[Prototype]] internal property of the Object prototype object is null, the value of the [[Class]] internal property is "Object", and the initial value of the [[Extensible]] internal property is true.

15.2.4.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is the standard built-in Object constructor.

15.2.4.2 Object.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. If the this value is undefined, return "[object Undefined]".
  2. If the this value is null, return "[object Null]".
  3. Let O be the result of calling ToObject passing the this value as the argument.
  4. Let class be the value of the [[Class]] internal property of O.
  5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

15.2.4.3 Object.prototype.toLocaleString ( )

When the toLocaleString method is called, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let toString be the result of calling the [[Get]] internal method of O passing "toString" as the argument.
  3. If IsCallable(toString) is false, throw a TypeError exception.
  4. Return the result of calling the [[Call]] internal method of toString passing O as the this value and no arguments.

NOTE 1 This function is provided to give all Objects a generic toLocaleString interface, even though not all may use it. Currently, Array, Number, and Date provide their own locale-sensitive toLocaleString methods.

NOTE 2 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.2.4.4 Object.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. If O is the result of calling the Object constructor with a host object (15.2.2.1), then
    1. Return either O or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.
  3. Return O.

15.2.4.5 Object.prototype.hasOwnProperty (V)

When the hasOwnProperty method is called with argument V, the following steps are taken:

  1. Let P be ToString(V).
  2. Let O be the result of calling ToObject passing the this value as the argument.
  3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
  4. If desc is undefined, return false.
  5. Return true.

NOTE 1 Unlike [[HasProperty]] (8.12.6), this method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

15.2.4.6 Object.prototype.isPrototypeOf (V)

When the isPrototypeOf method is called with argument V, the following steps are taken:

  1. If V is not an object, return false.
  2. Let O be the result of calling ToObject passing the this value as the argument.
  3. Repeat
    1. Let V be the value of the [[Prototype]] internal property of V.
    2. if V is null, return false
    3. If O and V refer to the same object, return true.

NOTE The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

15.2.4.7 Object.prototype.propertyIsEnumerable (V)

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

  1. Let P be ToString(V).
  2. Let O be the result of calling ToObject passing the this value as the argument.
  3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
  4. If desc is undefined, return false.
  5. Return the value of desc.[[Enumerable]].

NOTE 1 This method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

15.2.5 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

15.3 Function Objects

15.3.1 The Function Constructor Called as a Function

When Function is called as a function rather than as a constructor, it creates and initialises a new Function object. Thus the function call Function() is equivalent to the object creation expression new Function() with the same arguments.

15.3.1.1 Function (p1, p2, … , pn, body)

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

  1. Create and return a new Function object as if the standard built-in constructor Function was used in a new expression with the same arguments (15.3.2.1).

15.3.2 The Function Constructor

When Function is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.3.2.1 new Function (p1, p2, … , pn, body)

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function constructor is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

  1. Let argCount be the total number of arguments passed to this function invocation.
  2. Let P be the empty String.
  3. If argCount = 0, let body be the empty String.
  4. Else if argCount = 1, let body be that argument.
  5. Else, argCount > 1
    1. Let firstArg be the first argument.
    2. Let P be ToString(firstArg).
    3. Let k be 2.
    4. Repeat, while k < argCount
      1. Let nextArg be the k’th argument.
      2. Let P be the result of concatenating the previous value of P, the String "," (a comma), and ToString(nextArg).
      3. Increase k by 1.
    5. Let body be the k’th argument.
  6. Let body be ToString(body).
  7. If P is not parsable as a FormalParameterListopt then throw a SyntaxError exception.
  8. If body is not parsable as FunctionBody then throw a SyntaxError exception.
  9. If body is strict mode code (see 10.1.1) then let strict be true, else let strict be false.
  10. If strict is true, throw any exceptions specified in 13.1 that apply.
  11. Return a new Function object created as specified in 13.2 passing P as the FormalParameterListopt and body as the FunctionBody. Pass in the Global Environment as the Scope parameter and strict as the Strict flag.

A prototype property is automatically created for every function, to provide for the possibility that the function will be used as a constructor.

NOTE It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

15.3.3 Properties of the Function Constructor

The Function constructor is itself a Function object and its [[Class]] is "Function". The value of the [[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object (15.3.4).

The value of the [[Extensible]] internal property of the Function constructor is true.

The Function constructor has the following properties:

15.3.3.1 Function.prototype

The initial value of Function.prototype is the standard built-in Function prototype object (15.3.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.3.2 Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.4 Properties of the Function Prototype Object

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked, accepts any arguments and returns undefined.

The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object prototype object (15.2.4). The initial value of the [[Extensible]] internal property of the Function prototype object is true.

The Function prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype Object.

The length property of the Function prototype object is 0.

15.3.4.1 Function.prototype.constructor

The initial value of Function.prototype.constructor is the built-in Function constructor.

15.3.4.2 Function.prototype.toString ( )

An implementation-dependent representation of the function is returned. This representation has the syntax of a FunctionDeclaration. Note in particular that the use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.

The toString function is not generic; it throws a TypeError exception if its this value is not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.3.4.3 Function.prototype.apply (thisArg, argArray)

When the apply method is called on an object func with arguments thisArg and argArray, the following steps are taken:

  1. If IsCallable(func) is false, then throw a TypeError exception.
  2. If argArray is null or undefined, then
    1. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and an empty list of arguments.
  3. If Type(argArray) is not Object, then throw a TypeError exception.
  4. Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
  5. Let n be ToUint32(len).
  6. Let argList be an empty List.
  7. Let index be 0.
  8. Repeat while index < n
    1. Let indexName be ToString(index).
    2. Let nextArg be the result of calling the [[Get]] internal method of argArray with indexName as the argument.
    3. Append nextArg as the last element of argList.
    4. Set index to index + 1.
  9. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and argList as the list of arguments.

The length property of the apply method is 2.

NOTE The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

15.3.4.4 Function.prototype.call (thisArg [ , arg1 [ , arg2, … ] ] )

When the call method is called on an object func with argument thisArg and optional arguments arg1, arg2 etc, the following steps are taken:

  1. If IsCallable(func) is false, then throw a TypeError exception.
  2. Let argList be an empty List.
  3. If this method was called with more than one argument then in left to right order starting with arg1 append each argument as the last element of argList
  4. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and argList as the list of arguments.

The length property of the call method is 1.

NOTE The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

15.3.4.5 Function.prototype.bind (thisArg [, arg1 [, arg2, …]])

The bind method takes one or more arguments, thisArg and (optionally) arg1, arg2, etc, and returns a new function object by performing the following steps:

  1. Let Target be the this value.
  2. If IsCallable(Target) is false, throw a TypeError exception.
  3. Let A be a new (possibly empty) internal list of all of the argument values provided after thisArg (arg1, arg2 etc), in order.
  4. Let F be a new native ECMAScript object .
  5. Set all the internal methods, except for [[Get]], of F as specified in 8.12.
  6. Set the [[Get]] internal property of F as specified in 15.3.5.4.
  7. Set the [[TargetFunction]] internal property of F to Target.
  8. Set the [[BoundThis]] internal property of F to the value of thisArg.
  9. Set the [[BoundArgs]] internal property of F to A.
  10. Set the [[Class]] internal property of F to "Function".
  11. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
  12. Set the [[Call]] internal property of F as described in 15.3.4.5.1.
  13. Set the [[Construct]] internal property of F as described in 15.3.4.5.2.
  14. Set the [[HasInstance]] internal property of F as described in 15.3.4.5.3.
  15. If the [[Class]] internal property of Target is "Function", then
    1. Let L be the length property of Target minus the length of A.
    2. Set the length own property of F to either 0 or L, whichever is larger.
  16. Else set the length own property of F to 0.
  17. Set the attributes of the length own property of F to the values specified in 15.3.5.1.
  18. Set the [[Extensible]] internal property of F to true.
  19. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
  20. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  21. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
  22. Return F.

The length property of the bind method is 1.

NOTE Function objects created using Function.prototype.bind do not have a prototype property or the [[Code]], [[FormalParameters]], and [[Scope]] internal properties.

15.3.4.5.1 [[Call]]

When the [[Call]] internal method of a function object, F, which was created using the bind function is called with a this value and a list of arguments ExtraArgs, the following steps are taken:

  1. Let boundArgs be the value of F’s [[BoundArgs]] internal property.
  2. Let boundThis be the value of F’s [[BoundThis]] internal property.
  3. Let target be the value of F’s [[TargetFunction]] internal property.
  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 ExtraArgs in the same order.
  5. Return the result of calling the [[Call]] internal method of target providing boundThis as the this value and providing args as the arguments.

15.3.4.5.2 [[Construct]]

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

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

15.3.4.5.3 [[HasInstance]] (V)

When the [[HasInstance]] internal method of a function object F, that was created using the bind function is called with argument V, the following steps are taken:

  1. Let target be the value of F’s [[TargetFunction]] internal property.
  2. If target has no [[HasInstance]] internal method, a TypeError exception is thrown.
  3. Return the result of calling the [[HasInstance]] internal method of target providing V as the argument.

15.3.5 Properties of Function Instances

In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases uses a different version of the [[Get]] internal property. Depending on how they are created (see 8.6.2, 13.2, 15, and 15.3.4.5), function instances may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a [[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal property.

The value of the [[Class]] internal property is "Function".

Function instances that correspond to strict mode functions (13.2) and function instances created using the Function.prototype.bind method (15.3.4.5) have properties named “caller” and “arguments” that throw a TypeError exception. An ECMAScript implementation must not associate any implementation specific behaviour with accesses of these properties from strict mode function code.

15.3.5.1 length

The value of the length property is an integer that indicates the “typical” number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.5.2 prototype

The value of the prototype property is used to initialise the [[Prototype]] internal property of a newly created object before the Function object is invoked as a constructor for that newly created object. This property has the attribute { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE Function objects created using Function.prototype.bind do not have a prototype property.

15.3.5.3 [[HasInstance]] (V)

Assume F is a Function object.

When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:

  1. If V is not an object, return false.
  2. Let O be the result of calling the [[Get]] internal method of F with property name "prototype".
  3. If Type(O) is not Object, throw a TypeError exception.
  4. Repeat
    1. Let V be the value of the [[Prototype]] internal property of V.
    2. If V is null, return false.
    3. If O and V refer to the same object, return true.

NOTE Function objects created using Function.prototype.bind have a different implementation of [[HasInstance]] defined in 15.3.4.5.3.

15.3.5.4 [[Get]] (P)

Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects (8.12.3).

Assume F is a Function object. When the [[Get]] internal method of F is called with property name P, the following steps are taken:

  1. Let v be the result of calling the default [[Get]] internal method (8.12.3) on F passing P as the property name argument.
  2. If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
  3. Return v.

NOTE Function objects created using Function.prototype.bind use the default [[Get]] internal method.

15.4 Array Objects

Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232−1. A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 232. The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.

An object, O, is said to be sparse if the following algorithm returns true:

  1. Let len be the result of calling the [[Get]] internal method of O with argument "length".
  2. For each integer i in the range 0≤i<ToUint32(len)
    1. Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument ToString(i).
    2. If elem is undefined, return true.
  3. Return false.

15.4.1 The Array Constructor Called as a Function

When Array is called as a function rather than as a constructor, it creates and initialises a new Array object. Thus the function call Array() is equivalent to the object creation expression new Array() with the same arguments.

15.4.1.1 Array ( [ item1 [ , item2 [ , … ] ] ] )

When the Array function is called the following steps are taken:

  1. Create and return a new Array object exactly as if the standard built-in constructor Array was used in a new expression with the same arguments (15.4.2).

15.4.2 The Array Constructor

When Array is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.4.2.1 new Array ( [ item0 [ , item1 [ , … ] ] ] )

This description applies if and only if the Array constructor is given no arguments or at least two arguments.

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value of Array.prototype (15.4.3.1).

The [[Class]] internal property of the newly constructed object is set to "Array".

The [[Extensible]] internal property of the newly constructed object is set to true.

The length property of the newly constructed object is set to the number of arguments.

The 0 property of the newly constructed object is set to item0 (if supplied); the 1 property of the newly constructed object is set to item1 (if supplied); and, in general, for as many arguments as there are, the k property of the newly constructed object is set to argument k, where the first argument is considered to be argument number 0. These properties all have the attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

15.4.2.2 new Array (len)

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value of Array.prototype (15.4.3.1). The [[Class]] internal property of the newly constructed object is set to "Array". The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument len is a Number and ToUint32(len) is equal to len, then the length property of the newly constructed object is set to ToUint32(len). If the argument len is a Number and ToUint32(len) is not equal to len, a RangeError exception is thrown.

If the argument len is not a Number, then the length property of the newly constructed object is set to 1 and the 0 property of the newly constructed object is set to len with attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

15.4.3 Properties of the Array Constructor

The value of the [[Prototype]] internal property of the Array constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Array constructor has the following properties:

15.4.3.1 Array.prototype

The initial value of Array.prototype is the Array prototype object (15.4.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.4.3.2 Array.isArray ( arg )

The isArray function takes one argument arg, and returns the Boolean value true if the argument is an object whose class internal property is "Array"; otherwise it returns false. The following steps are taken:

  1. If Type(arg) is not Object, return false.
  2. If the value of the [[Class]] internal property of arg is "Array", then return true.
  3. Return false.

15.4.4 Properties of the Array Prototype Object

The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object prototype object (15.2.4).

The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length property (whose initial value is +0) and the special [[DefineOwnProperty]] internal method described in 15.4.5.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase “this object” refers to the object that is the this value for the invocation of the function. It is permitted for the this to be an object for which the value of the [[Class]] internal property is not "Array".

NOTE The Array prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the standard built-in Object prototype Object.

15.4.4.1 Array.prototype.constructor

The initial value of Array.prototype.constructor is the standard built-in Array constructor.

15.4.4.2 Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. Let array be the result of calling ToObject on the this value.
  2. Let func be the result of calling the [[Get]] internal method of array with argument "join".
  3. If IsCallable(func) is false, then let func be the standard built-in method Object.prototype.toString (15.2.4.2).
  4. Return the result of calling the [[Call]] internal method of func providing array as the this value and an empty arguments list.

NOTE The toString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the toString function can be applied successfully to a host object is implementation-dependent.

15.4.4.3 Array.prototype.toLocaleString ( )

The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.

The result is calculated as follows:

  1. Let array be the result of calling ToObject passing the this value as the argument.
  2. Let arrayLen be the result of calling the [[Get]] internal method of array with argument "length".
  3. Let len be ToUint32(arrayLen).
  4. Let separator be the String value for the list-separator String appropriate for the host environment’s current locale (this is derived in an implementation-defined way).
  5. If len is zero, return the empty String.
  6. Let firstElement be the result of calling the [[Get]] internal method of array with argument "0".
  7. If firstElement is undefined or null, then
    1. Let R be the empty String.
  8. Else
    1. Let elementObj be ToObject(firstElement).
    2. Let func be the result of calling the [[Get]] internal method of elementObj with argument "toLocaleString".
    3. If IsCallable(func) is false, throw a TypeError exception.
    4. Let R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.
  9. Let k be 1.
  10. Repeat, while k < len
    1. Let S be a String value produced by concatenating R and separator.
    2. Let nextElement be the result of calling the [[Get]] internal method of array with argument ToString(k).
    3. If nextElement is undefined or null, then
      1. Let R be the empty String.
    4. Else
      1. Let elementObj be ToObject(nextElement).
      2. Let func be the result of calling the [[Get]] internal method of elementObj with argument "toLocaleString".
      3. If IsCallable(func) is false, throw a TypeError exception.
      4. Let R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.
    5. Let R be a String value produced by concatenating S and R.
    6. Increase k by 1.
  11. Return R.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the toLocaleString function can be applied successfully to a host object is implementation-dependent.

15.4.4.4 Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )

When the concat method is called with zero or more arguments item1, item2, etc., it returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
  3. Let n be 0.
  4. Let items be an internal List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
  5. Repeat, while items is not empty
    1. Remove the first element from items and let E be the value of the element.
    2. If the value of the [[Class]] internal property of E is "Array", then
      1. Let k be 0.
      2. Let len be the result of calling the [[Get]] internal method of E with argument "length".
      3. Repeat, while k < len
        1. Let P be ToString(k).
        2. Let exists be the result of calling the [[HasProperty]] internal method of E with P.
        3. If exists is true, then
          1. Let subElement be the result of calling the [[Get]] internal method of E with argument P.
          2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: subElement, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
        4. Increase n by 1.
        5. Increase k by 1.
    3. Else, E is not an Array
      1. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: E, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
      2. Increase n by 1.
  6. Return A.

The length property of the concat method is 1.

NOTE The concat function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the concat function can be applied successfully to a host object is implementation-dependent.

15.4.4.5 Array.prototype.join (separator)

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

The join method takes one argument, separator, and performs the following steps:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let len be ToUint32(lenVal).
  4. If separator is undefined, let separator be the single-character String ",".
  5. Let sep be ToString(separator).
  6. If len is zero, return the empty String.
  7. Let element0 be the result of calling the [[Get]] internal method of O with argument "0".
  8. If element0 is undefined or null, let R be the empty String; otherwise, Let R be ToString(element0).
  9. Let k be 1.
  10. Repeat, while k < len
    1. Let S be the String value produced by concatenating R and sep.
    2. Let element be the result of calling the [[Get]] internal method of O with argument ToString(k).
    3. If element is undefined or null, Let next be the empty String; otherwise, let next be ToString(element).
    4. Let R be a String value produced by concatenating S and next.
    5. Increase k by 1.
  11. Return R.

The length property of the join method is 1.

NOTE The join function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the join function can be applied successfully to a host object is implementation-dependent.

15.4.4.6 Array.prototype.pop ( )

The last element of the array is removed from the array and returned.

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let len be ToUint32(lenVal).
  4. If len is zero,
    1. Call the [[Put]] internal method of O with arguments "length", 0, and true.
    2. Return undefined.
  5. Else, len > 0
    1. Let indx be ToString(len–1).
    2. Let element be the result of calling the [[Get]] internal method of O with argument indx.
    3. Call the [[Delete]] internal method of O with arguments indx and true.
    4. Call the [[Put]] internal method of O with arguments "length", indx, and true.
    5. Return element.

NOTE The pop function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the pop function can be applied successfully to a host object is implementation-dependent.

15.4.4.7 Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When the push method is called with zero or more arguments item1, item2, etc., the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let n be ToUint32(lenVal).
  4. Let items be an internal List whose elements are, in left to right order, the arguments that were passed to this function invocation.
  5. Repeat, while items is not empty
    1. Remove the first element from items and let E be the value of the element.
    2. Call the [[Put]] internal method of O with arguments ToString(n), E, and true.
    3. Increase n by 1.
  6. Call the [[Put]] internal method of O with arguments "length", n, and true.
  7. Return n.

The length property of the push method is 1.

NOTE The push function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the push function can be applied successfully to a host object is implementation-dependent.

15.4.4.8 Array.prototype.reverse ( )

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let len be ToUint32(lenVal).
  4. Let middle be floor(len/2).
  5. Let lower be 0.
  6. Repeat, while lowermiddle
    1. Let upper be lenlower −1.
    2. Let upperP be ToString(upper).
    3. Let lowerP be ToString(lower).
    4. Let lowerValue be the result of calling the [[Get]] internal method of O with argument lowerP.
    5. Let upperValue be the result of calling the [[Get]] internal method of O with argument upperP .
    6. Let lowerExists be the result of calling the [[HasProperty]] internal method of O with argument lowerP.
    7. Let upperExists be the result of calling the [[HasProperty]] internal method of O with argument upperP.
    8. If lowerExists is true and upperExists is true, then
      1. Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
      2. Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
    9. Else if lowerExists is false and upperExists is true, then
      1. Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
      2. Call the [[Delete]] internal method of O, with arguments upperP and true.
    10. Else if lowerExists is true and upperExists is false, then
      1. Call the [[Delete]] internal method of O, with arguments lowerP and true .
      2. Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
    11. Else, both lowerExists and upperExists are false
      1. No action is required.
    12. Increase lower by 1.
  7. Return O .

NOTE The reverse function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the reverse function can be applied successfully to a host object is implementation-dependent.

15.4.4.9 Array.prototype.shift ( )

The first element of the array is removed from the array and returned.

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let len be ToUint32(lenVal).
  4. If len is zero, then
    1. Call the [[Put]] internal method of O with arguments "length", 0, and true.
    2. Return undefined.
  5. Let first be the result of calling the [[Get]] internal method of O with argument "0".
  6. Let k be 1.
  7. Repeat, while k < len
    1. Let from be ToString(k).
    2. Let to be ToString(k–1).
    3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
    4. If fromPresent is true, then
      1. Let fromVal be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[Put]] internal method of O with arguments to, fromVal, and true.
    5. Else, fromPresent is false
      1. Call the [[Delete]] internal method of O with arguments to and true.
    6. Increase k by 1.
  8. Call the [[Delete]] internal method of O with arguments ToString(len–1) and true.
  9. Call the [[Put]] internal method of O with arguments "length", (len–1) , and true.
  10. Return first.

NOTE The shift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the shift function can be applied successfully to a host object is implementation-dependent.

15.4.4.10 Array.prototype.slice (start, end)

The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end where length is the length of the array. The following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
  3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  4. Let len be ToUint32(lenVal).
  5. Let relativeStart be ToInteger(start).
  6. If relativeStart is negative, let k be max((len + relativeStart),0); else let k be min(relativeStart, len).
  7. If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).
  8. If relativeEnd is negative, let final be max((len + relativeEnd),0); else let final be min(relativeEnd, len).
  9. Let n be 0.
  10. Repeat, while k < final
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4. Increase k by 1.
    5. Increase n by 1.
  11. Return A.

The length property of the slice method is 2.

NOTE The slice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the slice function can be applied successfully to a host object is implementation-dependent.

15.4.4.11 Array.prototype.sort (comparefn)

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.

Let obj be the result of calling ToObject passing the this value as the argument.

Let len be the result of applying Uint32 to the result of calling the [[Get]] internal method of obj with argument "length".

If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the behaviour of sort is implementation-defined.

Let proto be the value of the [[Prototype]] internal property of obj. If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the behaviour of sort is implementation-defined:

  • obj is sparse (15.4)
  • 0 ≤ j < len
  • The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) is true.

The behaviour of sort is also implementation defined if obj is sparse and any of the following conditions are true:

  • The [[Extensible]] internal property of obj is false.

  • Any array index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.

Otherwise, the following steps are taken.

  1. Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal methods of obj and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or [[Delete]] is a nonnegative integer less than len and where the arguments for calls to SortCompare are results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]] internal methods will be the value true. If obj is not sparse then [[Delete]] must not be called.
  2. Return obj.

The returned object must have the following two properties.

  • There must be some mathematical permutation π of the nonnegative integers less than len, such that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j],. But if property old[j] did not exist, then new[π(j)] does not exist.

  • Then for all nonnegative integers j and k, each less than len, if SortCompare(j,k) < 0 (see SortCompare below), then π(j) < π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j after this function has been executed.

A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CF b means comparefn(a,b) < 0; a =CF b means comparefn(a,b) = 0 (of either sign); and a >CF b means comparefn(a,b) > 0.

  • Calling comparefn(a,b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and b.

  • Calling comparefn(a,b) does not modify the this object.

  • a =CF a (reflexivity)

  • If a =CF b, then b =CF a (symmetry)

  • If a =CF b and b =CF c, then a =CF c (transitivity of =CF)

  • If a <CF b and b <CF c, then a <CF c (transitivity of <CF)

  • If a >CF b and b >CF c, then a >CF c (transitivity of >CF)

NOTE The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.

When the SortCompare abstract operation is called with two arguments j and k, the following steps are taken:

  1. Let jString be ToString(j).
  2. Let kString be ToString(k).
  3. Let hasj be the result of calling the [[HasProperty]] internal method of obj with argument jString.
  4. Let hask be the result of calling the [[HasProperty]] internal method of obj with argument kString.
  5. If hasj and hask are both false, then return +0.
  6. If hasj is false, then return 1.
  7. If hask is false, then return –1.
  8. Let x be the result of calling the [[Get]] internal method of obj with argument jString.
  9. Let y be the result of calling the [[Get]] internal method of obj with argument kString.
  10. If x and y are both undefined, return +0.
  11. If x is undefined, return 1.
  12. If y is undefined, return −1.
  13. If the argument comparefn is not undefined, then
    1. If IsCallable(comparefn) is false, throw a TypeError exception.
    2. Return the result of calling the [[Call]] internal method of comparefn passing undefined as the this value and with arguments x and y.
  14. Let xString be ToString(x).
  15. Let yString be ToString(y).
  16. If xString < yString, return −1.
  17. If xString > yString, return 1.
  18. Return +0.

NOTE 1 Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.

NOTE 2 The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the sort function can be applied successfully to a host object is implementation-dependent.

15.4.4.12 Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )

When the splice method is called with two or more arguments start, deleteCount and (optionally) item1, item2, etc., the deleteCount elements of the array starting at array index start are replaced by the arguments item1, item2, etc. An Array object containing the deleted elements (if any) is returned. The following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let A be a new array created as if by the expression new Array()where Array is the standard built-in constructor with that name.
  3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  4. Let len be ToUint32(lenVal).
  5. Let relativeStart be ToInteger(start).
  6. If relativeStart is negative, let actualStart be max((len + relativeStart),0); else let actualStart be min(relativeStart, len).
  7. Let actualDeleteCount be min(max(ToInteger(deleteCount),0), lenactualStart).
  8. Let k be 0.
  9. Repeat, while k < actualDeleteCount
    1. Let from be ToString(actualStart+k).
    2. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
    3. If fromPresent is true, then
      1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(k), Property Descriptor {[[Value]]: fromValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4. Increment k by 1.
  10. Let items be an internal List whose elements are, in left to right order, the portion of the actual argument list starting with item1. The list will be empty if no such items are present.
  11. Let itemCount be the number of elements in items.
  12. If itemCount < actualDeleteCount, then
    1. Let k be actualStart.
    2. Repeat, while k < (lenactualDeleteCount)
      1. Let from be ToString(k+actualDeleteCount).
      2. Let to be ToString(k+itemCount).
      3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
      4. If fromPresent is true, then
        1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
        2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
      5. Else, fromPresent is false
        1. Call the [[Delete]] internal method of O with arguments to and true.
      6. Increase k by 1.
    3. Let k be len.
    4. Repeat, while k > (lenactualDeleteCount + itemCount)
      1. Call the [[Delete]] internal method of O with arguments ToString(k–1) and true.
      2. Decrease k by 1.
  13. Else if itemCount > actualDeleteCount, then
    1. Let k be (lenactualDeleteCount).
    2. Repeat, while k > actualStart
      1. Let from be ToString(k + actualDeleteCount – 1).
      2. Let to be ToString(k + itemCount – 1)
      3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
      4. If fromPresent is true, then
        1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
        2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
      5. Else, fromPresent is false
        1. Call the [[Delete]] internal method of O with argument to and true.
      6. Decrease k by 1.
  14. Let k be actualStart.
  15. Repeat, while items is not empty
    1. Remove the first element from items and let E be the value of that element.
    2. Call the [[Put]] internal method of O with arguments ToString(k), E, and true.
    3. Increase k by 1.
  16. Call the [[Put]] internal method of O with arguments "length", (lenactualDeleteCount + itemCount), and true.
  17. Return A.

The length property of the splice method is 2.

NOTE The splice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the splice function can be applied successfully to a host object is implementation-dependent.

15.4.4.13 Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
  3. Let len be ToUint32(lenVal).
  4. Let argCount be the number of actual arguments.
  5. Let k be len.
  6. Repeat, while k > 0,
    1. Let from be ToString(k–1).
    2. Let to be ToString(k+argCount –1).
    3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
    4. If fromPresent is true, then
      1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
    5. Else, fromPresent is false
      1. Call the [[Delete]] internal method of O with arguments to, and true.
    6. Decrease k by 1.
  7. Let j be 0.
  8. Let items be an internal List whose elements are, in left to right order, the arguments that were passed to this function invocation.
  9. Repeat, while items is not empty
    1. Remove the first element from items and let E be the value of that element.
    2. Call the [[Put]] internal method of O with arguments ToString(j), E, and true.
    3. Increase j by 1.
  10. Call the [[Put]] internal method of O with arguments "length", len+argCount, and true.
  11. Return len+argCount.

The length property of the unshift method is 1.

NOTE The unshift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the unshift function can be applied successfully to a host object is implementation-dependent.

15.4.4.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )

indexOf compares searchElement to the elements of the array, in ascending order, using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the first such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.

When the indexOf method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If len is 0, return -1.
  5. If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be 0.
  6. If nlen, return -1.
  7. If n ≥ 0, then
    1. Let k be n.
  8. Else, n<0
    1. Let k be len - abs(n).
    2. If k is less than 0, then let k be 0.
  9. Repeat, while k<len
    1. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
    2. If kPresent is true, then
      1. Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
      2. Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement and elementK.
      3. If same is true, return k.
    3. Increase k by 1.
  10. Return -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the indexOf function can be applied successfully to a host object is implementation-dependent.

15.4.4.15 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

lastIndexOf compares searchElement to the elements of the array in descending order using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the last such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.

When the lastIndexOf method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If len is 0, return -1.
  5. If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len-1.
  6. If n0, then let k be min(n, len – 1).
  7. Else, n < 0
    1. Let k be len - abs(n).
  8. Repeat, while k0
    1. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
    2. If kPresent is true, then
      1. Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
      2. Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement and elementK.
      3. If same is true, return k.
    3. Decrease k by 1.
  9. Return -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the lastIndexOf function can be applied successfully to a host object is implementation-dependent.

15.4.4.16 Array.prototype.every ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.

When the every method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If thisArg was supplied, let T be thisArg; else let T be undefined.
  6. Let k be 0.
  7. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. If ToBoolean(testResult) is false, return false.
    4. Increase k by 1.
  8. Return true.

The length property of the every method is 1.

NOTE The every function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the every function can be applied successfully to a host object is implementation-dependent.

15.4.4.17 Array.prototype.some ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.

When the some method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If thisArg was supplied, let T be thisArg; else let T be undefined.
  6. Let k be 0.
  7. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. If ToBoolean(testResult) is true, return true.
    4. Increase k by 1.
  8. Return false.

The length property of the some method is 1.

NOTE The some function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the some function can be applied successfully to a host object is implementation-dependent.

15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callback will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.

When the forEach method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If thisArg was supplied, let T be thisArg; else let T be undefined.
  6. Let k be 0.
  7. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Call the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
    4. Increase k by 1.
  8. Return undefined.

The length property of the forEach method is 1.

NOTE The forEach function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the forEach function can be applied successfully to a host object is implementation-dependent.

15.4.4.19 Array.prototype.map ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.

When the map method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If thisArg was supplied, let T be thisArg; else let T be undefined.
  6. Let A be a new array created as if by the expression new Array(len) where Array is the standard built-in constructor with that name and len is the value of len.
  7. Let k be 0.
  8. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let mappedValue be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. Call the [[DefineOwnProperty]] internal method of A with arguments Pk, Property Descriptor {[[Value]]: mappedValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4. Increase k by 1.
  9. Return A.

The length property of the map method is 1.

NOTE The map function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the map function can be applied successfully to a host object is implementation-dependent.

15.4.4.20 Array.prototype.filter ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the filter method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If thisArg was supplied, let T be thisArg; else let T be undefined.
  6. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
  7. Let k be 0.
  8. Let to be 0.
  9. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let selected be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. If ToBoolean(selected) is true, then
        1. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
        2. Increase to by 1.
    4. Increase k by 1.
  10. Return A.

The length property of the filter method is 1.

NOTE The filter function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the filter function can be applied successfully to a host object is implementation-dependent.

15.4.4.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.

When the reduce method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If len is 0 and initialValue is not present, throw a TypeError exception.
  6. Let k be 0.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else, initialValue is not present
    1. Let kPresent be false.
    2. Repeat, while kPresent is false and k < len
      1. Let Pk be ToString(k).
      2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
      3. If kPresent is true, then
        1. Let accumulator be the result of calling the [[Get]] internal method of O with argument Pk.
      4. Increase k by 1.
    3. If kPresent is false, throw a TypeError exception.
  9. Repeat, while k < len
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let accumulator be the result of calling the [[Call]] internal method of callbackfn with undefined as the this value and argument list containing accumulator, kValue, k, and O.
    4. Increase k by 1.
  10. Return accumulator.

The length property of the reduce method is 1.

NOTE The reduce function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the reduce function can be applied successfully to a host object is implementation-dependent.

15.4.4.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.

When the reduceRight method is called with one or two arguments, the following steps are taken:

  1. Let O be the result of calling ToObject passing the this value as the argument.
  2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
  3. Let len be ToUint32(lenValue).
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If len is 0 and initialValue is not present, throw a TypeError exception.
  6. Let k be len-1.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else, initialValue is not present
    1. Let kPresent be false.
    2. Repeat, while kPresent is false and k ≥ 0
      1. Let Pk be ToString(k).
      2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
      3. If kPresent is true, then
        1. Let accumulator be the result of calling the [[Get]] internal method of O with argument Pk.
      4. Decrease k by 1.
    3. If kPresent is false, throw a TypeError exception.
  9. Repeat, while k ≥ 0
    1. Let Pk be ToString(k).
    2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
    3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let accumulator be the result of calling the [[Call]] internal method of callbackfn with undefined as the this value and argument list containing accumulator, kValue, k, and O.
    4. Decrease k by 1.
  10. Return accumulator.

The length property of the reduceRight method is 1.

NOTE The reduceRight function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the reduceRight function can be applied successfully to a host object is implementation-dependent.

15.4.5 Properties of Array Instances

Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is "Array". Array instances also have the following properties.

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

Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript objects (8.12.9).

Assume A is an Array object, Desc is a Property Descriptor, and Throw is a Boolean flag.

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

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

  1. Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument. The result will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
  2. Let oldLen be oldLenDesc.[[Value]].
  3. If P is "length", then
    1. If the [[Value]] field of Desc is absent, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Desc, and Throw as arguments.
    2. Let newLenDesc be a copy of Desc.
    3. Let newLen be ToUint32(Desc.[[Value]]).
    4. If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
    5. Set newLenDesc.[[Value] to newLen.
    6. If newLenoldLen, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
    7. Reject if oldLenDesc.[[Writable]] is false.
    8. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
    9. 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.
    10. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
    11. If succeeded is false, return false.
    12. While newLen < oldLen repeat,
      1. Set oldLen to oldLen – 1.
      2. Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing ToString(oldLen) and false as arguments.
      3. If deleteSucceeded is false, then
        1. Set newLenDesc.[[Value] to oldLen+1.
        2. If newWritable is false, set newLenDesc.[[Writable] to false.
        3. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and false as arguments.
        4. Reject.
    13. If newWritable is false, then
      1. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Property Descriptor{[[Writable]]: false}, and false as arguments. This call will always return true.
    14. Return true.
  4. Else if P is an array index (15.4), then
    1. Let index be ToUint32(P).
    2. Reject if indexoldLen and oldLenDesc.[[Writable]] is false.
    3. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P, Desc, and false as arguments.
    4. Reject if succeeded is false.
    5. If indexoldLen
      1. Set oldLenDesc.[[Value]] to index + 1.
      2. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", oldLenDesc, and false as arguments. This call will always return true.
    6. Return true.
  5. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P, Desc, and Throw as arguments.

15.4.5.2 length

The length property of this Array object is a data property whose value is always numerically greater than the name of every deletable property whose name is an array index.

The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array will result in the length being set to a numeric value that is one greater than that largest numeric property name. See 15.4.5.1.

15.5 String Objects

15.5.1 The String Constructor Called as a Function

When String is called as a function rather than as a constructor, it performs a type conversion.

15.5.1.1 String ( [ value ] )

Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty String "" is returned.

15.5.2 The String Constructor

When String is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.5.2.1 new String ( [ value ] )

The [[Prototype]] internal property of the newly constructed object is set to the standard built-in String prototype object that is the initial value of String.prototype (15.5.3.1).

The [[Class]] internal property of the newly constructed object is set to "String".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToString(value), or to the empty String if value is not supplied.

15.5.3 Properties of the String Constructor

The value of the [[Prototype]] internal property of the String constructor is the standard built-in Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the String constructor has the following properties:

15.5.3.1 String.prototype

The initial value of String.prototype is the standard built-in String prototype object (15.5.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.5.3.2 String.fromCharCode ( [ char0 [ , char1 [ , … ] ] ] )

Returns a String value containing as many characters as the number of arguments. Each argument specifies one character of the resulting String, with the first argument specifying the first character, and so on, from left to right. An argument is converted to a character by applying the operation ToUint16 (9.7) and regarding the resulting 16-bit integer as the code unit value of a character. If no arguments are supplied, the result is the empty String.

The length property of the fromCharCode function is 1.

15.5.4 Properties of the String Prototype Object

The String prototype object is itself a String object (its [[Class]] is "String") whose value is an empty String.

The value of the [[Prototype]] internal property of the String prototype object is the standard built-in Object prototype object (15.2.4).

15.5.4.1 String.prototype.constructor

The initial value of String.prototype.constructor is the built-in String constructor.

15.5.4.2 String.prototype.toString ( )

Returns this String value. (Note that, for a String object, the toString method happens to return the same thing as the valueOf method.)

The toString function is not generic; it throws a TypeError exception if its this value is not a String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.3 String.prototype.valueOf ( )

Returns this String value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.4 String.prototype.charAt (pos)

Returns a String containing the character at position pos in the String resulting from converting this object to a String. If there is no character at that position, the result is the empty String. The result is a String value, not a String object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let position be ToInteger(pos).
  4. Let size be the number of characters in S.
  5. If position < 0 or positionsize, return the empty String.
  6. Return a String of length 1, containing one character from S, namely the character at position position, where the first (leftmost) character in S is considered to be at position 0, the next one at position 1, and so on.

NOTE The charAt function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.5 String.prototype.charCodeAt (pos)

Returns a Number (a nonnegative integer less than 216) representing the code unit value of the character at position pos in the String resulting from converting this object to a String. If there is no character at that position, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let position be ToInteger(pos).
  4. Let size be the number of characters in S.
  5. If position < 0 or positionsize, return NaN.
  6. Return a value of Number type, whose value is the code unit value of the character at position position in the String S, where the first (leftmost) character in S is considered to be at position 0, the next one at position 1, and so on.

NOTE The charCodeAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.6 String.prototype.concat ( [ string1 [ , string2 [ , … ] ] ] )

When the concat method is called with zero or more arguments string1, string2, etc., it returns a String consisting of the characters of this object (converted to a String) followed by the characters of each of string1, string2, etc. (where each argument is converted to a String). The result is a String value, not a String object. The following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let args be an internal list that is a copy of the argument list passed to this function.
  4. Let R be S.
  5. Repeat, while args is not empty
    1. Remove the first element from args and let next be the value of that element.
    2. Let R be the String value consisting of the characters in the previous value of R followed by the characters of ToString(next).
  6. Return R.

The length property of the concat method is 1.

NOTE The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.7 String.prototype.indexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String, at one or more positions that are greater than or equal to position, then the index of the smallest such position is returned; otherwise, ‑1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let searchStr be ToString(searchString).
  4. Let pos be ToInteger(position). (If position is undefined, this step produces the value 0).
  5. Let len be the number of characters in S.
  6. Let start be min(max(pos, 0), len).
  7. Let searchLen be the number of characters in searchStr.
  8. Return the smallest possible integer k not smaller than start such that k+ searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as the character at position j of searchStr; but if there is no such integer k, then return the value -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.8 String.prototype.lastIndexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String at one or more positions that are smaller than or equal to position, then the index of the greatest such position is returned; otherwise, ‑1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let searchStr be ToString(searchString).
  4. Let numPos be ToNumber(position). (If position is undefined, this step produces the value NaN).
  5. If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
  6. Let len be the number of characters in S.
  7. Let start min(max(pos, 0), len).
  8. Let searchLen be the number of characters in searchStr.
  9. Return the largest possible nonnegative integer k not larger than start such that k+ searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as the character at position j of searchStr; but if there is no such integer k, then return the value -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.9 String.prototype.localeCompare (that)

When the localeCompare method is called with one argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by the system default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before perform the comparisons the following steps are performed to prepare the Strings:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let That be ToString(that).

The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 15.4.4.11) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings and to return 0 when comparing Strings that are considered canonically equivalent by the Unicode standard.

If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise comparison.

NOTE 1 The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

NOTE 2 This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment’s current locale. It is strongly recommended that this function treat Strings that are canonically equivalent according to the Unicode standard as identical (in other words, compare the Strings as if they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour Unicode compatibility equivalences or decompositions.

NOTE 3 The second parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 4 The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.10 String.prototype.match (regexp)

When the match method is called with argument regexp, the following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx be regexp;
  4. Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp is the standard built-in constructor with that name.
  5. Let global be the result of calling the [[Get]] internal method of rx with argument "global".
  6. Let exec be the standard built-in function RegExp.prototype.exec (see 15.10.6.2)
  7. If global is not true, then
    1. Return the result of calling the [[Call]] internal method of exec with rx as the this value and argument list containing S.
  8. Else, global is true
    1. Call the [[Put]] internal method of rx with arguments "lastIndex" and 0.
    2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
    3. Let previousLastIndex be 0.
    4. Let n be 0.
    5. Let lastMatch be true.
    6. Repeat, while lastMatch is true
      1. Let result be the result of calling the [[Call]] internal method of exec with rx as the this value and argument list containing S.
      2. If result is null, then set lastMatch to false.
      3. Else, result is not null
        1. Let thisIndex be the result of calling the [[Get]] internal method of rx with argument "lastIndex".
        2. If thisIndex = previousLastIndex then
          1. Call the [[Put]] internal method of rx with arguments "lastIndex" and thisIndex+1.
          2. Set previousLastIndex to thisIndex+1.
        3. Else, set previousLastIndex to thisIndex.
        4. Let matchStr be the result of calling the [[Get]] internal method of result with argument "0".
        5. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), the Property Descriptor {[[Value]]: matchStr, [[Writable]]: true, [[Enumerable]]: true, [[configurable]]: true}, and false.
        6. Increment n.
    7. If n = 0, then return null.
    8. Return A.

NOTE The match function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.11 String.prototype.replace (searchValue, replaceValue)

First set string according to the following steps:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let string be the result of calling ToString, giving it the this value as its argument.

If searchValue is a regular expression (an object whose [[Class]] internal property is "RegExp"), do the following: If searchValue.global is false, then search string for the first match of the regular expression searchValue. If searchValue.global is true, then search string for all matches of the regular expression searchValue. Do the search in the same manner as in String.prototype.match, including the update of searchValue.lastIndex. Let m be the number of left capturing parentheses in searchValue (using NcapturingParens as specified in 15.10.2.1).

If searchValue is not a regular expression, let searchString be ToString(searchValue) and search string for the first occurrence of searchString. Let m be 0.

If replaceValue is a function, then for each matched substring, call the function with the following m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, the next m arguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is the offset within string where the match occurred, and argument m + 3 is string. The result is a String value derived from the original input by replacing each matched substring with the corresponding return value of the function call, converted to a String if need be.

Otherwise, let newstring denote the result of converting replaceValue to a String. The result is a String value derived from the original input String by replacing each matched substring with a String derived from newstring by replacing characters in newstring by replacement text as specified in Table 22. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements. For example, "$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns "$1-$11,$1-$22". A $ in newstring that does not match any of the forms below is left as is.

Table 22 — Replacement Text Symbol Substitutions
Characters Replacement text
$$ $
$& The matched substring.
$‘ The portion of string that precedes the matched substring.
$’ The portion of string that follows the matched substring.
$n The nth capture, where n is a single digit in the range 1 to 9 and $n is not followed by a decimal digit. If nm and the nth capture is undefined, use the empty String instead. If n>m, the result is implementation-defined.
$nn The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. If nnm and the nnth capture is undefined, use the empty String instead. If nn>m, the result is implementation-defined.

NOTE The replace function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.12 String.prototype.search (regexp)

When the search method is called with argument regexp, the following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let string be the result of calling ToString, giving it the this value as its argument.
  3. If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx be regexp;
  4. Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp is the standard built-in constructor with that name.
  5. Search the value string from its beginning for an occurrence of the regular expression pattern rx. Let result be a Number indicating the offset within string where the pattern matched, or –1 if there was no match. The lastIndex and global properties of regexp are ignored when performing the search. The lastIndex property of regexp is left unchanged.
  6. Return result.

NOTE The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.13 String.prototype.slice (start, end)

The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from character position start and running to, but not including, character position end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let len be the number of characters in S.
  4. Let intStart be ToInteger(start).
  5. If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
  6. If intStart is negative, let from be max(len + intStart,0); else let from be min(intStart, len).
  7. If intEnd is negative, let to be max(len + intEnd,0); else let to be min(intEnd, len).
  8. Let span be max(tofrom,0).
  9. Return a String containing span consecutive characters from S beginning with the character at position from.

The length property of the slice method is 2.

NOTE The slice function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.14 String.prototype.split (separator, limit)

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be a RegExp object (i.e., an object whose [[Class]] internal property is "RegExp"; see 15.10).

The value of separator may be an empty String, an empty regular expression, or a regular expression that can match an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if separator is the empty String, the String is split up into individual characters; the length of the result array equals the length of the String, and each substring contains one character.) If separator is a regular expression, only the first match at a given position of the this String is considered, even if backtracking could yield a non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to the array ["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)

If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator is a regular expression that contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,

"A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/)

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined,
"CODE", "coded", "/", "CODE", ""]

If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

When the split method is called, the following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let A be a new array created as if by the expression new Array()where Array is the standard built-in constructor with that name.
  4. Let lengthA be 0.
  5. If limit is undefined, let lim = 232–1; else let lim = ToUint32(limit).
  6. Let s be the number of characters in S.
  7. Let p = 0.
  8. If separator is a RegExp object (its [[Class]] is "RegExp"), let R = separator; otherwise let R = ToString(separator).
  9. If lim = 0, return A.
  10. If separator is undefined, then
    1. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    2. Return A.
  11. If s = 0, then
    1. Call SplitMatch(S, 0, R) and let z be its MatchResult result.
    2. If z is not failure, return A.
    3. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4. Return A.
  12. Let q = p.
  13. Repeat, while qs
    1. Call SplitMatch(S, q, R) and let z be its MatchResult result.
    2. If z is failure, then let q = q+1.
    3. Else, z is not failure
      1. z must be a State. Let e be z's endIndex and let cap be z's captures array.
      2. If e = p, then let q = q+1.
      3. Else, ep
        1. Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through q (exclusive).
        2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
        3. Increment lengthA by 1.
        4. If lengthA = lim, return A.
        5. Let p = e.
        6. Let i = 0.
        7. Repeat, while i is not equal to the number of elements in cap.
          1. Let i = i+1.
          2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: cap[i], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
          3. Increment lengthA by 1.
          4. If lengthA = lim, return A.
        8. Let q = p.
  14. Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through s (exclusive).
  15. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
  16. Return A.

The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String or RegExp R, and performs the following in order to return a MatchResult (see 15.10.2.1):

  1. If R is a RegExp object (its [[Class]] is "RegExp"), then
    1. Call the [[Match]] internal method of R giving it the arguments S and q, and return the MatchResult result.
  2. Type(R) must be String. Let r be the number of characters in R.
  3. Let s be the number of characters in S.
  4. If q+r > s then return the MatchResult failure.
  5. If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at position q+i of S is different from the character at position i of R, then return failure.
  6. Let cap be an empty array of captures (see 15.10.2.1).
  7. Return the State (q+r, cap). (see 15.10.2.1)

The length property of the split method is 2.

NOTE 1 The split method ignores the value of separator.global for separators that are RegExp objects.

NOTE 2 The split function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.15 String.prototype.substring (start, end)

The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from character position start and running to, but not including, character position end of the String (or through the end of the String is end is undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let len be the number of characters in S.
  4. Let intStart be ToInteger(start).
  5. If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
  6. Let finalStart be min(max(intStart, 0), len).
  7. Let finalEnd be min(max(intEnd, 0), len).
  8. Let from be min(finalStart, finalEnd).
  9. Let to be max(finalStart, finalEnd).
  10. Return a String whose length is to - from, containing characters from S, namely the characters with indices from through to −1, in ascending order.

The length property of the substring method is 2.

NOTE The substring function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.16 String.prototype.toLowerCase ( )

The following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let L be a String where each character of L is either the Unicode lowercase equivalent of the corresponding character of S or the actual corresponding character of S if no Unicode lowercase equivalent exists.
  4. Return L.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the Unicode Basic Multilingual Plane. Surrogate code points are directly transferred from S to L without any mapping.

The result must be derived according to the case mappings in the Unicode character database (this explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode 2.1.8 and later).

NOTE 1 The case mapping of some characters may produce multiple characters. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

NOTE 2 The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.17 String.prototype.toLocaleLowerCase ( )

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.18 String.prototype.toUpperCase ( )

This function behaves in exactly the same way as String.prototype.toLowerCase, except that characters are mapped to their uppercase equivalents as specified in the Unicode Character Database.

NOTE The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.19 String.prototype.toLocaleUpperCase ( )

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.20 String.prototype.trim ( )

The following steps are taken:

  1. Call CheckObjectCoercible passing the this value as its argument.
  2. Let S be the result of calling ToString, giving it the this value as its argument.
  3. Let T be a String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator.
  4. Return T.

NOTE The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.5 Properties of String Instances

String instances inherit properties from the String prototype object and their [[Class]] internal property value is "String". String instances also have a [[PrimitiveValue]] internal property, a length property, and a set of enumerable properties with array index names.

The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index named properties correspond to the individual characters of the String value. A special [[GetOwnProperty]] internal method is used to specify the number, values, and attributes of the array index named properties.

15.5.5.1 length

The number of characters in the String value represented by this String object.

Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.5.5.2 [[GetOwnProperty]] ( P )

String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method provides access to named properties corresponding to the individual characters of String objects.

Assume S is a String object and P is a String.

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

  1. Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S with argument P.
  2. If desc is not undefined return desc.
  3. If ToString(abs(ToInteger(P))) is not the same value as P, return undefined.
  4. Let str be the String value of the [[PrimitiveValue]] internal property of S.
  5. Let index be ToInteger(P).
  6. Let len be the number of characters in str.
  7. If lenindex, return undefined.
  8. Let resultStr be a String of length 1, containing one character from str, specifically the character at position index, where the first (leftmost) character in str is considered to be at position 0, the next one at position 1, and so on.
  9. Return a Property Descriptor { [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false }

15.6 Boolean Objects

15.6.1 The Boolean Constructor Called as a Function

When Boolean is called as a function rather than as a constructor, it performs a type conversion.

15.6.1.1 Boolean (value)

Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).

15.6.2 The Boolean Constructor

When Boolean is called as part of a new expression it is a constructor: it initialises the newly created object.

15.6.2.1 new Boolean (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Boolean prototype object, the one that is the initial value of Boolean.prototype (15.6.3.1).

The [[Class]] internal property of the newly constructed Boolean object is set to "Boolean".

The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set to ToBoolean(value).

The [[Extensible]] internal property of the newly constructed object is set to true.

15.6.3 Properties of the Boolean Constructor

The value of the [[Prototype]] internal property of the Boolean constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Boolean constructor has the following property:

15.6.3.1 Boolean.prototype

The initial value of Boolean.prototype is the Boolean prototype object (15.6.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.6.4 Properties of the Boolean Prototype Object

The Boolean prototype object is itself a Boolean object (its [[Class]] is "Boolean") whose value is false.

The value of the [[Prototype]] internal property of the Boolean prototype object is the standard built-in Object prototype object (15.2.4).

15.6.4.1 Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is the built-in Boolean constructor.

15.6.4.2 Boolean.prototype.toString ( )

The following steps are taken:

  1. Let B be the this value.
  2. If Type(B) is Boolean, then let b be B.
  3. Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be the value of the [[PrimitiveValue]] internal property of B.
  4. Else throw a TypeError exception.
  5. If b is true, then return "true"; else return "false".

15.6.4.3 Boolean.prototype.valueOf ( )

The following steps are taken:

  1. Let B be the this value.
  2. If Type(B) is Boolean, then let b be B.
  3. Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be the value of the [[PrimitiveValue]] internal property of B.
  4. Else throw a TypeError exception.
  5. Return b.

15.6.5 Properties of Boolean Instances

Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal property value is "Boolean". Boolean instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.

15.7 Number Objects

15.7.1 The Number Constructor Called as a Function

When Number is called as a function rather than as a constructor, it performs a type conversion.

15.7.1.1 Number ( [ value ] )

Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else returns +0.

15.7.2 The Number Constructor

When Number is called as part of a new expression it is a constructor: it initialises the newly created object.

15.7.2.1 new Number ( [ value ] )

The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype object, the one that is the initial value of Number.prototype (15.7.3.1).

The [[Class]] internal property of the newly constructed object is set to "Number".

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToNumber(value) if value was supplied, else to +0.

The [[Extensible]] internal property of the newly constructed object is set to true.

15.7.3 Properties of the Number Constructor

The value of the [[Prototype]] internal property of the Number constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Number constructor has the following properties:

15.7.3.1 Number.prototype

The initial value of Number.prototype is the Number prototype object (15.7.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.2 Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10308.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.3 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10‑324.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.4 Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.5 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is −∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.6 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.4 Properties of the Number Prototype Object

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object prototype object (15.2.4).

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an Object for which the value of the [[Class]] internal property is "Number".

In the following descriptions of functions that are properties of the Number prototype object, the phrase “this Number object” refers to either the object that is the this value for the invocation of the function or, if Type(this value) is Number, an object that is created as if by the expression new Number(this value) where Number is the standard built-in constructor with that name. Also, the phrase “this Number value” refers to either the Number value represented by this Number object, that is, the value of the [[PrimitiveValue]] internal property of this Number object or the this value if its type is Number. A TypeError exception is thrown if the this value is neither an object for which the value of the [[Class]] internal property is "Number" or a value whose type is Number.

15.7.4.1 Number.prototype.constructor

The initial value of Number.prototype.constructor is the built-in Number constructor.

15.7.4.2 Number.prototype.toString ( [ radix ] )

The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix. If ToInteger(radix) is the Number 10 then this Number value is given as an argument to the ToString abstract operation; the resulting String value is returned.

If ToInteger(radix) is not an integer between 2 and 36 inclusive throw a RangeError exception. If ToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String representation of this Number value using the specified radix. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm should be a generalisation of that specified in 9.8.1.

The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.3 Number.prototype.toLocaleString()

Produces a String value that represents this Number value formatted according to the conventions of the host environment’s current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.7.4.4 Number.prototype.valueOf ( )

Returns this Number value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.5 Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the following steps:

  1. Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0).
  2. If f < 0 or f > 20, throw a RangeError exception.
  3. Let x be this Number value.
  4. If x is NaN, return the String "NaN".
  5. Let s be the empty String.
  6. If x < 0, then
    1. Let s be "-".
    2. Let x = –x.
  7. If x ≥ 1021, then
    1. Let m = ToString(x).
  8. Else, x < 1021
    1. Let n be an integer for which the exact mathematical value of n ÷ 10fx is as close to zero as possible. If there are two such n, pick the larger n.
    2. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If f ≠ 0, then
      1. Let k be the number of characters in m.
      2. If kf, then
        1. Let z be the String consisting of f+1–k occurrences of the character ‘0’.
        2. Let m be the concatenation of Strings z and m.
        3. Let k = f + 1.
      3. Let a be the first kf characters of m, and let b be the remaining f characters of m.
      4. Let m be the concatenation of the three Strings a, ".", and b.
  9. Return the concatenation of the Strings s and m.

The length property of the toFixed method is 1.

If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such values.

NOTE The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,

(1000000000000000128).toString() returns "1000000000000000100",
while (1000000000000000128).toFixed(0) returns "1000000000000000128".

15.7.4.6 Number.prototype.toExponential (fractionDigits)

Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

  1. Let x be this Number value.
  2. Let f be ToInteger(fractionDigits).
  3. If x is NaN, return the String "NaN".
  4. Let s be the empty String.
  5. If x < 0, then
    1. Let s be "-".
    2. Let x = –x.
  6. If x = +∞, then
    1. Return the concatenation of the Strings s and "Infinity".
  7. If fractionDigits is not undefined and (f < 0 or f > 20), throw a RangeError exception.
  8. If x = 0, then
    1. Let f = 0.
    2. Let m be the String consisting of f+1 occurrences of the character ‘0’.
    3. Let e = 0.
  9. Else, x ≠ 0
    1. If fractionDigits is not undefined, then
      1. Let e and n be integers such that 10fn < 10f+1 and for which the exact mathematical value of n × 10efx is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10ef is larger.
    2. Else, fractionDigits is undefined
      1. Let e, n, and f be integers such that f ≥ 0, 10fn < 10f+1, the number value for n × 10ef is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
    3. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
  10. If f ≠ 0, then
    1. Let a be the first character of m, and let b be the remaining f characters of m.
    2. Let m be the concatenation of the three Strings a, ".", and b.
  11. If e = 0, then
    1. Let c = "+".
    2. Let d = "0".
  12. Else
    1. If e > 0, then let c = "+".
    2. Else, e ≤ 0
      1. Let c = "-".
      2. Let e = –e.
    3. Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
  13. Let m be the concatenation of the four Strings m, "e", c, and d.
  14. Return the concatenation of the Strings s and m.

The length property of the toExponential method is 1.

If the toExponential method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toExponential for values of fractionDigits less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for such values.

NOTE For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 9.b.i be used as a guideline:

  1. Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10ef is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10ef is closest in value to x. If there are two such possible values of n, choose the one that is even.

15.7.4.7 Number.prototype.toPrecision (precision)

Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision–1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString (9.8.1) instead. Specifically, perform the following steps:

  1. Let x be this Number value.
  2. If precision is undefined, return ToString(x).
  3. Let p be ToInteger(precision).
  4. If x is NaN, return the String "NaN".
  5. Let s be the empty String.
  6. If x < 0, then
    1. Let s be "-".
    2. Let x = –x.
  7. If x = +∞, then
    1. Return the concatenation of the Strings s and "Infinity".
  8. If p < 1 or p > 21, throw a RangeError exception.
  9. If x = 0, then
    1. Let m be the String consisting of p occurrences of the character ‘0’.
    2. Let e = 0.
  10. Else x ≠ 0,
    1. Let e and n be integers such that 10p–1n < 10p and for which the exact mathematical value of n × 10ep+1x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10ep+1 is larger.
    2. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If e < –6 or ep, then
      1. Let a be the first character of m, and let b be the remaining p–1 characters of m.
      2. Let m be the concatenation of the three Strings a, ".", and b.
      3. If e = 0, then
        1. Let c = "+" and d = "0".
      4. Else e ≠ 0,
        1. If e > 0, then
          1. Let c = "+".
        2. Else e < 0,
          1. Let c = "-".
          2. Let e = –e.
        3. Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
      5. Let m be the concatenation of the five Strings s, m, "e", c, and d.
  11. If e = p–1, then return the concatenation of the Strings s and m.
  12. If e ≥ 0, then
    1. Let m be the concatenation of the first e+1 characters of m, the character ‘.’, and the remaining p– (e+1) characters of m.
  13. Else e < 0,
    1. Let m be the concatenation of the String "0.", –(e+1) occurrences of the character ‘0’, and the String m.
  14. Return the concatenation of the Strings s and m.

The length property of the toPrecision method is 1.

If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toPrecision for values of precision less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.

15.7.5 Properties of Number Instances

Number instances inherit properties from the Number prototype object and their [[Class]] internal property value is "Number". Number instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Number value represented by this Number object.

15.8 The Math Object

The Math object is a single object that has some named properties, some of which are functions.

The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the Math object is "Math".

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor with the new operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.

NOTE In this specification, the phrase “the Number value for x” has a technical meaning defined in 8.5.

15.8.1 Value Properties of the Math Object

15.8.1.1 E

The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.2 LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.3 LN2

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.4 LOG2E

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

15.8.1.5 LOG10E

The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

15.8.1.6 PI

The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.7 SQRT1_2

The Number value for the square root of ½, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

15.8.1.8 SQRT2

The Number value for the square root of 2, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.2 Function Properties of the Math Object

Each of the following Math object functions applies the ToNumber abstract operator to each of its arguments (in left-to-right order if there is more than one) and then performs a computation on the resulting Number value(s).

In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values described in 8.5.

NOTE The behaviour of the functions acos, asin, atan, atan2, cos, exp, log, pow, sin, sqrt, and tan is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

15.8.2.1 abs (x)

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

  • If x is NaN, the result is NaN.
  • If x is −0, the result is +0.
  • If x is −∞, the result is +∞.

15.8.2.2 acos (x)

Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to .

  • If x is NaN, the result is NaN.
  • If x is greater than 1, the result is NaN.
  • If x is less than 1, the result is NaN.
  • If x is exactly 1, the result is +0.

15.8.2.3 asin (x)

Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians and ranges from −π/2 to +π/2.

  • If x is NaN, the result is NaN.
  • If x is greater than 1, the result is NaN.
  • If x is less than –1, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.

15.8.2.4 atan (x)

Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed in radians and ranges from −π/2 to +π/2.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞, the result is an implementation-dependent approximation to +π/2.
  • If x is −∞, the result is an implementation-dependent approximation to −π/2.

15.8.2.5 atan2 (y, x)

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from −π to .

  • If either x or y is NaN, the result is NaN.
  • If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
  • If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.
  • If y is +0 and x>0, the result is +0.
  • If y is +0 and x is +0, the result is +0.
  • If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.
  • If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
  • If y is −0 and x>0, the result is −0.
  • If y is −0 and x is +0, the result is −0.
  • If y is −0 and x is −0, the result is an implementation-dependent approximation to −π.
  • If y is −0 and x<0, the result is an implementation-dependent approximation to −π.
  • If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.
  • If y<0 and x is −0, the result is an implementation-dependent approximation to −π/2.
  • If y>0 and y is finite and x is +∞, the result is +0.
  • If y>0 and y is finite and x is −∞, the result if an implementation-dependent approximation to +π.
  • If y<0 and y is finite and x is +∞, the result is −0.
  • If y<0 and y is finite and x is −∞, the result is an implementation-dependent approximation to −π.
  • If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
  • If y is −∞ and x is finite, the result is an implementation-dependent approximation to −π/2.
  • If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
  • If y is +∞ and x is −∞, the result is an implementation-dependent approximation to +3π/4.
  • If y is −∞ and x is +∞, the result is an implementation-dependent approximation to −π/4.
  • If y is −∞ and x is −∞, the result is an implementation-dependent approximation to −3π/4.

15.8.2.6 ceil (x)

Returns the smallest (closest to −∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞, the result is +∞.
  • If x is −∞, the result is −∞.
  • If x is less than 0 but greater than -1, the result is −0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

15.8.2.7 cos (x)

Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is 1.
  • If x is −0, the result is 1.
  • If x is +∞, the result is NaN.
  • If x is −∞, the result is NaN.

15.8.2.8 exp (x)

Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

  • If x is NaN, the result is NaN.
  • If x is +0, the result is 1.
  • If x is −0, the result is 1.
  • If x is +∞, the result is +∞.
  • If x is −∞, the result is +0.

15.8.2.9 floor (x)

Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞, the result is +∞.
  • If x is −∞, the result is −∞.
  • If x is greater than 0 but less than 1, the result is +0.

NOTE The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

15.8.2.10 log (x)

  • Returns an implementation-dependent approximation to the natural logarithm of x.
  • If x is NaN, the result is NaN.
  • If x is less than 0, the result is NaN.
  • If x is +0 or −0, the result is −∞.
  • If x is 1, the result is +0.
  • If x is +∞, the result is +∞.

15.8.2.11 max ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

  • If no arguments are given, the result is −∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the largest value is done as in 11.8.5 except that +0 is considered to be larger than −0.

The length property of the max method is 2.

15.8.2.12 min ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.

  • If no arguments are given, the result is +∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the smallest value is done as in 11.8.5 except that +0 is considered to be larger than −0.

The length property of the min method is 2.

15.8.2.13 pow (x, y)

Returns an implementation-dependent approximation to the result of raising x to the power y.

  • If y is NaN, the result is NaN.
  • If y is +0, the result is 1, even if x is NaN.
  • If y is −0, the result is 1, even if x is NaN.
  • If x is NaN and y is nonzero, the result is NaN.
  • If abs(x)>1 and y is +∞, the result is +∞.
  • If abs(x)>1 and y is −∞, the result is +0.
  • If abs(x)==1 and y is +∞, the result is NaN.
  • If abs(x)==1 and y is −∞, the result is NaN.
  • If abs(x)<1 and y is +∞, the result is +0.
  • If abs(x)<1 and y is −∞, the result is +∞.
  • If x is +∞ and y>0, the result is +∞.
  • If x is +∞ and y<0, the result is +0.
  • If x is −∞ and y>0 and y is an odd integer, the result is −∞.
  • If x is −∞ and y>0 and y is not an odd integer, the result is +∞.
  • If x is −∞ and y<0 and y is an odd integer, the result is −0.
  • If x is −∞ and y<0 and y is not an odd integer, the result is +0.
  • If x is +0 and y>0, the result is +0.
  • If x is +0 and y<0, the result is +∞.
  • If x is −0 and y>0 and y is an odd integer, the result is −0.
  • If x is −0 and y>0 and y is not an odd integer, the result is +0.
  • If x is −0 and y<0 and y is an odd integer, the result is −∞.
  • If x is −0 and y<0 and y is not an odd integer, the result is +∞.
  • If x<0 and x is finite and y is finite and y is not an integer, the result is NaN.

15.8.2.14 random ( )

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

15.8.2.15 round (x)

Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞, the result is +∞.
  • If x is −∞, the result is −∞.
  • If x is greater than 0 but less than 0.5, the result is +0.
  • If x is less than 0 but greater than or equal to -0.5, the result is −0.

NOTE 1 Math.round(3.5) returns 4, but Math.round(–3.5) returns –3.

NOTE 2 The value of Math.round(x) is the same as the value of Math.floor(x+0.5), except when x is −0 or is less than 0 but greater than or equal to -0.5; for these cases Math.round(x) returns −0, but Math.floor(x+0.5) returns +0.

15.8.2.16 sin (x)

Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞ or −∞, the result is NaN.

15.8.2.17 sqrt (x)

Returns an implementation-dependent approximation to the square root of x.

  • If x is NaN, the result is NaN.
  • If x is less than 0, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞, the result is +∞.

15.8.2.18 tan (x)

Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.

  • If x is NaN, the result is NaN.
  • If x is +0, the result is +0.
  • If x is −0, the result is −0.
  • If x is +∞ or −∞, the result is NaN.

15.9 Date Objects

15.9.1 Overview of Date Objects and Definitions of Abstract Operators

The following functions are abstract operations that operate on time values (defined in 15.9.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

15.9.1.1 Time Values and Time Range

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called a time value. A time value may also be NaN, indicating that the Date object does not represent a specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from –9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.

15.9.1.2 Day Number and Time within Day

A given time value t belongs to day number

Day(t) = floor(t / msPerDay)

where the number of milliseconds per day is

msPerDay = 86400000

The remainder is called the time within the day:

TimeWithinDay(t) = t modulo msPerDay

15.9.1.3 Year Number

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y) = 365 if (y modulo 4) ≠ 0
= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0
= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0
= 366 if (y modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) = 365 × (y−1970) + floor((y−1969)/4) − floor((y−1901)/100) + floor((y−1601)/400)

The time value of the start of a year is:

TimeFromYear(y) = msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t) = 0 if DaysInYear(YearFromTime(t)) = 365
= 1 if DaysInYear(YearFromTime(t)) = 366

15.9.1.4 Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:

MonthFromTime(t) = 0 if 0 ≤ DayWithinYear(t) < 31
= 1 if 31 ≤ DayWithinYear (t) < 59+InLeapYear(t)
= 2 if 59+InLeapYear(t) ≤ DayWithinYear (t) < 90+InLeapYear(t)
= 3 if 90+InLeapYear(t) ≤ DayWithinYear (t) < 120+InLeapYear(t)
= 4 if 120+InLeapYear(t) ≤ DayWithinYear (t) < 151+InLeapYear(t)
= 5 if 151+InLeapYear(t) ≤ DayWithinYear (t) < 181+InLeapYear(t)
= 6 if 181+InLeapYear(t) ≤ DayWithinYear (t) < 212+InLeapYear(t)
= 7 if 212+InLeapYear(t) ≤ DayWithinYear (t) < 243+InLeapYear(t)
= 8 if 243+InLeapYear(t) ≤ DayWithinYear (t) < 273+InLeapYear(t)
= 9 if 273+InLeapYear(t) ≤ DayWithinYear (t) < 304+InLeapYear(t)
= 10 if 304+InLeapYear(t) ≤ DayWithinYear (t) < 334+InLeapYear(t)
= 11 if 334+InLeapYear(t) ≤ DayWithinYear (t) < 365+InLeapYear(t)

where

DayWithinYear(t) = Day(t)−DayFromYear(YearFromTime(t))

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

15.9.1.5 Date Number

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time value t to a month number is defined by:

DateFromTime(t) = DayWithinYear(t)+1 if MonthFromTime(t)=0
= DayWithinYear(t)−30 if MonthFromTime(t)=1
= DayWithinYear(t)−58−InLeapYear(t) if MonthFromTime(t)=2
= DayWithinYear(t)−89−InLeapYear(t) if MonthFromTime(t)=3
= DayWithinYear(t)−119−InLeapYear(t) if MonthFromTime(t)=4
= DayWithinYear(t)−150−InLeapYear(t) if MonthFromTime(t)=5
= DayWithinYear(t)−180−InLeapYear(t) if MonthFromTime(t)=6
= DayWithinYear(t)−211−InLeapYear(t) if MonthFromTime(t)=7
= DayWithinYear(t)−242−InLeapYear(t) if MonthFromTime(t)=8
= DayWithinYear(t)−272−InLeapYear(t) if MonthFromTime(t)=9
= DayWithinYear(t)−303−InLeapYear(t) if MonthFromTime(t)=10
= DayWithinYear(t)−333−InLeapYear(t) if MonthFromTime(t)=11

15.9.1.6 Week Day

The weekday for a particular time value t is defined as

WeekDay(t) = (Day(t) + 4) modulo 7

A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

15.9.1.7 Local Time Zone Adjustment

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight saving time is not reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the geographic location.

15.9.1.8 Daylight Saving Time Adjustment

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four things:

(1) the time since the beginning of the year

(2) whether t is in a leap year

(3) the week day of the beginning of the year

and (4) the geographic location.

The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round.

If the host environment provides functionality for determining daylight saving time, the implementation of ECMAScript is free to map the year in question to an equivalent year (same leap-year-ness and same starting week day for the year) for which the host environment provides daylight saving time information. The only restriction is that all equivalent years should produce the same result.

15.9.1.9 Local Time

Conversion from UTC to local time is defined by

LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)

Conversion from local time to UTC is defined by

UTC(t) = tLocalTZADaylightSavingTA(tLocalTZA)

Note that UTC(LocalTime(t)) is not necessarily always equal to t.

15.9.1.10 Hours, Minutes, Second, and Milliseconds

The following functions are useful in decomposing time values:

HourFromTime(t) = floor(t / msPerHour) modulo HoursPerDay
MinFromTime(t) = floor(t / msPerMinute) modulo MinutesPerHour
SecFromTime(t) = floor(t / msPerSecond) modulo SecondsPerMinute
msFromTime(t) = t modulo msPerSecond

where

HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000
msPerMinute = 60000 = msPerSecond × SecondsPerMinute
msPerHour = 3600000 = msPerMinute × MinutesPerHour

15.9.1.11 MakeTime (hour, min, sec, ms)

The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
  2. Let h be ToInteger(hour).
  3. Let m be ToInteger(min).
  4. Let s be ToInteger(sec).
  5. Let milli be ToInteger(ms).
  6. Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according to IEEE 754 rules (that is, as if using the ECMAScript operators * and +).
  7. Return t.

15.9.1.12 MakeDay (year, month, date)

The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. If year is not finite or month is not finite or date is not finite, return NaN.
  2. Let y be ToInteger(year).
  3. Let m be ToInteger(month).
  4. Let dt be ToInteger(date).
  5. Let ym be y + floor(m /12).
  6. Let mn be m modulo 12.
  7. Find a value t such that YearFromTime(t) == ym and MonthFromTime(t) == mn and DateFromTime(t) == 1; but if this is not possible (because some argument is out of range), return NaN.
  8. Return Day(t) + dt − 1.

15.9.1.13 MakeDate (day, time)

The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. If day is not finite or time is not finite, return NaN.
  2. Return day × msPerDay + time.

15.9.1.14 TimeClip (time)

The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:

  1. If time is not finite, return NaN.
  2. If abs(time) > 8.64 x 1015, return NaN.
  3. Return an implementation-dependent choice of either ToInteger(time) or ToInteger(time) + (+0). (Adding a positive zero converts −0 to +0.)

NOTE The point of step 3 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish −0 and +0.

15.9.1.15 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYY is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.

- -” (hyphen) appears literally twice in the string.

MM is the month of the year from 01 (January) to 12 (December).

DD is the day of the month from 01 to 31.

T T” appears literally in the string, to indicate the beginning of the time element.

HH is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.

: :” (colon) appears literally twice in the string.

mm is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.

ss is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.

. .” (dot) appears literally in the string.

sss is the number of complete milliseconds since the start of the second as three decimal digits.

Z is the time zone offset specified as “Z” (for UTC) or either “+” or “-” followed by a time expression HH:mm

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

All numbers must be base 10. If the MM or DD fields are absent “01” is used as the value. If the HH, mm, or ss fields are absent “00” is used as the value and the value of an absent sss field is “000”. The value of an absent time zone offset is “Z”.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.

NOTE 1 As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00

NOTE 2 There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.

15.9.1.15.1 Extended years

ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or – sign. The year 0 is considered positive and hence prefixed with a + sign.

NOTE Examples of extended years:

-283457-03-21T15:00:59.008Z   283458 B.C.
-000001-01-01T00:00:00Z          2 B.C.
+000000-01-01T00:00:00Z         1 B.C.
+000001-01-01T00:00:00Z         1 A.D.
+001970-01-01T00:00:00Z         1970 A.D.
+002009-12-15T00:00:00Z         2009 A.D.
+287396-10-12T08:59:00.992Z 287396 A.D.

15.9.2 The Date Constructor Called as a Function

When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).

NOTE The function call Date() is not equivalent to the object creation expression new Date() with the same arguments.

15.9.2.1 Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A String is created and returned as if by the expression (new Date()).toString() where Date is the standard built-in constructor with that name and toString is the standard built-in method Date.prototype.toString.

15.9.3 The Date Constructor

When Date is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.9.3.1 new Date (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

When Date is called with two to seven arguments, it computes the date from year, month, and (optionally) date, hours, minutes, seconds and ms.

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

  1. Let y be ToNumber(year).
  2. Let m be ToNumber(month).
  3. If date is supplied then let dt be ToNumber(date); else let dt be 1.
  4. If hours is supplied then let h be ToNumber(hours); else let h be 0.
  5. If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
  6. If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
  7. If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
  8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
  9. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
  10. Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(UTC(finalDate)).

15.9.3.2 new Date (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

  1. Let v be ToPrimitive(value).
  2. If Type(v) is String, then
    1. Parse v as a date, in exactly the same manner as for the parse method (15.9.4.2); let V be the time value for this date.
  3. Else, let V be ToNumber(v).
  4. Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(V) and return.

15.9.3.3 new Date ( )

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to the time value (UTC) identifying the current time.

15.9.4 Properties of the Date Constructor

The value of the [[Prototype]] internal property of the Date constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 7), the Date constructor has the following properties:

15.9.4.1 Date.prototype

The initial value of Date.prototype is the built-in Date prototype object (15.9.5).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.9.4.2 Date.parse (string)

The parse function applies the ToString operator to its argument and interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognisable Strings or dates containing illegal element values in the format String shall cause Date.parse to return NaN.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (15.9.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

15.9.4.3 Date.UTC (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When the UTC function is called with two to seven arguments, it computes the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps are taken:

  1. Let y be ToNumber(year).
  2. Let m be ToNumber(month).
  3. If date is supplied then let dt be ToNumber(date); else let dt be 1.
  4. If hours is supplied then let h be ToNumber(hours); else let h be 0.
  5. If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
  6. If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
  7. If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
  8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
  9. Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The length property of the UTC function is 7.

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

15.9.4.4 Date.now ( )

The now function return a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.

15.9.5 Properties of the Date Prototype Object

The Date prototype object is itself a Date object (its [[Class]] is "Date") whose [[PrimitiveValue]] is NaN.

The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object prototype object (15.2.4).