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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 3rdEdition
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.

1Scope

This Standard defines the ECMAScript scripting language.

2Conformance

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.

3Normative 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

4Overview

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.

4.1Web 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.

4.2Language 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 Stringobject, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the JSONobject, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError andURIError.

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.

4.2.1Objects

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.

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.

4.2.2The 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.

4.3Terms and definitions

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

4.3.1type

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

4.3.2primitive value

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

4.3.3object

member of the type Object

4.3.4constructor

function object that creates and initialises objects

4.3.5prototype

object that provides shared properties for other objects

4.3.6native object

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

4.3.7built-in object

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

4.3.8host object

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

4.3.9undefined value

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

4.3.10Undefined type

type whose sole value is the undefined value

4.3.11null value

primitive value that represents the intentional absence of any object value

4.3.12Null type

type whose sole value is the null value

4.3.13Boolean value

member of the Boolean type

4.3.14Boolean type

type consisting of the primitive values true and false

4.3.15Boolean object

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

4.3.16String value

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

4.3.17String type

set of all possible String values

4.3.18String object

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

4.3.19Number value

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

4.3.20Number type

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

4.3.21Number object

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

4.3.22Infinity

number value that is the positive infinite Number value

4.3.23NaN

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

4.3.24function

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

4.3.25built-in function

built-in object that is a function

4.3.26property

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

4.3.27method

function that is the value of a property

4.3.28built-in method

method that is a built-in function

4.3.29attribute

internal value that defines some characteristic of a property

4.3.30own property

property that is directly contained by its object

4.3.31inherited property

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

5Notational Conventions

5.1Syntactic and Lexical Grammars

5.1.1Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called anonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as itsright-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.

5.1.2The 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.

5.1.3The 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.

5.1.4The 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.

5.1.5The 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.

5.1.6Grammar Notation

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

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

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

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

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

is a convenient abbreviation for:

and that:

is a convenient abbreviation for:

which in turn is an abbreviation for:

which in turn is an abbreviation for:

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

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

which is merely a convenient abbreviation for:

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

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

the definition

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

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

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

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

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

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

means that the nonterminal Identifier may be replaced by any sequence of characters that could replaceIdentifierName 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:

5.2Algorithm 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 abstractoperations, 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 x−k = q × y for some integer q.

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

6Source 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

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.

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.

7Lexical 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. TheInputElementRegExp symbol is used in other syntactic grammar contexts.

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

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

Syntax

7.1Unicode 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

7.2White 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 aRegularExpressionLiteral (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

\uFEFF

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

7.3Line 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 StringLiteraltoken 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

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

7.4Comments

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

7.5Tokens

Syntax

7.6Identifier 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 theUnicodeEscapeSequence 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

The definitions of the nonterminal UnicodeEscapeSequence is given in 7.8.4

7.6.1Reserved Words

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

Syntax

7.6.1.1Keywords

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

Syntax

7.6.1.2Future 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

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:

7.7Punctuators

Syntax

7.8Literals

Syntax

7.8.1Null Literals

Syntax

Semantics

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

7.8.2Boolean Literals

Syntax

Semantics

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

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

7.8.3Numeric Literals

Syntax

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

3in

Semantics

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

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

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

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

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

• The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteraltimes 10e, where e is the MV of ExponentPart.

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

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

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

• The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteraltimes 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 ofNumericLiteral to include OctalIntegerLiteral as described in B.1.1.

7.8.4String Literals

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

Syntax

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

Semantics

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

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

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

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

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

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

• 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 ofSingleStringCharacter.

• 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

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

• The CV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter orLineTerminator 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 secondHexDigit) 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 ofEscapeSequence to include OctalEscapeSequence as described in B.1.2.

7.8.5Regular 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 theRegularExpressionBody 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 andRegularExpressionFlags productions or the productions used by these productions.

Syntax

Semantics

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

7.9Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-whilestatement, 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.

7.9.1Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

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

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

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least oneLineTerminator 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 throwtoken.

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 continuetoken.

7.9.2Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

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

{ 1
2 } 3

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

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

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

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

The source

if (a > b)
else c = d

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

The source

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

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

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

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

8Types

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.

8.1The Undefined Type

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

8.2The Null Type

The Null type has exactly one value, called null.

8.3The Boolean Type

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

8.4The 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.

8.5The 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

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

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 range0 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.

8.6The Object Type

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

• A named data property associates a name with an ECMAScript language value and a set of Boolean attributes.

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

• An internal property has no name and is not directly accessible via ECMAScript language operators. Internal properties exist purely for specification purposes.

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

8.6.1Property 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

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

Table 6 — Attributes of a Named Accessor Property

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

8.6.2Object 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

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 anull value). Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for put access. Named accessor properties are inherited for both get access and put access.

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

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

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

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

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

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

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

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

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

• 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 falsethen it must not subsequently become true.

Table 9 — Internal Properties Only Defined for Some Objects

8.7The 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, thereferenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value of undefined indicates that the reference could not be resolved to a binding. The referenced name is a String.

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

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

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

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

• HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.

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

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

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

8.7.1GetValue (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 getbe the special [[Get]] internal method defined below.
   2. Return the result of calling the get internal method using base as its this value, and passingGetReferencedName(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 passingGetReferencedName(V) and IsStrictReference(V) as arguments.

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

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

8.7.2PutValue (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 putbe the special [[Put]] internal method defined below.
   2. Call the put internal method using base as its this value, and passing GetReferencedName(V) for the property name, W for the value, and IsStrictReference(V) for the Throw flag.
5. Else base must be a reference whose base is an environment record. So,
   1. Call the SetMutableBinding (10.2.1) concrete method of base, passing GetReferencedName(V), W, and IsStrictReference(V) as arguments.
6. Return.

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

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

8.8The 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.

8.9The 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.

8.10The 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:

8.10.1IsAccessorDescriptor ( 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.

8.10.2IsDataDescriptor ( 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.

8.10.3IsGenericDescriptor ( 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.

8.10.4FromPropertyDescriptor ( 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.

8.10.5ToPropertyDescriptor ( 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" istrue, 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.

8.11The 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.

8.12Algorithms 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 in15.5.5.2.

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 Descfor 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 descriptorDesc, 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 ofDesc 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 andDesc 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]]) isfalse.
       2. Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) isfalse.
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 in15.4.5.1.

9Type 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.

9.1ToPrimitive

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

9.2ToBoolean

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

Table 11 — ToBoolean Conversions

9.3ToNumber

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

Table 12 — To Number Conversions

9.3.1ToNumber Applied to the String Type

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

Syntax

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

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

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

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

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

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

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

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

• The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV ofStrNumericLiteral, 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 ofStrUnsignedDecimalLiteral. (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 firstDecimalDigits plus (the MV of the second DecimalDigits times 10−n), where n is the number of characters in the second DecimalDigits.

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

• The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits ExponentPart is (the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10−n)) times 10e, where n is the number of characters in the second DecimalDigits and e is the MV of ExponentPart.

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

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

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

• The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits times 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 StrUnsignedDecimalLiteraland 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 anExponentPart 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.

9.4ToInteger

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)).

9.5ToInt32: (Signed 32 Bit Integer)

The abstract operation ToInt32 converts its argument to one of 232 integer values in the range −231 through231−1, 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.

9.6ToUint32: (Unsigned 32 Bit Integer)

The abstract operation ToUint32 converts its argument to one of 232 integer values in the range 0 through232−1, 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.

9.7ToUint16: (Unsigned 16 Bit Integer)

The abstract operation ToUint16 converts its argument to one of 216 integer values in the range 0 through216−1, 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.

9.8ToString

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

Table 13 — ToString Conversions

9.8.1ToString 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−1 ≤ s < 10k, the Number value for s × 10n−k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that sis not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
6. If k ≤ n ≤ 21, return the String consisting of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n−k occurrences of the character ‘0’.
7. If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal representation ofs, 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).

9.9ToObject

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

Table 14 — ToObject

9.10CheckObjectCoercible

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

9.11IsCallable

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

9.12The 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.

10Executable Code and Execution Contexts

10.1Types of Executable Code

There are three types of ECMAScript executable code:

• Global code is source text that is treated as an ECMAScript Program. The global code of a particularProgram does not include any source text that is parsed as part of a FunctionBody.

• Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Program. The eval code for a particular invocation of eval is the global code portion of that Program.

• Function code is source text that is parsed as part of a FunctionBody. The function code of a particularFunctionBody does not include any source text that is parsed as part of a nested FunctionBody. Function code also denotes the source text supplied when using the built-in Function object as a constructor. More precisely, the last parameter provided to the Function constructor is converted to a String and treated as the FunctionBody. If more than one parameter is provided to the Function constructor, all parameters except the last one are converted to Strings and concatenated together, separated by commas. The resulting String is interpreted as the FormalParameterList for the FunctionBody defined by the last parameter. The function code for a particular instantiation of a Function does not include any source text that is parsed as part of a nested FunctionBody.

10.1.1Strict Mode Code

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

• Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive(see 14.1).

• Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is contained in strict mode code.

• Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.

• Function code that is supplied as the last argument to the built-in Function constructor is strict function code if the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

10.2Lexical 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 aFunctionDeclaration, 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.

10.2.1Environment Records

There are two kinds of Environment Record values used in this specification: declarative environment recordsand 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

10.2.1.1Declarative 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

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

10.2.1.1.1HasBinding(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.

10.2.1.1.2CreateMutableBinding (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.

10.2.1.1.3SetMutableBinding (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 Nto the value of argument V. A binding for N must already exist. If the binding is an immutable binding, aTypeError 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 aTypeError exception.

10.2.1.1.4GetBindingValue(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.

10.2.1.1.5DeleteBinding (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.

10.2.1.1.6ImplicitThisValue()

Declarative Environment Records always return undefined as their ImplicitThisValue.

1. Return undefined.

10.2.1.1.7CreateImmutableBinding (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.

10.2.1.1.8InitializeImmutableBinding (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 Nto 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.

10.2.1.2Object 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.

10.2.1.2.1HasBinding(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.

10.2.1.2.2CreateMutableBinding (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 argumentD 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 trueas arguments.

10.2.1.2.3SetMutableBinding (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.

10.2.1.2.4GetBindingValue(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.

10.2.1.2.5DeleteBinding (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.

10.2.1.2.6ImplicitThisValue()

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.

10.2.2Lexical Environment Operations

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

10.2.2.1GetIdentifierReference (lex, name, strict)

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, an identifier Stringname, 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.

10.2.2.2NewDeclarativeEnvironment (E)

When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or nullas 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.

10.2.2.3NewObjectEnvironment (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.

10.2.3The 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.

10.3Execution 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

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.

10.3.1Identifier 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 productionPrimaryExpression : 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 betrue, 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.

10.4Establishing 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 VariableEnvironmentand 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.

10.4.1Entering 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.

10.4.1.1Initial 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.

10.4.2Entering 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 theLexicalEnvironment 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.

10.4.2.1Strict 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.

10.4.3Entering 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.

10.5Declaration 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’sEnvironment 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 codeand, 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 passingargName as the argument.
      4. If argAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passingargName 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 andconfigurableBindings 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) passingfunc, 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.

10.6Arguments 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 argumentsfunc 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 isstrict 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}, andfalse 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 nameand env.
          3. Let p be the result of calling the MakeArgSetter abstract operation with arguments nameand env.
          4. Call the [[DefineOwnProperty]] internal method of map passing ToString(indx), theProperty 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 objto 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