ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 59

;

genContext

is now the

9. Resume the suspended evaluation of

genContext

using

NormalCompletion

value

) as the result of the operation

that suspended it. Let

result

be the value returned by the resumed computation.

10.

: When we return here,

genContext

has already been removed from the

and

methodContext

is the currently

11. Return

(

result

The abstract operation GeneratorResumeAbrupt takes arguments

generator

abruptCompletion

Completion Record

whose [[Type]] is

return

throw

), and

generatorBrand

. It performs the following steps when called:

1. Let

state

be ?

GeneratorValidate

(

generator

generatorBrand

2. If

state

suspendedStart

, then

a. Set

generator

.[[GeneratorState]] to

completed

b. 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.

c. Set

state

completed

3. If

state

completed

, then

a. If

abruptCompletion

.[[Type]] is

return

, then

i. Return

CreateIterResultObject

abruptCompletion

.[[Value]],

true

b. Return

(

abruptCompletion

state

suspendedYield

5. Let

genContext

generator

.[[GeneratorContext]].

6. Let

methodContext

be the

7. Suspend

methodContext

8. Set

generator

.[[GeneratorState]] to

executing

9. Push

genContext

onto the

;

genContext

is now the

10. Resume the suspended evaluation of

genContext

using

abruptCompletion

as the result of the operation that

suspended it. Let

result

be the completion record returned by the resumed computation.

11.

: When we return here,

genContext

has already been removed from the

and

methodContext

is the currently

12. Return

(

result

The abstract operation GetGeneratorKind takes no arguments. It performs the following steps when called:

1. Let

genContext

2. If

genContext

does not have a Generator component, return

non-generator

3. Let

generator

be the Generator component of

genContext

4. If

generator

has an [[AsyncGeneratorState]] internal slot, return

async

5. Else, return

sync

27.5.3.4 GeneratorResumeAbrupt (

generator

abruptCompletion

generatorBrand

)

27.5.3.5 GetGeneratorKind ( )

27.5.3.6 GeneratorYield (

iterNextObj

)

784

The abstract operation GeneratorYield takes argument

iterNextObj

. It performs the following steps when called:

iterNextObj

is an Object that implements the

IteratorResult

interface.

2. Let

genContext

genContext

4. Let

generator

be the value of the Generator component of

genContext

GetGeneratorKind

() is

sync

6. Set

generator

.[[GeneratorState]] to

suspendedYield

7. Remove

genContext

from the

and restore the

that is at the top of the

8. Set the code evaluation state of

genContext

such that when evaluation is resumed with a

that originally called this abstract operation.

resumptionValue

the following steps will be performed:

a. Return

resumptionValue

b. NOTE: This returns to the evaluation of the

YieldExpression

9. Return

NormalCompletion

(

iterNextObj

10. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of

genContext

The abstract operation Yield takes argument

value

(an

ECMAScript language value

). It performs the following steps

when called:

1. Let

generatorKind

be !

GetGeneratorKind

().

2. If

generatorKind

async

, return ?

AsyncGeneratorYield

(

value

3. Otherwise, return ?

GeneratorYield

CreateIterResultObject

(

value

false

)).

The abstract operation CreateIteratorFromClosure takes arguments

closure

(an

Abstract Closure

with no parameters),

generatorBrand

, and

generatorPrototype

(an Object). It performs the following steps when called:

1. NOTE:

closure

can contain uses of the

Yield

shorthand to yield an IteratorResult object.

2. Let

internalSlotsList

be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».

3. Let

generator

be !

OrdinaryObjectCreate

generatorPrototype

internalSlotsList

4. Set

generator

.[[GeneratorBrand]] to

generatorBrand

5. Set

generator

.[[GeneratorState]] to

undefined

6. Perform !

GeneratorStart

%AsyncGeneratorFunction.prototype.prototype%

generator

closure

7. Return

generator

An AsyncGenerator object is an instance of an async generator function and conforms to both the AsyncIterator and
AsyncIterable interfaces.

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

"prototype"

property of the AsyncGenerator function that created the instance. AsyncGenerator instances indirectly inherit
properties from the AsyncGenerator Prototype intrinsic,

27.5.3.7 Yield (

value

)

27.5.3.8 CreateIteratorFromClosure (

closure

generatorBrand

generatorPrototype

)

27.6 AsyncGenerator Objects

785

The

AsyncGenerator prototype object

%AsyncGeneratorFunction.prototype.prototype%

is an

ordinary object

is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is

%AsyncIteratorPrototype%

has properties that are indirectly inherited by all AsyncGenerator instances.

The initial value of

AsyncGenerator.prototype.constructor

%AsyncGeneratorFunction.prototype%

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

1. Let

generator

be the

this

value.

2. Let

completion

NormalCompletion

(

value

3. Return !

AsyncGeneratorEnqueue

generator

completion

empty

1. Let

generator

be the

this

value.

2. Let

completion

{ [[Type]]:

return

, [[Value]]:

value

, [[Target]]:

empty

3. Return !

AsyncGeneratorEnqueue

generator

completion

empty

1. Let

generator

be the

this

value.

2. Let

completion

ThrowCompletion

exception

3. Return !

AsyncGeneratorEnqueue

generator

completion

empty

The initial value of the

@@toStringTag

property is the String value

"AsyncGenerator"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

AsyncGenerator instances are initially created with the internal slots described below:

27.6.1 Properties of the AsyncGenerator Prototype Object

27.6.1.1 AsyncGenerator.prototype.constructor

27.6.1.2 AsyncGenerator.prototype.next (

value

)

27.6.1.3 AsyncGenerator.prototype.return (

value

)

27.6.1.4 AsyncGenerator.prototype.throw (

exception

)

27.6.1.5 AsyncGenerator.prototype [ @@toStringTag ]

27.6.2 Properties of AsyncGenerator Instances

786

Table 74: Internal Slots of AsyncGenerator Instances

Internal Slot

Description

[[AsyncGeneratorState]]

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

undefined

suspendedStart

suspendedYield

executing

awaiting-return

, and

completed

[[AsyncGeneratorContext]] The

that is used when executing the code of this async generator.

[[AsyncGeneratorQueue]]

of AsyncGeneratorRequest records which represent requests to resume the

async generator.

[[GeneratorBrand]]

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

empty

The AsyncGeneratorRequest is a

Record

value used to store information about how an async generator should be

resumed and contains capabilities for fulfilling or rejecting the corresponding promise.

They have the following fields:

Table 75: AsyncGeneratorRequest

Record

Fields

Field Name

Value

Meaning

[[Completion]] A

The abstract operation AsyncGeneratorStart takes arguments

record

The completion which should be used to resume the async
generator.

[[Capability]]

PromiseCapability

Record

The promise capabilities associated with this request.

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

generator

is an AsyncGenerator instance.

generator

.[[AsyncGeneratorState]] is

undefined

3. Let

genContext

4. Set the Generator component of

genContext

generator

5. 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.

27.6.3 AsyncGenerator Abstract Operations

27.6.3.1 AsyncGeneratorRequest Records

27.6.3.2 AsyncGeneratorStart (

generator

generatorBody

)

787

ii. Let

result

generatorBody

().

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

or explicit return.

d. Remove

genContext

from the

and restore the

that is at the top

of the

e. Set

generator

.[[AsyncGeneratorState]] to

completed

f. If

result

is a normal completion, let

resultValue

undefined

g. Else,

i. Let

resultValue

result

.[[Value]].

ii. If

result

.[[Type]] is not

return

, then

1. Return !

AsyncGeneratorReject

generator

resultValue

h. Return !

.[[AsyncGeneratorQueue]] to a new empty

generator

resultValue

true

6. Set

generator

.[[AsyncGeneratorContext]] to

genContext

7. Set

generator

.[[AsyncGeneratorState]] to

suspendedStart

8. Set

generator

9. Return

undefined

The abstract operation AsyncGeneratorValidate takes arguments

generator

and

generatorBrand

. It performs the

following steps when called:

1. Perform ?

RequireInternalSlot

(

generator

, [[AsyncGeneratorContext]]).

2. Perform ?

RequireInternalSlot

(

generator

, [[AsyncGeneratorState]]).

3. Perform ?

RequireInternalSlot

(

generator

, [[AsyncGeneratorQueue]]).

4. If

generator

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

generatorBrand

, throw a

TypeError

exception.

The abstract operation AsyncGeneratorResolve takes arguments

generator

value

, and

done

(a Boolean). It performs the

following steps when called:

generator

is an AsyncGenerator instance.

2. Let

queue

generator

.[[AsyncGeneratorQueue]].

queue

is not an empty

4. Let

next

be the first element of

queue

5. Remove the first element from

queue

6. Let

promiseCapability

next

.[[Capability]].

7. Let

iteratorResult

be !

CreateIterResultObject

value

done

8. Perform !

promiseCapability

.[[Resolve]],

undefined

, «

iteratorResult

»).

9. Perform !

generator

10. Return

undefined

The abstract operation AsyncGeneratorReject takes arguments

generator

and

exception

. It performs the following steps

when called:

generator

is an AsyncGenerator instance.

27.6.3.3 AsyncGeneratorValidate (

generator

generatorBrand

)

27.6.3.4 AsyncGeneratorResolve (

generator

value

done

)

27.6.3.5 AsyncGeneratorReject (

generator

exception

)

788

2. Let

queue

generator

.[[AsyncGeneratorQueue]].

queue

is not an empty

4. Let

next

be the first element of

queue

5. Remove the first element from

queue

6. Let

promiseCapability

next

.[[Capability]].

7. Perform !

promiseCapability

.[[Reject]],

undefined

, «

exception

»).

8. Perform !

generator

9. Return

undefined

The abstract operation AsyncGeneratorResumeNext takes argument

generator

. It performs the following steps when

called:

generator

is an AsyncGenerator instance.

2. Let

state

generator

.[[AsyncGeneratorState]].

state

is not

executing

4. If

state

awaiting-return

, return

undefined

5. Let

queue

generator

.[[AsyncGeneratorQueue]].

6. If

queue

is an empty

, return

undefined

7. Let

next

be the value of the first element of

queue

next

is an AsyncGeneratorRequest record.

9. Let

completion

next

.[[Completion]].

10. If

completion

is an

abrupt completion

, then

a. If

state

suspendedStart

, then

i. Set

generator

.[[AsyncGeneratorState]] to

completed

ii. Set

state

completed

b. If

state

completed

, then

i. If

completion

.[[Type]] is

return

, then

1. Set

generator

.[[AsyncGeneratorState]] to

awaiting-return

2. Let

promise

be ?

PromiseResolve

be the algorithm steps defined in

%Promise%

completion

.[[Value]]).

3. Let

stepsFulfilled

AsyncGeneratorResumeNext Return

Processor Fulfilled Functions

4. Let

lengthFulfilled

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

AsyncGeneratorResumeNext Return Processor Fulfilled Functions

5. Let

onFulfilled

be !

CreateBuiltinFunction

(

stepsFulfilled

lengthFulfilled

, « [[Generator]] »).

6. Set

onFulfilled

.[[Generator]] to

generator

7. Let

stepsRejected

be the algorithm steps defined in

AsyncGeneratorResumeNext Return

Processor Rejected Functions

8. Let

lengthRejected

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

AsyncGeneratorResumeNext Return Processor Rejected Functions

9. Let

onRejected

be !

CreateBuiltinFunction

stepsRejected

lengthRejected

, « [[Generator]] »).

10. Set

onRejected

.[[Generator]] to

generator

11. Perform !

PerformPromiseThen

(

promise

onFulfilled

onRejected

12. Return

undefined

ii. Else,

completion

.[[Type]] is

throw

2. Perform !

AsyncGeneratorReject

generator

completion

.[[Value]]).

27.6.3.6 AsyncGeneratorResumeNext (

generator

)

789

3. Return

undefined

11. Else if

state

completed

, return !

(

generator

undefined

true

12.

state

is either

suspendedStart

suspendedYield

13. Let

genContext

generator

.[[AsyncGeneratorContext]].

14. Let

callerContext

be the

15. Suspend

callerContext

16. Set

generator

.[[AsyncGeneratorState]] to

executing

17. Push

genContext

onto the

;

genContext

is now the

18. Resume the suspended evaluation of

genContext

using

completion

as the result of the operation that suspended

it. Let

result

be the completion record returned by the resumed computation.

19.

result

is never an

abrupt completion

20.

: When we return here,

genContext

has already been removed from the

and

callerContext

is the currently

21. Return

undefined

return processor fulfilled function is an anonymous built-in function that is used as

part of the

AsyncGenerator.prototype.return (

specification device to unwrap promises passed in to the

value

)

method. Each

return processor fulfilled

function has a [[Generator]] internal slot.

When an

return processor fulfilled function is called with argument

value

, the following

steps are taken:

1. Let

active function object

2. Set

.[[Generator]].[[AsyncGeneratorState]] to

completed

3. Return !

.[[Generator]],

value

true

The

"length"

property of an

return processor fulfilled function is

𝔽

return processor rejected function is an anonymous built-in function that is used as

part of the

AsyncGenerator.prototype.return (

specification device to unwrap promises passed in to the

value

)

method. Each

return processor rejected

function has a [[Generator]] internal slot.

When an

return processor rejected function is called with argument

reason

, the following

steps are taken:

1. Let

active function object

2. Set

.[[Generator]].[[AsyncGeneratorState]] to

completed

3. Return !

AsyncGeneratorReject

.[[Generator]],

reason

The

"length"

property of an

return processor rejected function is

𝔽

The abstract operation AsyncGeneratorEnqueue takes arguments

generator

completion

Completion Record

), and

27.6.3.6.1 AsyncGeneratorResumeNext Return Processor Fulfilled Functions

27.6.3.6.2 AsyncGeneratorResumeNext Return Processor Rejected Functions

27.6.3.7 AsyncGeneratorEnqueue (

generator

completion

generatorBrand

)

790

generatorBrand

. It performs the following steps when called:

1. Let

promiseCapability

be !

NewPromiseCapability

%Promise%

2. Let

check

AsyncGeneratorValidate

(

generator

generatorBrand

3. If

check

is an

abrupt completion

, then

a. Let

badGeneratorError

be a newly created

TypeError

object.

b. Perform !

promiseCapability

.[[Reject]],

undefined

, «

badGeneratorError

»).

c. Return

promiseCapability

.[[Promise]].

4. Let

queue

generator

.[[AsyncGeneratorQueue]].

5. Let

request

be AsyncGeneratorRequest { [[Completion]]:

completion

, [[Capability]]:

promiseCapability

6. Append

request

to the end of

queue

7. Let

state

generator

.[[AsyncGeneratorState]].

8. If

state

is not

executing

, then

a. Perform !

generator

9. Return

promiseCapability

.[[Promise]].

The abstract operation AsyncGeneratorYield takes argument

value

. It performs the following steps when called:

1. Let

genContext

genContext

3. Let

generator

be the value of the Generator component of

genContext

GetGeneratorKind

() is

async

5. Set

value

to ?

(

value

6. Set

generator

.[[AsyncGeneratorState]] to

suspendedYield

7. Remove

genContext

from the

and restore the

that is at the top of the

8. Set the code evaluation state of

genContext

such that when evaluation is resumed with a

resumptionValue

the following steps will be performed:

a. If

resumptionValue

.[[Type]] is not

return

, return

resumptionValue

b. Let

awaited

resumptionValue

.[[Value]]).

c. If

awaited

.[[Type]] is

throw

, return

(

awaited

awaited

.[[Type]] is

normal

e. Return

{ [[Type]]:

return

, [[Value]]:

awaited

.[[Value]], [[Target]]:

empty

f. NOTE: When one of the above steps returns, it returns to the evaluation of the

YieldExpression

production that originally called this abstract operation.

9. Return !

generator

value

false

10. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of

genContext

The abstract operation CreateAsyncIteratorFromClosure takes arguments

closure

(an

Abstract Closure

with no

parameters),

generatorBrand

, and

generatorPrototype

(an Object). It performs the following steps when called:

1. NOTE:

closure

can contain uses of the

shorthand and uses of the

Yield

shorthand to yield an

IteratorResult object.

2. Let

internalSlotsList

be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]],

27.6.3.8 AsyncGeneratorYield (

value

)

27.6.3.9 CreateAsyncIteratorFromClosure (

closure

generatorBrand

generatorPrototype

)

791

[[GeneratorBrand]] ».

3. Let

generator

be !

OrdinaryObjectCreate

AsyncFunctionDeclaration

generatorPrototype

internalSlotsList

4. Set

generator

.[[GeneratorBrand]] to

generatorBrand

5. Set

generator

.[[AsyncGeneratorState]] to

undefined

6. Perform !

AsyncGeneratorStart

(

generator

closure

7. Return

generator

AsyncFunction objects are functions that are usually created by evaluating

AsyncFunctionExpression

AsyncMethod

s, and

AsyncArrowFunction

s. They may also be created by calling the

%AsyncFunction%

intrinsic.

The AsyncFunction

%AsyncFunction%

is a subclass of

Function

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

. Thus

the function call

AsyncFunction(…)

is equivalent to the object creation expression

new AsyncFunction(…)

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 AsyncFunction behaviour must include a

super

call to the

AsyncFunction

to create and initialize a subclass instance with the internal slots necessary for built-

in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct
instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.

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

When the

AsyncFunction

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

active function object

2. Let

args

be the

argumentsList

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

3. Return

CreateDynamicFunction

(

, NewTarget,

async

args

NOTE

The AsyncFunction

is a standard built-in

that inherits from the Function

AsyncFunction prototype object

See NOTE for

20.2.1.1

27.7 AsyncFunction Objects

27.7.1 The AsyncFunction Constructor

27.7.1.1 AsyncFunction (

, … ,

body

)

27.7.2 Properties of the AsyncFunction Constructor

792

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

%Function%

has a

"name"

property whose value is

"AsyncFunction"

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

AsyncFunction.prototype

is the

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

AsyncFunction prototype object

%AsyncFunction.prototype%

is an

ordinary object

is not a

Every AsyncFunction instance is an ECMAScript

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

listed in

Table 29

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

%Function.prototype%

The initial value of

AsyncFunction.prototype.constructor

%AsyncFunction%

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The initial value of the

@@toStringTag

property is the String value

"AsyncFunction"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

and has the internal slots listed in

Table 29

. The value

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

false

. AsyncFunction instances are not constructors

and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they
are not constructible.

Each AsyncFunction instance has the following own properties:

The specification for the

"length"

property of Function instances given in

20.2.4.1

also applies to AsyncFunction

27.7.2.1 AsyncFunction.length

27.7.2.2 AsyncFunction.prototype

27.7.3 Properties of the AsyncFunction Prototype Object

27.7.3.1 AsyncFunction.prototype.constructor

27.7.3.2 AsyncFunction.prototype [ @@toStringTag ]

27.7.4 AsyncFunction Instances

27.7.4.1 length

793

instances.

The specification for the

"name"

property of Function instances given in

20.2.4.2

also applies to AsyncFunction

instances.

The abstract operation AsyncFunctionStart takes arguments

promiseCapability

PromiseCapability Record

) and

asyncFunctionBody

. It performs the following steps when called:

1. Let

runningContext

be the

2. Let

asyncContext

be a copy of

runningContext

3. NOTE: Copying the execution state is required for the step below to resume its execution. It is ill-defined to

resume a currently executing context.

4. Set the code evaluation state of

asyncContext

such that when evaluation is resumed for that

the following steps will be performed:

a. Let

result

be the result of evaluating

asyncFunctionBody

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

explicit return; all awaiting is done.

c. Remove

asyncContext

from the

and restore the

that is at the top

of the

d. If

result

.[[Type]] is

normal

, then

i. Perform !

promiseCapability

.[[Resolve]],

undefined

, «

undefined

»).

e. Else if

result

.[[Type]] is

return

, then

i. Perform !

promiseCapability

.[[Resolve]],

undefined

, «

result

.[[Value]] »).

f. Else,

result

.[[Type]] is

throw

ii. Perform !

promiseCapability

.[[Reject]],

undefined

, «

result

.[[Value]] »).

g. Return.

5. Push

asyncContext

onto the

;

asyncContext

is now the

6. Resume the suspended evaluation of

asyncContext

. Let

result

be the value returned by the resumed

computation.

: When we return here,

asyncContext

has already been removed from the

and

runningContext

is the currently

result

is a normal completion with a value of

undefined

. The possible sources of completion values are

or, if the async function doesn't await anything, step

4.g

above.

9. Return.

27.7.4.2 name

27.7.5 Async Functions Abstract Operations

27.7.5.1 AsyncFunctionStart (

promiseCapability

asyncFunctionBody

)

28 Reflection

794

The Reflect object:

%Reflect%

is the initial value of the

"Reflect"

property of the

global object

is an

ordinary object

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

%Object.prototype%

is not a

does not have a [[Construct]] internal method; it cannot be used as a

with the

new

operator.

does not have a [[Call]] internal method; it cannot be invoked as a function.

When the

apply

function is called with arguments

target

thisArgument

, and

argumentsList

, the following steps are

taken:

1. If

IsCallable

target

) is

false

, throw a

TypeError

exception.

2. Let

args

be ?

CreateListFromArrayLike

argumentsList

3. Perform

PrepareForTailCall

().

4. Return ?

(

target

thisArgument

args

When the

construct

function is called with arguments

target

argumentsList

, and

newTarget

, the following steps are

taken:

1. If

IsConstructor

target

) is

false

, throw a

TypeError

exception.

2. If

newTarget

is not present, set

newTarget

target

3. Else if

IsConstructor

(

newTarget

) is

false

, throw a

TypeError

exception.

4. Let

args

be ?

CreateListFromArrayLike

argumentsList

5. Return ?

Construct

(

target

args

newTarget

When the

defineProperty

function is called with arguments

target

propertyKey

, and

attributes

, the following

steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

propertyKey

3. Let

desc

be ?

ToPropertyDescriptor

(

attributes

4. Return ?

target

.[[DefineOwnProperty]](

key

desc

When the

deleteProperty

function is called with arguments

target

and

propertyKey

, the following steps are

taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

28.1 The Reflect Object

28.1.1 Reflect.apply (

target

thisArgument

argumentsList

)

28.1.2 Reflect.construct (

target

argumentsList

[ ,

newTarget

] )

28.1.3 Reflect.defineProperty (

target

propertyKey

attributes

)

28.1.4 Reflect.deleteProperty (

target

propertyKey

)

795

2. Let

key

be ?

(

propertyKey

3. Return ?

target

.[[Delete]](

key

When the

get

function is called with arguments

target

propertyKey

, and

receiver

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

propertyKey

3. If

receiver

is not present, then

a. Set

receiver

target

4. Return ?

target

.[[Get]](

key

receiver

When the

getOwnPropertyDescriptor

function is called with arguments

target

and

propertyKey

, the following

steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

propertyKey

3. Let

desc

be ?

target

.[[GetOwnProperty]](

key

4. Return

FromPropertyDescriptor

desc

When the

getPrototypeOf

function is called with argument

target

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Return ?

target

.[[GetPrototypeOf]]().

When the

has

function is called with arguments

target

and

propertyKey

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

propertyKey

3. Return ?

target

.[[HasProperty]](

key

When the

isExtensible

function is called with argument

target

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Return ?

target

.[[IsExtensible]]().

When the

ownKeys

function is called with argument

target

, the following steps are taken:

28.1.5 Reflect.get (

target

propertyKey

[ ,

receiver

] )

28.1.6 Reflect.getOwnPropertyDescriptor (

target

propertyKey

)

28.1.7 Reflect.getPrototypeOf (

target

)

28.1.8 Reflect.has (

target

propertyKey

)

28.1.9 Reflect.isExtensible (

target

)

28.1.10 Reflect.ownKeys (

target

)

796

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

keys

be ?

target

.[[OwnPropertyKeys]]().

3. Return

CreateArrayFromList

keys

When the

preventExtensions

function is called with argument

target

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Return ?

target

.[[PreventExtensions]]().

When the

set

function is called with arguments

target

propertyKey

, and

receiver

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

propertyKey

3. If

receiver

is not present, then

a. Set

receiver

target

4. Return ?

target

.[[Set]](

key

receiver

When the

setPrototypeOf

function is called with arguments

target

and

proto

, the following steps are taken:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. If

(

proto

) is not Object and

proto

is not

null

, throw a

TypeError

exception.

3. Return ?

target

.[[SetPrototypeOf]](

proto

The initial value of the

@@toStringTag

property is the String value

"Reflect"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

The Proxy

creates and initializes a new

%Proxy%

is the initial value of the

"Proxy"

property of the

global object

Proxy exotic object

when called as a