ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 58

(

promiseCapability

.[[Resolve]],

undefined

, «

valuesArray

»).

15. Return

undefined

The

"length"

property of a

Promise.allSettled

reject element function is

𝔽

The

any

function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an

AggregateError

holding the rejection reasons if all of the given promises are rejected. It resolves all elements of

the passed iterable to promises as it runs this algorithm.

1. Let

be the

this

value.

2. Let

promiseCapability

NewPromiseCapability

3. Let

promiseResolve

GetPromiseResolve

(

(

promiseResolve

promiseCapability

5. Let

iteratorRecord

GetIterator

(

iterable

(

iteratorRecord

promiseCapability

7. Let

result

PerformPromiseAny

iteratorRecord

promiseCapability

promiseResolve

8. If

result

is an

, then

a. If

iteratorRecord

.[[Done]] is

false

, set

result

IteratorClose

(

iteratorRecord

result

(

result

promiseCapability

9. Return

(

result

NOTE

The abstract operation PerformPromiseAny takes arguments

iteratorRecord

constructor

resultCapability

PromiseCapability Record

), and

promiseResolve

. It performs the following steps when called:

: !

(

constructor

) is

true

: !

(

promiseResolve

) is

true

3. Let

errors

be a new empty

4. Let

remainingElementsCount

Record

{ [[Value]]: 1 }.

5. Let

index

be 0.

6. Repeat,

a. Let

next

IteratorStep

(

iteratorRecord

The

any

function requires its

this

value to be a

function that supports the parameter

conventions of the

Promise

27.2.4.3 Promise.any (

iterable

)

27.2.4.3.1 PerformPromiseAny (

iteratorRecord

constructor

resultCapability

promiseResolve

)

768

b. If

next

is an

iteratorRecord

.[[Done]] to

true

next

d. If

next

false

, then

i. Set

iteratorRecord

.[[Done]] to

true

ii. Set

remainingElementsCount

.[[Value]] to

remainingElementsCount

.[[Value]] - 1.

iii. If

remainingElementsCount

.[[Value]] is 0, then

1. Let

error

be a newly created

AggregateError

object.

2. Perform !

DefinePropertyOrThrow

(

error

"errors"

, PropertyDescriptor { [[Configurable]]:

true

, [[Enumerable]]:

false

, [[Writable]]:

true

, [[Value]]: !

CreateArrayFromList

(

errors

) }).

3. Return

ThrowCompletion

error

iv. Return

resultCapability

.[[Promise]].

e. Let

nextValue

IteratorValue

(

next

f. If

nextValue

is an

iteratorRecord

.[[Done]] to

true

nextValue

h. Append

undefined

errors

i. Let

nextPromise

(

promiseResolve

constructor

, «

nextValue

»).

j. Let

stepsRejected

be the algorithm steps defined in

Promise.any

Reject Element Functions

k. Let

lengthRejected

be the number of non-optional parameters of the function definition in

Promise.any

Reject Element Functions

l. Let

onRejected

be !

CreateBuiltinFunction

stepsRejected

lengthRejected

, « [[AlreadyCalled]], [[Index]],

[[Errors]], [[Capability]], [[RemainingElements]] »).

m. Set

onRejected

.[[AlreadyCalled]] to

false

n. Set

onRejected

.[[Index]] to

index

o. Set

onRejected

.[[Errors]] to

errors

p. Set

onRejected

.[[Capability]] to

resultCapability

q. Set

onRejected

.[[RemainingElements]] to

remainingElementsCount

r. Set

remainingElementsCount

.[[Value]] to

remainingElementsCount

.[[Value]] + 1.

s. Perform ?

(

nextPromise

"then"

, «

resultCapability

.[[Resolve]],

onRejected

»).

t. Set

index

+ 1.

Promise.any

reject element function is an anonymous built-in function that is used to reject a specific

Promise.any

element. Each

Promise.any

reject element function has [[Index]], [[Errors]], [[Capability]],

[[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a

Promise.any

reject element function is called with argument

, the following steps are taken:

1. Let

2. If

.[[AlreadyCalled]] is

true

, return

undefined

3. Set

.[[AlreadyCalled]] to

true

4. Let

index

.[[Index]].

5. Let

errors

.[[Errors]].

6. Let

promiseCapability

.[[Capability]].

7. Let

remainingElementsCount

.[[RemainingElements]].

8. Set

errors

[

index

] to

9. Set

remainingElementsCount

.[[Value]] to

remainingElementsCount

.[[Value]] - 1.

10. If

remainingElementsCount

.[[Value]] is 0, then

27.2.4.3.2

Promise.any

Reject Element Functions

769

a. Let

error

be a newly created

AggregateError

object.

b. Perform !

DefinePropertyOrThrow

(

error

"errors"

, PropertyDescriptor { [[Configurable]]:

true

[[Enumerable]]:

false

, [[Writable]]:

true

, [[Value]]: !

CreateArrayFromList

errors

) }).

c. Return ?

(

promiseCapability

.[[Reject]],

undefined

, «

error

»).

11. Return

undefined

The

"length"

property of a

Promise.any

reject element function is

𝔽

The initial value of

Promise.prototype

is the

Promise prototype object

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

race

function returns a new promise which is settled in the same way as the first passed promise to settle. It

resolves all elements of the passed

iterable

to promises as it runs this algorithm.

1. Let

be the

this

value.

2. Let

promiseCapability

NewPromiseCapability

3. Let

promiseResolve

GetPromiseResolve

(

(

promiseResolve

promiseCapability

5. Let

iteratorRecord

GetIterator

(

iterable

(

iteratorRecord

promiseCapability

7. Let

result

PerformPromiseRace

iteratorRecord

promiseCapability

promiseResolve

8. If

result

is an

, then

a. If

iteratorRecord

.[[Done]] is

false

, set

result

IteratorClose

(

iteratorRecord

result

(

result

promiseCapability

9. Return

(

result

NOTE 1

NOTE 2

The abstract operation PerformPromiseRace takes arguments

iteratorRecord

constructor

resultCapability

PromiseCapability Record

), and

promiseResolve

. It performs the following steps when called:

(

constructor

) is

true

(

promiseResolve

) is

true

3. Repeat,

a. Let

next

IteratorStep

(

iteratorRecord

b. If

next

is an

iteratorRecord

.[[Done]] to

true

If the

iterable

argument is empty or if none of the promises in

iterable

ever settle then the pending

promise returned by this method will never be settled.

The

race

function expects its

this

value to be a

function that supports the parameter

conventions of the Promise

. It also expects that its

this

value provides a

resolve

method.

27.2.4.4 Promise.prototype

27.2.4.5 Promise.race (

iterable

)

27.2.4.5.1 PerformPromiseRace (

iteratorRecord

constructor

resultCapability

promiseResolve

)

770

next

d. If

next

false

, then

i. Set

iteratorRecord

.[[Done]] to

true

ii. Return

resultCapability

.[[Promise]].

e. Let

nextValue

IteratorValue

(

next

f. If

nextValue

is an

iteratorRecord

.[[Done]] to

true

nextValue

h. Let

nextPromise

(

promiseResolve

constructor

, «

nextValue

»).

i. Perform ?

(

nextPromise

"then"

, «

resultCapability

.[[Resolve]],

resultCapability

.[[Reject]] »).

The

reject

function returns a new promise rejected with the passed argument.

1. Let

be the

this

value.

2. Let

promiseCapability

3. Perform ?

(

promiseCapability

.[[Reject]],

undefined

, «

»).

4. Return

promiseCapability

.[[Promise]].

NOTE

The

resolve

function returns either a new promise resolved with the passed argument, or the argument itself if the

argument is a promise produced by this

1. Let

be the

this

value.

2. If

Type

(

) is not Object, throw a

TypeError

exception.

PromiseResolve

(

NOTE

The abstract operation PromiseResolve takes arguments

ECMAScript language value

) and

(an

). It

returns a new promise resolved with

. It performs the following steps when called:

Type

) is Object.

2. If

IsPromise

(

) is

true

, then

a. Let

xConstructor

Get

"constructor"

b. If

SameValue

xConstructor

) is

true

, return

3. Let

promiseCapability

4. Perform ?

(

promiseCapability

.[[Resolve]],

undefined

, «

»).

5. Return

promiseCapability

.[[Promise]].

The

reject

function expects its

this

value to be a

function that supports the

parameter conventions of the Promise

The

resolve

function expects its

this

value to be a

function that supports the

parameter conventions of the Promise

27.2.4.6 Promise.reject (

)

27.2.4.7 Promise.resolve (

)

27.2.4.7.1 PromiseResolve (

)

771

Promise[@@species]

is an

accessor property

whose set accessor function is

undefined

. Its get accessor function

performs the following steps:

1. Return the

this

value.

The value of the

"name"

property of this function is

"get [Symbol.species]"

NOTE

The

Promise prototype object

%Promise.prototype%

has a [[Prototype]] internal slot whose value is

%Object.prototype%

is an

does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.

When the

catch

method is called with argument

onRejected

, the following steps are taken:

1. Let

promise

be the

this

value.

2. Return ?

(

promise

"then"

, «

undefined

onRejected

»).

The initial value of

Promise.prototype.constructor

%Promise%

When the

finally

method is called with argument

onFinally

, the following steps are taken:

1. Let

promise

be the

this

value.

2. If

Type

(

promise

) is not Object, throw a

TypeError

exception.

3. Let

SpeciesConstructor

(

promise

%Promise%

(

) is

true

5. If

be the number of non-optional parameters of the function definition in

onFinally

) is

false

, then

a. Let

thenFinally

onFinally

b. Let

catchFinally

onFinally

6. Else,

a. Let

stepsThenFinally

be the algorithm steps defined in

Then Finally Functions

b. Let

lengthThenFinally

Then

Finally Functions

c. Let

thenFinally

be !

CreateBuiltinFunction

stepsThenFinally

lengthThenFinally

, « [[Constructor]],

Promise prototype methods normally use their

this

value's

to create a derived object.

However, a subclass

be the algorithm steps defined in

may over-ride that default behaviour by redefining its

@@species

property.

27.2.4.8 get Promise [ @@species ]

27.2.5 Properties of the Promise Prototype Object

27.2.5.1 Promise.prototype.catch (

onRejected

)

27.2.5.2 Promise.prototype.constructor

27.2.5.3 Promise.prototype.finally (

onFinally

)

772

[[OnFinally]] »).

d. Set

thenFinally

.[[Constructor]] to

e. Set

thenFinally

.[[OnFinally]] to

onFinally

f. Let

stepsCatchFinally

Catch Finally Functions

g. Let

lengthCatchFinally

be the number of non-optional parameters of the function definition in

Catch

Finally Functions

h. Let

catchFinally

be !

CreateBuiltinFunction

stepsCatchFinally

lengthCatchFinally

, « [[Constructor]],

[[OnFinally]] »).

i. Set

catchFinally

.[[Constructor]] to

j. Set

catchFinally

.[[OnFinally]] to

onFinally

7. Return ?

(

promise

"then"

, «

thenFinally

catchFinally

»).

A Then Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot.
The value of the [[Constructor]] internal slot is a

Promise

-like

, and the value of the

[[OnFinally]] internal slot is a

When a Then Finally function is called with argument

value

, the following steps are taken:

1. Let

2. Let

onFinally

.[[OnFinally]].

(

onFinally

) is

true

4. Let

result

be ?

onFinally

undefined

5. Let

.[[Constructor]].

(

) is

true

7. Let

promise

be ?

PromiseResolve

result

8. Let

valueThunk

be equivalent to a function that returns

value

9. Return ?

(

promise

"then"

, «

valueThunk

»).

The

"length"

property of a Then Finally function is

𝔽

A Catch Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal
slot. The value of the [[Constructor]] internal slot is a

Promise

-like

, and the value of the

[[OnFinally]] internal slot is a

When a Catch Finally function is called with argument

reason

, the following steps are taken:

1. Let

2. Let

onFinally

.[[OnFinally]].

(

onFinally

) is

true

4. Let

result

be ?

onFinally

undefined

5. Let

.[[Constructor]].

(

) is

true

7. Let

promise

be ?

PromiseResolve

result

8. Let

thrower

be equivalent to a function that throws

reason

9. Return ?

(

promise

"then"

, «

thrower

»).

27.2.5.3.1 Then Finally Functions

27.2.5.3.2 Catch Finally Functions

773

The

"length"

property of a Catch Finally function is

𝔽

When the

then

method is called with arguments

onFulfilled

and

onRejected

, the following steps are taken:

1. Let

promise

be the

this

value.

2. If

IsPromise

(

promise

) is

false

, throw a

TypeError

exception.

3. Let

SpeciesConstructor

(

promise

%Promise%

4. Let

resultCapability

5. Return

promise

onFulfilled

onRejected

resultCapability

The abstract operation PerformPromiseThen takes arguments

promise

onFulfilled

, and

onRejected

and optional

argument

resultCapability

PromiseCapability Record

). It performs the “then” operation on

promise

using

onFulfilled

and

onRejected

as its settlement actions. If

resultCapability

is passed, the result is stored by updating

resultCapability

promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the
result does not matter. It performs the following steps when called:

IsPromise

promise

) is

true

2. If

resultCapability

is not present, then

a. Set

resultCapability

undefined

3. If

onFulfilled

) is

false

, then

a. Let

onFulfilledJobCallback

empty

4. Else,

a. Let

onFulfilledJobCallback

HostMakeJobCallback

onFulfilled

5. If

onRejected

) is

false

, then

a. Let

onRejectedJobCallback

empty

6. Else,

a. Let

onRejectedJobCallback

HostMakeJobCallback

onRejected

7. Let

fulfillReaction

be the PromiseReaction { [[Capability]]:

resultCapability

, [[Type]]:

Fulfill

, [[Handler]]:

onFulfilledJobCallback

8. Let

rejectReaction

be the PromiseReaction { [[Capability]]:

resultCapability

, [[Type]]:

Reject

, [[Handler]]:

onRejectedJobCallback

9. If

promise

.[[PromiseState]] is

pending

, then

a. Append

fulfillReaction

as the last element of the

as the last element of the

that is

promise

.[[PromiseFulfillReactions]].

b. Append

rejectReaction

that is

promise

.[[PromiseRejectReactions]].

10. Else if

promise

.[[PromiseState]] is

fulfilled

, then

a. Let

value

promise

.[[PromiseResult]].

b. Let

fulfillJob

NewPromiseReactionJob

(

fulfillReaction

value

c. Perform

HostEnqueuePromiseJob

(

fulfillJob

.[[Job]],

fulfillJob

.[[Realm]]).

11. Else,

HostPromiseRejectionTracker

: The value of

promise

.[[PromiseState]] is

rejected

b. Let

reason

promise

.[[PromiseResult]].

c. If

promise

.[[PromiseIsHandled]] is

false

, perform

(

promise

"handle"

d. Let

rejectJob

NewPromiseReactionJob

rejectReaction

reason

e. Perform

HostEnqueuePromiseJob

(

rejectJob

.[[Job]],

rejectJob

.[[Realm]]).

27.2.5.4 Promise.prototype.then (

onFulfilled

onRejected

)

27.2.5.4.1 PerformPromiseThen (

promise

onFulfilled

onRejected

[ ,

resultCapability

] )

774

12. Set

promise

.[[PromiseIsHandled]] to

true

13. If

resultCapability

undefined

, then

a. Return

undefined

14. Else,

a. Return

resultCapability

.[[Promise]].

property is the String value

"Promise"

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

Promise instances are ordinary objects that inherit properties from the

Promise prototype object

(the intrinsic,

%Promise.prototype%

). Promise instances are initially created with the internal slots described in

Table 72

Table 72: Internal Slots of Promise Instances

Internal Slot

Description

[[PromiseState]]

One of

pending

fulfilled

, or

rejected

. Governs how a promise will react to incoming

calls to its

then

method.

[[PromiseResult]]

The value with which the promise has been fulfilled or rejected, if any. Only
meaningful if [[PromiseState]] is not

pending

[[PromiseFulfillReactions]] A

of PromiseReaction records to be processed when/if the promise transitions

from the

pending

state to the

fulfilled

state.

[[PromiseRejectReactions]] A

s. They may also be cr eated by calling the

of PromiseReaction records to be processed when/if the promise transitions

from the

pending

state to the

rejected

state.

[[PromiseIsHandled]]

A boolean indicating whether the promise has ever had a fulfillment or rejection
handler; used in unhandled rejection tracking.

GeneratorFunction objects are functions that are usually created by evaluating

GeneratorDeclaration

GeneratorExpression

s, and

GeneratorMethod

%GeneratorFunction%

intrinsic.

27.2.5.5 Promise.prototype [ @@toStringTag ]

27.2.6 Properties of Promise Instances

27.3 GeneratorFunction Objects

775

GeneratorFunction

to create and initialize subclass instances with the internal slots necessary for

built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects
create direct instances of GeneratorFunction. There is no syntactic means to create instances of
GeneratorFunction subclasses.

The last argument specifies the body (executable code) of a generator function; any preceding arguments specify
formal parameters.

When the

GeneratorFunction

function is called with some arguments

, … ,

body

(where

might be 0,

that is, there are no “

” arguments, and where

body

might also not be provided), the following steps are taken:

1. Let

be the

2. Let

args

be the

argumentsList

that was passed to this function by [[Call]] or [[Construct]].

CreateDynamicFunction

(

, NewTarget,

generator

args

NOTE

The GeneratorFunction

is a standard built-in

that inherits from the Function

GeneratorFunction prototype object

has a [[Prototype]] internal slot whose value is

%Function%

has a

"name"

property whose value is

"GeneratorFunction"

has the following properties:

This is a

data property

with a value of 1. This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

[[Configurable]]:

true

The initial value of

GeneratorFunction.prototype

is the

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

GeneratorFunction prototype object

%GeneratorFunction.prototype%

(see

is an

and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots

listed in

Generator prototype object

Table 73

has a [[Prototype]] internal slot whose value is

%Function.prototype%

See NOTE for

20.2.1.1

27.3.1.1 GeneratorFunction (

, … ,

body

)

27.3.2 Properties of the GeneratorFunction Constructor

27.3.2.1 GeneratorFunction.length

27.3.2.2 GeneratorFunction.prototype

27.3.3 Properties of the GeneratorFunction Prototype Object

777

The initial value of

GeneratorFunction.prototype.constructor

%GeneratorFunction%

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The initial value of

GeneratorFunction.prototype.prototype

is the

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

Every GeneratorFunction instance is an ECMAScript

property is the String value

"GeneratorFunction"

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

and has the internal slots listed in

property of Function instances given in

. The

value of the [[IsClassConstructor]] internal slot for all such instances is

false

Each GeneratorFunction instance has the following own properties:

The specification for the

"length"

20.2.4.1

also applies to GeneratorFunction

instances.

The specification for the

"name"

property of Function instances given in

20.2.4.2

also applies to GeneratorFunction

instances.

Whenever a GeneratorFunction instance is created another

is also created and is the initial value of the

generator function's

"prototype"

property. The value of the prototype property is used to initialize the [[Prototype]]

internal slot of a newly created Generator object when the generator

is invoked using [[Call]].

This property has the attributes { [[Writable]]:

true

, [[Enumerable]]:

false

, [[Configurable]]:

false

NOTE

AsyncGeneratorFunction objects are functions that are usually created by evaluating

AsyncGeneratorDeclaration

Unlike Function instances, the object that is the value of the a GeneratorFunction's

"prototype"

property does not have a

"constructor"

property whose value is the GeneratorFunction instance.

27.3.3.1 GeneratorFunction.prototype.constructor

27.3.3.2 GeneratorFunction.prototype.prototype

27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]

27.3.4 GeneratorFunction Instances

27.3.4.1 length

27.3.4.2 name

27.3.4.3 prototype

27.4 AsyncGeneratorFunction Objects

778

AsyncGeneratorExpression

, and

AsyncGeneratorMethod

syntactic productions. They may also be created by calling the

%AsyncGeneratorFunction%

intrinsic.

The AsyncGeneratorFunction

%AsyncGeneratorFunction%

is a subclass of

Function

creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as a

to the AsyncGeneratorFunction

. Thus the function call

AsyncGeneratorFunction (...)

is equivalent to the object

creation expression

new AsyncGeneratorFunction (...)

with the same arguments.

is designed to be subclassable. It may be used as the value of an

extends

clause of a class definition. Subclass

constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a

super

call

to create and initialize subclass instances with the internal slots

necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async
generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to
create instances of AsyncGeneratorFunction subclasses.

The last argument specifies the body (executable code) of an async generator function; any preceding arguments
specify formal parameters.

When the

AsyncGeneratorFunction

function is called with some arguments

, … ,

body

(where

might be 0, that is, there are no "

" arguments, and where

body

might also not be provided), the following steps are

taken:

1. Let

be the

2. Let

args

be the

argumentsList

that was passed to this function by [[Call]] or [[Construct]].

The AsyncGeneratorFunction

CreateDynamicFunction

(

, NewTarget,

asyncGenerator

args

NOTE

is a standard built-in

that inherits from the Function

AsyncGeneratorFunction prototype object

has a [[Prototype]] internal slot whose value is

%Function%

has a

"name"

property whose value is

"AsyncGeneratorFunction"

has the following properties:

This is a

data property

with a value of 1. This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

[[Configurable]]:

true

See NOTE for

20.2.1.1

27.4.1 The AsyncGeneratorFunction Constructor

27.4.1.1 AsyncGeneratorFunction (

, … ,

body

)

27.4.2 Properties of the AsyncGeneratorFunction Constructor

27.4.2.1 AsyncGeneratorFunction.length

779

The initial value of

AsyncGeneratorFunction.prototype

is the

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

AsyncGeneratorFunction prototype object

%AsyncGeneratorFunction.prototype%

is an

is not a

and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots

listed in

AsyncGeneratorFunction.prototype.prototype

Table 74

has a [[Prototype]] internal slot whose value is

%Function.prototype%

The initial value of

AsyncGeneratorFunction.prototype.constructor

%AsyncGeneratorFunction%

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The initial value of

is the

AsyncGenerator prototype

object

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

property is the String value

"AsyncGeneratorFunction"

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

Every AsyncGeneratorFunction instance is an ECMAScript

and has the internal slots listed in

property of Function instances given in

The value of the [[IsClassConstructor]] internal slot for all such instances is

false

Each AsyncGeneratorFunction instance has the following own properties:

The value of the

"length"

property is an

integral Number

that indicates the typical number of arguments expected by

the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of
arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the
number specified by its

"length"

property depends on the function.

27.4.2.2 AsyncGeneratorFunction.prototype

27.4.3 Properties of the AsyncGeneratorFunction Prototype Object

27.4.3.1 AsyncGeneratorFunction.prototype.constructor

27.4.3.2 AsyncGeneratorFunction.prototype.prototype

27.4.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]

27.4.4 AsyncGeneratorFunction Instances

27.4.4.1 length

780

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The specification for the

"name"

20.2.4.2

also applies to

AsyncGeneratorFunction instances.

Whenever an AsyncGeneratorFunction instance is created another

is also created and is the initial

value of the async generator function's

"prototype"

property. The value of the prototype property is used to initialize

the [[Prototype]] internal slot of a newly created AsyncGenerator object when the generator

%GeneratorFunction.prototype.prototype%

is invoked

using [[Call]].

This property has the attributes { [[Writable]]:

true

, [[Enumerable]]:

false

, [[Configurable]]:

false

NOTE

A Generator object is an instance of a generator function and conforms to both the

Iterator

and

Iterable

interfaces.

Generator instances directly inherit properties from the object that is the initial value of the

"prototype"

property of

the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator
Prototype intrinsic,

The

Generator prototype object

%GeneratorFunction.prototype.prototype%

is an

%GeneratorFunction.prototype%

is not a Generator instance and does not have a [[GeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is

%IteratorPrototype%

has properties that are indirectly inherited by all Generator instances.

The initial value of

Generator.prototype.constructor

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The

method performs the following steps:

Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's

"prototype"

property does not have a

"constructor"

property whose value is the

AsyncGeneratorFunction instance.

27.4.4.2 name

27.4.4.3 prototype

27.5 Generator Objects

27.5.1 Properties of the Generator Prototype Object

27.5.1.1 Generator.prototype.constructor

27.5.1.2 Generator.prototype.next (

value

)

781

1. Let

be the

this

value.

2. Return ?

GeneratorResume

(

value

empty

The

return

method performs the following steps:

1. Let

be the

this

value.

2. Let

{ [[Type]]:

return

, [[Value]]:

value

, [[Target]]:

empty

GeneratorResumeAbrupt

empty

The

throw

method performs the following steps:

1. Let

be the

this

value.

2. Let

ThrowCompletion

(

exception

GeneratorResumeAbrupt

empty

property is the String value

"Generator"

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

Generator instances are initially created with the internal slots described in

Table 73

Table 73: Internal Slots of Generator Instances

Internal Slot

Description

[[GeneratorState]]

The current execution state of the generator. The possible values are:

undefined

suspendedStart

suspendedYield

executing

, and

completed

[[GeneratorContext]] The

The abstract operation GeneratorStart takes arguments

that is used when executing the code of this generator.

[[GeneratorBrand]]

A brand used to distinguish different kinds of generators. The [[GeneratorBrand]] of
generators declared by ECMAScript source text is always

empty

with no parameters). It performs the following steps when called:

: The value of

generator

.[[GeneratorState]] is

undefined

2. Let

genContext

running execution context

27.5.1.3 Generator.prototype.return (

value

)

27.5.1.4 Generator.prototype.throw (

exception

)

27.5.1.5 Generator.prototype [ @@toStringTag ]

27.5.2 Properties of Generator Instances

27.5.3 Generator Abstract Operations

27.5.3.1 GeneratorStart (

generator

generatorBody

)

782

3. Set the Generator component of

genContext

generator

4. Set the code evaluation state of

genContext

such that when evaluation is resumed for that

the

following steps will be performed:

a. If

generatorBody

is a

Parse Node

, then

i. Let

result

be the result of evaluating

generatorBody

b. Else,

generatorBody

is an

Abstract Closure

with no parameters.

ii. Let

result

generatorBody

().

: If we return here, the generator either threw an exception or performed either an implicit or

explicit return.

d. Remove

genContext

from the

execution context stack

and restore the

running execution context

that is at the top

of the

execution context stack

as the

e. Set

generator

.[[GeneratorState]] to

completed

f. Once a generator enters the

completed

state it never leaves it and its associated

never resumed. Any execution state associated with

generator

can be discarded at this point.

g. If

result

.[[Type]] is

normal

, let

resultValue

undefined

h. Else if

result

.[[Type]] is

return

, let

resultValue

result

.[[Value]].

i. Else,

result

.[[Type]] is

throw

ii. Return

(

result

j. Return

CreateIterResultObject

resultValue

true

5. Set

generator

.[[GeneratorContext]] to

genContext

6. Set

generator

.[[GeneratorState]] to

suspendedStart

7. Return

NormalCompletion

(

undefined

The abstract operation GeneratorValidate takes arguments

generator

and

generatorBrand

. It performs the following

steps when called:

1. Perform ?

RequireInternalSlot

(

generator

, [[GeneratorState]]).

2. Perform ?

RequireInternalSlot

(

generator

, [[GeneratorBrand]]).

3. If

generator

.[[GeneratorBrand]] is not the same value as

generatorBrand

, throw a

TypeError

exception.

generator

also has a [[GeneratorContext]] internal slot.

5. Let

state

generator

.[[GeneratorState]].

6. If

state

executing

, throw a

TypeError

exception.

7. Return

state

The abstract operation GeneratorResume takes arguments

generator

value

, and

generatorBrand

. It performs the

following steps when called:

1. Let

state

GeneratorValidate

(

generator

generatorBrand

2. If

state

completed

, return

CreateIterResultObject

undefined

true