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

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

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

The Number type has exactly 18437736874454810627_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ + 3_ℝ) values, representing the double-precision 64-bit format IEEE 754-2019 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990_ℝ (that is, 2_ℝ^53_ℝ - 2_ℝ) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see 24.1) or a SharedArrayBuffer (see 24.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

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

The other 18437736874454810624_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ) 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_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^53_ℝ - 2_ℝ) finite nonzero values are of two kinds:

18428729675200069632_ℝ (that is, 2_ℝ^64_ℝ - 2_ℝ^54_ℝ) of them are normalized, having the form

where s is +1_ℝ or -1_ℝ, m is a positive mathematical integer less than 2_ℝ^53_ℝ but not less than 2_ℝ^52_ℝ, and e is a mathematical integer ranging from -1074_ℝ to 971_ℝ, inclusive.

The remaining 9007199254740990_ℝ (that is, 2_ℝ^53_ℝ - 2_ℝ) values are denormalized, having the form

where s is +1_ℝ or -1_ℝ, m is a positive mathematical integer less than 2_ℝ^52_ℝ, and e is -1074_ℝ.

Note that all the positive and negative mathematical integers whose magnitude is no greater than 2⁵³ are representable in the Number type (indeed, the mathematical integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the mathematical 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 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 2_ℝ^1024_ℝ (which is +1_ℝ × 2_ℝ^53_ℝ × 2_ℝ^971_ℝ) and -2_ℝ^1024_ℝ (which is -1_ℝ × 2_ℝ^53_ℝ × 2_ℝ^971_ℝ). 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 2_ℝ^1024_ℝ and -2_ℝ^1024_ℝ are considered to have even significands. Finally, if 2_ℝ^1024_ℝ was chosen, replace it with +∞; if -2_ℝ^1024_ℝ 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 754-2019 roundTiesToEven mode.)

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

The Number::unit value is 1.

The BigInt type represents a mathematical integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

The BigInt::unit value is 1n.

Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 3.

Attribute Name	Value Domain	Description
[[Value]]	Any ECMAScript language type	The value retrieved by a get access of the property.
[[Writable]]	Boolean	If false, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]	Boolean	If true, the property will be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

An accessor property associates a key value with the attributes listed in Table 4.

Attribute Name	Value Domain	Description
[[Get]]	Object | Undefined	If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 7) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]	Object | Undefined	If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 7) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]	Boolean	If true, the property is to be enumerated by a for-in enumeration (see 13.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined in Table 5 is used.

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

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

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

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

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

Table 6 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

• For the internal methods listed in Table 6, the object use those defined in 9.1.
• If the object has a [[Call]] internal method, it uses the one defined in 9.2.1.
• If the object has a [[Construct]] internal method, it uses the one defined in 9.2.2.

An exotic object is an object that is not an ordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.

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

In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

An internal method implicitly returns a Completion Record, either a normal completion that wraps a value of the return type shown in its invocation pattern, or a throw completion.

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

Table 7 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.

Internal Method	Signature	Description
[[Call]]	(any, a List of any) → any	Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
[[Construct]]	(a List of any, Object) → Object	Creates an object. Invoked via the new operator or a super call. The first argument to the internal method is a list containing the arguments of the constructor invocation or the super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructor function objects do not have a [[Construct]] internal method.

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

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

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

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

Definitions:

• The target of an internal method is the object upon which the internal method is called.
• A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
• A non-existent property is a property that does not exist as an own property on a non-extensible target.
• All references to SameValue are according to the definition of the SameValue algorithm.

Return value:

The value returned by any internal method must be a Completion Record with either:

• [[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
• [[Type]] = throw, [[Target]] = empty, and [[Value]] = any ECMAScript language value.

Note 1

An internal method must not return a completion with [[Type]] = continue, break, or return.

[[GetPrototypeOf]] ( )

• The normal return type is either Object or Null.
• If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the SameValue as V.

Note 2

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

[[SetPrototypeOf]] ( V )

• The normal return type is Boolean.
• If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.

[[IsExtensible]] ( )

• The normal return type is Boolean.
• If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.

[[PreventExtensions]] ( )

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

[[GetOwnProperty]] ( P )

• The normal return type is either Property Descriptor or Undefined.
• If the Type of the return value is Property Descriptor, the return value must be a complete property descriptor.
• If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descritor whose [[Value]] is SameValue as P's [[Value]] attribute.
• If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must be true.
• If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
• If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).

Note 3

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

[[DefineOwnProperty]] ( P, Desc )

• The normal return type is Boolean.
• [[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
  1. P is a writable data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
  2. All attributes of Desc are the SameValue as P's attributes.
• [[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

[[HasProperty]] ( P )

• The normal return type is Boolean.
• If P was previously observed as a non-configurable own data or accessor property of the target, [[HasProperty]] must return true.

[[Get]] ( P, Receiver )

• The normal return type is any ECMAScript language type.
• If P was previously observed as a non-configurable, non-writable own data property of the target with value V, then [[Get]] must return the SameValue as V.
• If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

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

• The normal return type is Boolean.
• If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
• If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

[[Delete]] ( P )

• The normal return type is Boolean.
• If P was previously observed as a non-configurable own data or accessor property of the target, [[Delete]] must return false.

[[OwnPropertyKeys]] ( )

• The normal return type is List.
• The returned List must not contain any duplicate entries.
• The Type of each element of the returned List is either String or Symbol.
• The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
• If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].

[[Call]] ( )

• The normal return type is any ECMAScript language type.

[[Construct]] ( )

• The normal return type is Object.

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

Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 8.3. The well-known intrinsics are listed in Table 8.

Intrinsic Name	Global Name	ECMAScript Language Association
%Array%	Array	The Array constructor (22.1.1)
%ArrayBuffer%	ArrayBuffer	The ArrayBuffer constructor (24.1.2)
%ArrayBufferPrototype%	ArrayBuffer.prototype	The initial value of the "prototype" data property of %ArrayBuffer%; i.e., %ArrayBuffer.prototype%
%ArrayIteratorPrototype%		The prototype of Array iterator objects (22.1.5); i.e., %ArrayIterator.prototype%
%ArrayPrototype%	Array.prototype	The initial value of the "prototype" data property of %Array% (22.1.3); i.e., %Array.prototype%
%ArrayProto_entries%	Array.prototype.entries	The initial value of the "entries" data property of %Array.prototype% (22.1.3.4); i.e., %Array.prototype.entries%
%ArrayProto_forEach%	Array.prototype.forEach	The initial value of the "forEach" data property of %Array.prototype% (22.1.3.12); i.e., %Array.prototype.forEach%
%ArrayProto_keys%	Array.prototype.keys	The initial value of the "keys" data property of %Array.prototype% (22.1.3.16); i.e., %Array.prototype.keys%
%ArrayProto_values%	Array.prototype.values	The initial value of the "values" data property of %Array.prototype% (22.1.3.32); i.e., %Array.prototype.values%
%AsyncFromSyncIteratorPrototype%		The prototype of async-from-sync iterator objects (25.1.4)
%AsyncFunction%		The constructor of async function objects (25.7.1)
%AsyncFunctionPrototype%		The initial value of the "prototype" data property of %AsyncFunction%; i.e., %AsyncFunction.prototype%
%AsyncGenerator%		The initial value of the "prototype" property of %AsyncGeneratorFunction%; i.e., %AsyncGeneratorFunction.prototype%
%AsyncGeneratorFunction%		The constructor of async iterator objects (25.3.1)
%AsyncGeneratorPrototype%		The initial value of the "prototype" property of %AsyncGenerator%; i.e., %AsyncGenerator.prototype%
%AsyncIteratorPrototype%		An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%	Atomics	The Atomics object (24.4)
%BigInt%	BigInt	The BigInt constructor (20.2.1)
%BigInt64Array%	BigInt64Array	The BigInt64Array constructor (22.2)
%BigUint64Array%	BigUint64Array	The BigUint64Array constructor (22.2)
%Boolean%	Boolean	The Boolean constructor (19.3.1)
%BooleanPrototype%	Boolean.prototype	The initial value of the "prototype" data property of %Boolean% (19.3.3); i.e., %Boolean.prototype%
%DataView%	DataView	The DataView constructor (24.3.2)
%DataViewPrototype%	DataView.prototype	The initial value of the "prototype" data property of %DataView%; i.e., %DataView.prototype%
%Date%	Date	The Date constructor (20.4.2)
%DatePrototype%	Date.prototype	The initial value of the "prototype" data property of %Date%.; i.e., %Date.prototype%
%decodeURI%	decodeURI	The decodeURI function (18.2.6.2)
%decodeURIComponent%	decodeURIComponent	The decodeURIComponent function (18.2.6.3)
%encodeURI%	encodeURI	The encodeURI function (18.2.6.4)
%encodeURIComponent%	encodeURIComponent	The encodeURIComponent function (18.2.6.5)
%Error%	Error	The Error constructor (19.5.1)
%ErrorPrototype%	Error.prototype	The initial value of the "prototype" data property of %Error%; i.e., %Error.prototype%
%eval%	eval	The eval function (18.2.1)
%EvalError%	EvalError	The EvalError constructor (19.5.5.1)
%EvalErrorPrototype%	EvalError.prototype	The initial value of the "prototype" data property of %EvalError%; i.e., %EvalError.prototype%
%Float32Array%	Float32Array	The Float32Array constructor (22.2)
%Float32ArrayPrototype%	Float32Array.prototype	The initial value of the "prototype" data property of %Float32Array%; i.e., %Float32Array.prototype%
%Float64Array%	Float64Array	The Float64Array constructor (22.2)
%Float64ArrayPrototype%	Float64Array.prototype	The initial value of the "prototype" data property of %Float64Array%; i.e., %Float64Array.prototype%
%ForInIteratorPrototype%		The prototype of For-In iterator objects (13.7.5.16)
%Function%	Function	The Function constructor (19.2.1)
%FunctionPrototype%	Function.prototype	The initial value of the "prototype" data property of %Function%; i.e., %Function.prototype%
%Generator%		The initial value of the "prototype" data property of %GeneratorFunction%
%GeneratorFunction%		The constructor of generator objects (25.2.1)
%GeneratorPrototype%		The initial value of the "prototype" data property of %Generator%; i.e., %Generator.prototype%
%Int8Array%	Int8Array	The Int8Array constructor (22.2)
%Int8ArrayPrototype%	Int8Array.prototype	The initial value of the "prototype" data property of %Int8Array%; i.e., %Int8Array.prototype%
%Int16Array%	Int16Array	The Int16Array constructor (22.2)
%Int16ArrayPrototype%	Int16Array.prototype	The initial value of the "prototype" data property of %Int16Array%; i.e., %Int16Array.prototype%
%Int32Array%	Int32Array	The Int32Array constructor (22.2)
%Int32ArrayPrototype%	Int32Array.prototype	The initial value of the "prototype" data property of %Int32Array%; i.e., %Int32Array.prototype%
%isFinite%	isFinite	The isFinite function (18.2.2)
%isNaN%	isNaN	The isNaN function (18.2.3)
%IteratorPrototype%		An object that all standard built-in iterator objects indirectly inherit from
%JSON%	JSON	The JSON object (24.5)
%JSONParse%	JSON.parse	The initial value of the "parse" data property of %JSON%; i.e., %JSON.parse%
%JSONStringify%	JSON.stringify	The initial value of the "stringify" data property of %JSON%; i.e., %JSON.stringify%
%Map%	Map	The Map constructor (23.1.1)
%MapIteratorPrototype%		The prototype of Map iterator objects (23.1.5)
%MapPrototype%	Map.prototype	The initial value of the "prototype" data property of %Map%; i.e., %Map.prototype%
%Math%	Math	The Math object (20.3)
%Number%	Number	The Number constructor (20.1.1)
%NumberPrototype%	Number.prototype	The initial value of the "prototype" data property of %Number%; i.e., %Number.prototype%
%Object%	Object	The Object constructor (19.1.1)
%ObjectPrototype%	Object.prototype	The initial value of the "prototype" data property of %Object% (19.1.3); i.e., %Object.prototype%
%ObjProto_toString%	Object.prototype.toString	The initial value of the "toString" data property of %Object.prototype% (19.1.3.6); i.e., %Object.prototype.toString%
%ObjProto_valueOf%	Object.prototype.valueOf	The initial value of the "valueOf" data property of %Object.prototype% (19.1.3.7); i.e., %Object.prototype.valueOf%
%parseFloat%	parseFloat	The parseFloat function (18.2.4)
%parseInt%	parseInt	The parseInt function (18.2.5)
%Promise%	Promise	The Promise constructor (25.6.3)
%PromisePrototype%	Promise.prototype	The initial value of the "prototype" data property of %Promise%; i.e., %Promise.prototype%
%PromiseProto_then%	Promise.prototype.then	The initial value of the "then" data property of %Promise.prototype% (25.6.5.4); i.e., %Promise.prototype.then%
%Promise_all%	Promise.all	The initial value of the "all" data property of %Promise% (25.6.4.1); i.e., %Promise.all%
%Promise_reject%	Promise.reject	The initial value of the "reject" data property of %Promise% (25.6.4.5); i.e., %Promise.reject%
%Promise_resolve%	Promise.resolve	The initial value of the "resolve" data property of %Promise% (25.6.4.6); i.e., %Promise.resolve%
%Proxy%	Proxy	The Proxy constructor (26.2.1)
%RangeError%	RangeError	The RangeError constructor (19.5.5.2)
%RangeErrorPrototype%	RangeError.prototype	The initial value of the "prototype" data property of %RangeError%; i.e., %RangeError.prototype%
%ReferenceError%	ReferenceError	The ReferenceError constructor (19.5.5.3)
%ReferenceErrorPrototype%	ReferenceError.prototype	The initial value of the "prototype" data property of %ReferenceError%; i.e., %ReferenceError.prototype%
%Reflect%	Reflect	The Reflect object (26.1)
%RegExp%	RegExp	The RegExp constructor (21.2.3)
%RegExpPrototype%	RegExp.prototype	The initial value of the "prototype" data property of %RegExp%; i.e., %RegExp.prototype%
%RegExpStringIteratorPrototype%		The prototype of RegExp String Iterator objects (21.2.7)
%Set%	Set	The Set constructor (23.2.1)
%SetIteratorPrototype%		The prototype of Set iterator objects (23.2.5)
%SetPrototype%	Set.prototype	The initial value of the "prototype" data property of %Set%; i.e., %Set.prototype%
%SharedArrayBuffer%	SharedArrayBuffer	The SharedArrayBuffer constructor (24.2.2)
%SharedArrayBufferPrototype%	SharedArrayBuffer.prototype	The initial value of the "prototype" data property of %SharedArrayBuffer%; i.e., %SharedArrayBuffer.prototype%
%String%	String	The String constructor (21.1.1)
%StringIteratorPrototype%		The prototype of String iterator objects (21.1.5)
%StringPrototype%	String.prototype	The initial value of the "prototype" data property of %String%; i.e., %String.prototype%
%Symbol%	Symbol	The Symbol constructor (19.4.1)
%SymbolPrototype%	Symbol.prototype	The initial value of the "prototype" data property of %Symbol% (19.4.3); i.e., %Symbol.prototype%
%SyntaxError%	SyntaxError	The SyntaxError constructor (19.5.5.4)
%SyntaxErrorPrototype%	SyntaxError.prototype	The initial value of the "prototype" data property of %SyntaxError%; i.e., %SyntaxError.prototype%
%ThrowTypeError%		A function object that unconditionally throws a new instance of %TypeError%
%TypedArray%		The super class of all typed Array constructors (22.2.1)
%TypedArrayPrototype%		The initial value of the "prototype" data property of %TypedArray%; i.e., %TypedArray.prototype%
%TypeError%	TypeError	The TypeError constructor (19.5.5.5)
%TypeErrorPrototype%	TypeError.prototype	The initial value of the "prototype" data property of %TypeError%; i.e., %TypeError.prototype%
%Uint8Array%	Uint8Array	The Uint8Array constructor (22.2)
%Uint8ArrayPrototype%	Uint8Array.prototype	The initial value of the "prototype" data property of %Uint8Array%; i.e., %Uint8Array.prototype%
%Uint8ClampedArray%	Uint8ClampedArray	The Uint8ClampedArray constructor (22.2)
%Uint8ClampedArrayPrototype%	Uint8ClampedArray.prototype	The initial value of the "prototype" data property of %Uint8ClampedArray%; i.e., %Uint8ClampedArray.prototype%
%Uint16Array%	Uint16Array	The Uint16Array constructor (22.2)
%Uint16ArrayPrototype%	Uint16Array.prototype	The initial value of the "prototype" data property of %Uint16Array%; i.e., %Uint16Array.prototype%
%Uint32Array%	Uint32Array	The Uint32Array constructor (22.2)
%Uint32ArrayPrototype%	Uint32Array.prototype	The initial value of the "prototype" data property of %Uint32Array%; i.e., %Uint32Array.prototype%
%URIError%	URIError	The URIError constructor (19.5.5.6)
%URIErrorPrototype%	URIError.prototype	The initial value of the "prototype" data property of %URIError%; i.e., %URIError.prototype%
%WeakMap%	WeakMap	The WeakMap constructor (23.3.1)
%WeakMapPrototype%	WeakMap.prototype	The initial value of the "prototype" data property of %WeakMap%; i.e., %WeakMap.prototype%
%WeakSet%	WeakSet	The WeakSet constructor (23.4.1)
%WeakSetPrototype%	WeakSet.prototype	The initial value of the "prototype" data property of %WeakSet%; i.e., %WeakSet.prototype%

Algorithm steps that say

1. Let completion be Await(value).

mean the same thing as:

1. Let asyncContext be the running execution context.
2. Let promise be ? PromiseResolve(%Promise%, value).
3. Let stepsFulfilled be the algorithm steps defined in Await Fulfilled Functions.
4. Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, « [[AsyncContext]] »).
5. Set onFulfilled.[[AsyncContext]] to asyncContext.
6. Let stepsRejected be the algorithm steps defined in Await Rejected Functions.
7. Let onRejected be ! CreateBuiltinFunction(stepsRejected, « [[AsyncContext]] »).
8. Set onRejected.[[AsyncContext]] to asyncContext.
9. Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).
10. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
11. Set the code evaluation state of asyncContext such that when evaluation is resumed with a Completion completion, the following steps of the algorithm that invoked Await will be performed, with completion available.
12. Return.
13. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of asyncContext.

where all variables in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.

Note

Await can be combined with the ? and ! prefixes, so that for example

1. Let result be ? Await(value).

means the same thing as:

1. Let result be Await(value).
2. ReturnIfAbrupt(result).

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

1. Return NormalCompletion(argument).

Is a shorthand that is defined as follows:

1. Return Completion { [[Type]]: normal, [[Value]]: argument, [[Target]]: empty }.

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

1. Return ThrowCompletion(argument).

Is a shorthand that is defined as follows:

1. Return Completion { [[Type]]: throw, [[Value]]: argument, [[Target]]: empty }.

The abstract operation UpdateEmpty with arguments completionRecord and value performs the following steps:

1. Assert: If completionRecord.[[Type]] is either return or throw, then completionRecord.[[Value]] is not empty.
2. If completionRecord.[[Value]] is not empty, return Completion(completionRecord).
3. Return Completion { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

1. Assert: Type(V) is Reference.
2. Return the base value component of V.

1. Assert: Type(V) is Reference.
2. Return the referenced name component of V.

1. Assert: Type(V) is Reference.
2. Return the strict reference flag of V.

1. Assert: Type(V) is Reference.
2. If Type(V's base value component) is Boolean, String, Symbol, BigInt, or Number, return true; otherwise return false.

1. Assert: Type(V) is Reference.
2. If either the base value component of V is an Object or HasPrimitiveBase(V) is true, return true; otherwise return false.

1. Assert: Type(V) is Reference.
2. If the base value component of V is undefined, return true; otherwise return false.

1. Assert: Type(V) is Reference.
2. If V has a thisValue component, return true; otherwise return false.

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

Note

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

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

Note

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

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

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

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

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

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

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

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

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

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

1. If Desc is undefined, return undefined.
2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
3. Assert: obj is an extensible ordinary object with no own properties.
4. If Desc has a [[Value]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
5. If Desc has a [[Writable]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "writable", Desc.[[Writable]]).
6. If Desc has a [[Get]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
7. If Desc has a [[Set]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
8. If Desc has an [[Enumerable]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "enumerable", Desc.[[Enumerable]]).
9. If Desc has a [[Configurable]] field, then
   1. Perform ! CreateDataPropertyOrThrow(obj, "configurable", Desc.[[Configurable]]).
10. Return 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 a new Property Descriptor that initially has no fields.
3. Let hasEnumerable be ? HasProperty(Obj, "enumerable").
4. If hasEnumerable is true, then
   1. Let enumerable be ! ToBoolean(? Get(Obj, "enumerable")).
   2. Set desc.[[Enumerable]] to enumerable.
5. Let hasConfigurable be ? HasProperty(Obj, "configurable").
6. If hasConfigurable is true, then
   1. Let configurable be ! ToBoolean(? Get(Obj, "configurable")).
   2. Set desc.[[Configurable]] to configurable.
7. Let hasValue be ? HasProperty(Obj, "value").
8. If hasValue is true, then
   1. Let value be ? Get(Obj, "value").
   2. Set desc.[[Value]] to value.
9. Let hasWritable be ? HasProperty(Obj, "writable").
10. If hasWritable is true, then
    1. Let writable be ! ToBoolean(? Get(Obj, "writable")).
    2. Set desc.[[Writable]] to writable.
11. Let hasGet be ? HasProperty(Obj, "get").
12. If hasGet is true, then
    1. Let getter be ? Get(Obj, "get").
    2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
    3. Set desc.[[Get]] to getter.
13. Let hasSet be ? HasProperty(Obj, "set").
14. If hasSet is true, then
    1. Let setter be ? Get(Obj, "set").
    2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
    3. Set desc.[[Set]] to setter.
15. If desc.[[Get]] is present or desc.[[Set]] is present, then
    1. If desc.[[Value]] is present or desc.[[Writable]] is present, throw a TypeError exception.
16. Return desc.

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

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

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

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

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

1. Assert: size ≥ 0.
2. Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
3. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
4. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
5. Let zero be « 0 ».
6. For each index i of db, do
   1. Append WriteSharedMemory { [[Order]]: Init, [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventList.
7. Return db.

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

1. Assert: fromBlock and toBlock are distinct Data Block or Shared Data Block values.
2. Assert: fromIndex, toIndex, and count are integer values ≥ 0.
3. Let fromSize be the number of bytes in fromBlock.
4. Assert: fromIndex + count ≤ fromSize.
5. Let toSize be the number of bytes in toBlock.
6. Assert: toIndex + count ≤ toSize.
7. Repeat, while count > 0
   1. If fromBlock is a Shared Data Block, then
      1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
      2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
      3. Let bytes be a List of length 1 that contains a nondeterministically chosen byte value.
      4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
      5. Let readEvent be ReadSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
      6. Append readEvent to eventList.
      7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
      8. If toBlock is a Shared Data Block, then
         1. Append WriteSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventList.
      9. Else,
         1. Set toBlock[toIndex] to bytes[0].
   2. Else,
      1. Assert: toBlock is not a Shared Data Block.
      2. Set toBlock[toIndex] to fromBlock[fromIndex].
   3. Set toIndex to toIndex + 1.
   4. Set fromIndex to fromIndex + 1.
   5. Set count to count - 1.
8. Return NormalCompletion(empty).