ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 20

. The specific steps are

The specific choice of

is up to the

and

is only in use before any callback function is invoked. When a callback function related to

, like a Promise handler, is invoked, the invocation pushes its own

execution context

and

Realm

9.4 Jobs and Host Operations to Enqueue Jobs

9.4.1 JobCallback Records

211

context. To propagate the state,

The function to invoke when the

Abstract Closures should not capture and call function objects directly. Instead, use

HostMakeJobCallback

and

HostCallJobCallback

NOTE

JobCallback Records have the fields listed in

Table 27

Table 27:

JobCallback Record

Fields

Field Name

Value

Meaning

[[Callback]]

function object

is invoked.

[[HostDefined]] Any, default value is

empty

. Field reserved for use by hosts.

The

abstract operation HostMakeJobCallback takes argument

callback

function object

The implementation of HostMakeJobCallback must conform to the following requirements:

It must always complete normally (i.e., not return an

abrupt completion

It must always return a

JobCallback Record

whose [[Callback]] field is

callback

The default implementation of HostMakeJobCallback performs the following steps when called:

IsCallable

(

callback

) is

true

2. Return the

JobCallback Record

{ [[Callback]]:

callback

, [[HostDefined]]:

empty

ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.

NOTE

The

ECMAScript language value

abstract operation HostCallJobCallback takes arguments

jobCallback

JobCallback Record

(an

), and

argumentsList

of ECMAScript language values).

The implementation of HostCallJobCallback must conform to the following requirements:

It must always perform and return the result of

https://html.spec.whatwg.org/

(

jobCallback

.[[Callback]],

argumentsList

NOTE

The WHATWG HTML specification (

), for example, uses the

value to propagate the incumbent settings object for Promise callbacks.

This is called at the time that the callback is passed to the function that is responsible for its being
eventually scheduled and run. For example,

promise.then(thenAction)

calls

MakeJobCallback on

thenAction

at the time of invoking

Promise.prototype.then

not at the time of scheduling the reaction

This requirement means that hosts cannot change the [[Call]] behaviour of function objects
defined in this specification.

9.4.2 HostMakeJobCallback (

callback

)

9.4.3 HostCallJobCallback (

jobCallback

argumentsList

)

212

The default implementation of HostCallJobCallback performs the following steps when called:

IsCallable

(

jobCallback

.[[Callback]]) is

true

2. Return ?

(

jobCallback

.[[Callback]],

argumentsList

ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.

The

as well as the following:

abstract operation HostEnqueuePromiseJob takes arguments

job

realm

null

). It schedules

job

to be performed at some future time. The Abstract Closures used with this

algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to
Promise handling operations.

The implementation of HostEnqueuePromiseJob must conform to the requirements in

9.4

realm

is not

null

, each time

job

is invoked the implementation must perform

prepared to evaluate ECMAScript code

steps

such that execution is

at the time of

job

's invocation.

Let

scriptOrModule

GetActiveScriptOrModule

() at the time HostEnqueuePromiseJob is invoked. If

realm

not

null

, each time

job

is invoked the implementation must perform

steps such that

scriptOrModule

is the

active script or module

at the time of

job

's invocation.

Jobs must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.

NOTE

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

1. Let

realm

CreateRealm

().

2. Let

newContext

be a new

execution context

3. Set the Function of

newContext

null

4. Set the

Realm

newContext

realm

5. Set the ScriptOrModule of

newContext

null

6. Push

newContext

onto the

running execution context

;

newContext

is now the

7. If the

requires use of an

realm

global

be such an object created in

manner. Otherwise, let

global

undefined

, indicating that an

should be created

as the

8. If the

host

requires that the

this

binding in

realm

's global scope return an object other than the

let

thisValue

be such an object created in a

NewPromiseResolveThenableJob

manner. Otherwise, let

thisValue

undefined

The

realm

for Jobs returned by

is usually the result of calling

GetFunctionRealm

on the

then

function object

. The

realm

for Jobs returned by

NewPromiseReactionJob

is usually the result of calling

GetFunctionRealm

on the handler if the

handler is not

undefined

. If the handler is

undefined

realm

null

. For both kinds of Jobs, when

GetFunctionRealm

completes abnormally (i.e. called on a revoked Proxy),

realm

is the current

realm

null

, no user ECMAScript

code will be evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The
WHATWG HTML specification (

https://html.spec.whatwg.org/

), for example, uses

realm

check for the ability to run script and for the

entry

concept.

9.4.4 HostEnqueuePromiseJob (

job

realm

)

9.5 InitializeHostDefinedRealm ( )

213

indicating that

realm

's global

this

binding should be the

9. Perform

SetRealmGlobalObject

(

realm

global

thisValue

10. Let

globalObj

be ?

SetDefaultGlobalBindings

realm

11. Create any

comprises a set of ECMAScript execution contexts, an

properties on

globalObj

12. Return

NormalCompletion

(

empty

agent

running execution context

, a

Agent Record

, and an

executing thread

. Except for the

, the constituents of an

executes a job on the

's execution contexts independently of other agents, except that

may be used as the

by multiple agents, provided none of the agents sharing the

thread have an

to access the specification level execution objects held within the

whose [[CanBlock]] property is

true

NOTE 1

While an

is the

surrounding agent

for the code in those jobs. The code

uses the

surrounding agent

, and the

's fields.

Some web browsers share a single

across multiple unrelated tabs of a browser

window, for example.

9.6 Agents

214

Table 28:

Agent Record

Fields

Field Name

Value

Meaning

[[LittleEndian]]

Boolean

The default value computed for the

isLittleEndian

parameter when it is needed

by the algorithms

and should be the alternative that is most efficient for

the implementation. Once the value has been observed it cannot change.

[[CanBlock]]

Boolean

Determines whether the

can block or not.

[[Signifier]]

Any
globally-
unique
value

Uniquely identifies the

candidate
execution
Record

within its

agent cluster

[[IsLockFree1]]

Boolean

true

if atomic operations on one-byte values are lock-free,

false

otherwise.

[[IsLockFree2]]

Boolean

true

if atomic operations on two-byte values are lock-free,

false

otherwise.

[[IsLockFree8]]

Boolean

true

if atomic operations on eight-byte values are lock-free,

false

otherwise.

[[CandidateExecution]] A

See the

memory model

[[KeptAlive]]

objects

Initially a new empty

, representing the list of objects to be kept alive until

the end of the current

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any

NOTE 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the
hardware, but may also reflect implementation choices that can vary over time and between
ECMAScript implementations.

There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-
free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps
may be required to complete a lock-free atomic operation.

That an atomic access of size

is lock-free does not imply anything about the (perceived)

atomicity of non-atomic accesses of size

, specifically, non-atomic accesses may still be

performed as a sequence of several separate memory accesses. See

ReadSharedMemory

and

WriteSharedMemory

for details.

215

NOTE 3

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

1. Let

2. Return

.[[Signifier]].

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

1. Let

2. Return

.[[CanBlock]].

NOTE

agent cluster

is a maximal set of agents that can communicate by operating on shared memory.

NOTE 1

Every

belongs to exactly one agent cluster.

NOTE 2

All agents within a cluster must have the same value for the [[LittleEndian]] property in their respective

Agent

Records.

NOTE 3

All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respective

Agent

Records; similarly for the [[IsLockFree2]] property.

is a specification mechanism and need not correspond to any particular artefact of an

ECMAScript implementation.

In some environments it may not be reasonable for a given

to suspend. For example, in a

web browser environment, it may be reasonable to disallow suspending a document's main
event handling thread, while still allowing workers' event handling threads to suspend.

Programs within different agents may share memory by unspecified means. At a minimum, the
backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they
are never in the same agent cluster.

The agents in a cluster need not all be alive at some particular point in time. If

creates

another

, after which

terminates and

creates

, the three agents are in the same

cluster if

could share some memory with

and

could share some memory with

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes
hard to use shared memory for multi-byte data.

9.6.1 AgentSignifier ( )

9.6.2 AgentCanSuspend ( )

9.7 Agent Clusters

216

All agents within a cluster must have different values for the [[Signifier]] property in their respective

Agent

Records.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) an

without the

An embedding may terminate an

's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while

other agents in the cluster are deactivated indefinitely.

NOTE 4

without any of the

's cluster's other agents' prior knowledge or

cooperation. If an

is terminated not by programmatic action of its own or of another

in the cluster but by

forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in
the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining
member of the cluster will be able to detect the termination, with the termination data containing enough information
to identify the

Prior to any evaluation of any ECMAScript code by any

that was terminated.

NOTE 5

in a cluster, the [[CandidateExecution]] field of the

empty candidate execution

for all agents in the cluster is set to the initial

whose [[EventsRecords]] field is a

, an

whose [[AgentSignifier]] field is that

's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields

are empty Lists.

NOTE 6

NOTE 7

The purpose of the preceding restriction is to avoid a situation where an

deadlocks or

starves because another

has been deactivated. For example, if an HTML shared worker that

has a lifetime independent of documents in any windows were allowed to share memory with
the dedicated worker of such an independent document, and the document and its dedicated
worker were to be deactivated while the dedicated worker holds a lock (say, the document is
pushed into its window's history), and the shared worker then tries to acquire the lock, then the
shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile
other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents
that don't belong to the same suspend/wake collective within the embedding.

Examples of that type of termination are: operating systems or users terminating agents that are
running in separate processes; the embedding itself terminating an

that is running in-

process with the other agents when per-

resource accounting indicates that the

runaway.

All agents in an agent cluster share the same

candidate execution

in its

[[CandidateExecution]] field. The

candidate execution

is a specification mechanism used by the

memory model

An agent cluster is a specification mechanism and need not correspond to any particular artefact
of an ECMAScript implementation.

9.8 Forward Progress

217

For an

make forward progress

is for it to perform an evaluation step according to this specification.

running execution context

becomes

blocked

when its

waits synchronously and indefinitely for an external

event. Only agents whose

's [[CanBlock]] property is

true

can become blocked in this sense. An

unblocked

is one that is not blocked.

Implementations must ensure that:

eventually makes forward progress

in a set of agents that share an

, one

eventually makes forward progress

does not cause another

to become blocked except via explicit APIs that provide blocking.

NOTE

This specification does not make any guarantees that any object will be garbage collected. Objects which are not

may be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when
describing behaviour triggered by garbage collection.

objects is based on two operations which happen at particular

points in time:

When

WeakRef.prototype.deref

is called, the referent (if

undefined

is not returned) is kept alive so

that subsequent, synchronous accesses also return the object. This list is reset when synchronous work is done
using the

abstract operation.

When an object which is registered with a

becomes unreachable, a call of the

's cleanup callback may eventually be made, after synchronous ECMAScript execution

completes. The

cleanup is performed with the

abstract

operation.

Neither of these actions (

) may interrupt synchronous ECMAScript

execution. Because hosts may assemble longer, synchronous ECMAScript execution runs, this specification defers the
scheduling of

and

to the

host environment

Some ECMAScript implementations include garbage collector implementations which run in the background,
including when ECMAScript is idle. Letting the

host environment

schedule

This, along with the liveness guarantee in the

allows it to

resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall
memory usage.

For some set of objects

, a

hypothetical WeakRef-oblivious

execution with respect to

is an execution whereby the

whose referent is an element of

always returns

undefined

memory model

, ensures that all

SeqCst

writes

eventually become observable to all agents.

9.9 Processing Model of WeakRef and FinalizationRegistry Objects

9.9.1 Objectives

9.9.2 Liveness

218

NOTE 1

NOTE 2

NOTE 3

At any point during evaluation, a set of objects

is considered

live

if either of the following conditions is met:

Any element in

is included in any

's [[KeptAlive]]

There exists a valid future hypothetical WeakRef-oblivious execution with respect to

that observes the Object

value of any object in

NOTE 4

NOTE 5

NOTE 6

At any time, if a set of objects

is not

, an ECMAScript implementation may perform the following steps

atomically:

1. For each element

obj

, do

a. For each

ref

such that

ref

.[[WeakRefTarget]] is

obj

, do

i. Set

ref

.[[WeakRefTarget]] to

empty

-obliviousness, together with liveness, capture two notions. One, that a

itself

does not keep an object alive. Two, that cycles in liveness does not imply that an object is live. To
be concrete, if determining

obj

's liveness depends on determining the liveness of another

referent,

obj2

's liveness cannot assume

obj

's liveness, which would be circular

reasoning.

-obliviousness is defined on sets of objects instead of individual objects to account for

cycles. If it were defined on individual objects, then an object in a cycle will be considered live
even though its Object value is only observed via WeakRefs of other objects in the cycle.

Colloquially, we say that an individual object is live if every set of objects containing it is live.

The intuition the second condition above intends to capture is that an object is live if its identity
is observable via non-

means. An object's identity may be observed by observing a strict

equality comparison between objects or observing the object being used as key in a Map.

Presence of an object in a field, an internal slot, or a property does not imply that the object is
live. For example if the object in question is never passed back to the program, then it cannot be
observed.

This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]]
and [[UnregisterToken]] fields of a

Cell record.

The above definition implies that, if a key in a WeakMap is not live, then its corresponding value
is not necessarily live either.

Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness
as defined here is undecidable. In practice, engines use conservative approximations such as
reachability. There is expected to be significant implementation leeway.

9.9.3 Execution

219

b. For each

HostEnqueueFinalizationRegistryCleanupJob

such that

.[[Cells]] contains a

Record

cell

such that

cell

[[WeakRefTarget]] is

obj

, do

i. Set

cell

.[[WeakRefTarget]] to

empty

ii. Optionally, perform !

(

c. For each WeakMap

map

such that

map

.[[WeakMapData]] contains a

Record

such that

.[[Key]] is

obj

, do

i. Set

.[[Key]] to

empty

ii. Set

.[[Value]] to

empty

d. For each WeakSet

set

such that

set

.[[WeakSetData]] contains

obj

, do

i. Replace the element of

set

.[[WeakSetData]] whose value is

obj

with an element whose value is

empty

NOTE 1

NOTE 2

The abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument

finalizationRegistry

). HostEnqueueFinalizationRegistryCleanupJob is an

abstract operation

that is expected to call

finalizationRegistry

) at some point in the future, if possible. The

host

's responsibility is to make this call at a time which does not interrupt synchronous ECMAScript code execution.

Together with the definition of liveness, this clause prescribes legal optimizations that an
implementation may apply regarding WeakRefs.

It is possible to access an object without observing its identity. Optimizations such as dead
variable elimination and scalar replacement on properties of non-escaping objects whose identity
is not observed are allowed. These optimizations are thus allowed to observably empty
WeakRefs that point to such objects.

On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]]
internal slot of a

, optimizations such as rematerialization that observably empty the

HostEnqueueFinalizationRegistryCleanupJob

are prohibited.

Because calling

is optional, registered objects in a

do not necessarily hold that

. Implementations may

omit

callbacks for any reason, e.g., if the

itself becomes

dead, or if the application is shutting down.

Implementations are not obligated to empty WeakRefs for maximal sets of non-

If an implementation chooses a non-

objects.

set

in which to empty WeakRefs, it must empty

WeakRefs for all objects in

simultaneously. In other words, an implementation must not empty

pointing to an object

obj

without emptying out other WeakRefs that, if not emptied,

could result in an execution that observes the Object value of

obj

9.9.4 Host Hooks

9.9.4.1 HostEnqueueFinalizationRegistryCleanupJob (

finalizationRegistry

)

9.10 ClearKeptObjects ( )

220

The abstract operation ClearKeptObjects takes no arguments. ECMAScript implementations are expected to call
ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps
when called:

1. Let

agentRecord

be the

surrounding agent

.[[KeptAlive]] to a new empty

2. Set

agentRecord

The abstract operation AddToKeptObjects takes argument

object

(an Object). It performs the following steps when

called:

1. Let

agentRecord

be the

surrounding agent

2. Append

object

agentRecord

.[[KeptAlive]].

NOTE

The abstract operation CleanupFinalizationRegistry takes argument

finalizationRegistry

). It

performs the following steps when called:

finalizationRegistry

has [[Cells]] and [[CleanupCallback]] internal slots.

2. Let

callback

finalizationRegistry

.[[CleanupCallback]].

3. While

finalizationRegistry

.[[Cells]] contains a

Record

cell

such that

cell

.[[WeakRefTarget]] is

empty

, an

implementation may perform the following steps:

a. Choose any such

cell

b. Remove

cell

from

finalizationRegistry

.[[Cells]].

c. Perform ?

(

callback

undefined

, «

cell

.[[HeldValue]] »).

4. Return

NormalCompletion

(

undefined

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either

null

or an object

and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (and visible as
properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited
for both get access and set access.

Every

has a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related

internal method invariants specified in

6.1.7.3

. Namely, once the value of an object's [[Extensible]] internal slot has

been set to

false

, it is no longer possible to add properties to the object, to modify the value of the object's

When the abstract operation AddToKeptObjects is called with a target object reference, it adds
the target to a list that will point strongly at the target until

is called.

9.11 AddToKeptObjects (

object

)

9.12 CleanupFinalizationRegistry (

finalizationRegistry

)

10 Ordinary and Exotic Objects Behaviours

10.1 Ordinary Object Internal Methods and Internal Slots

221

[[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to

true

In the following algorithm descriptions, assume

is an

ECMAScript language value

is a property key value,

is any

, and

Desc

record.

internal method delegates to a similarly-named abstract operation. If such an abstract operation

depends on another internal method, then the internal method is invoked on

rather than calling the similarly-

named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods
invoked when

internal methods are applied to them.

The [[GetPrototypeOf]] internal method of an

takes no arguments. It performs the following steps

when called:

1. Return !

OrdinaryGetPrototypeOf

The abstract operation OrdinaryGetPrototypeOf takes argument

(an Object). It performs the following steps when

called:

1. Return

.[[Prototype]].

The [[SetPrototypeOf]] internal method of an

takes argument

(an Object or

null

). It performs the

following steps when called:

1. Return !

OrdinarySetPrototypeOf

The abstract operation OrdinarySetPrototypeOf takes arguments

: Either

(

(

) is Null.

2. Let

current

.[[Prototype]].

3. If

current

) is

true

, return

true

4. Let

extensible

.[[Extensible]].

5. If

extensible

false

, return

false

6. Let

7. Let

done

false

8. Repeat, while

done

false

a. If

null

, set

done

true

b. Else if

) is

true

, return

false

c. Else,

i. If

.[[GetPrototypeOf]] is not the

internal method defined in

10.1.1

, set

done

true

ii. Else, set

.[[Prototype]].

10.1.1 [[GetPrototypeOf]] ( )

10.1.1.1 OrdinaryGetPrototypeOf (

)

10.1.2 [[SetPrototypeOf]] (

)

10.1.2.1 OrdinarySetPrototypeOf (

)

222

9. Set

.[[Prototype]] to

10. Return

true

NOTE

The [[IsExtensible]] internal method of an

takes no arguments. It performs the following steps when

called:

1. Return !

OrdinaryIsExtensible

The abstract operation OrdinaryIsExtensible takes argument

(an Object). It performs the following steps when

called:

1. Return

.[[Extensible]].

The [[PreventExtensions]] internal method of an

OrdinaryPreventExtensions

takes no arguments. It performs the following steps

when called:

1. Return !

(

The abstract operation OrdinaryPreventExtensions takes argument

(an Object). It performs the following steps

when called:

1. Set

.[[Extensible]] to

false

2. Return

true

The [[GetOwnProperty]] internal method of an

takes argument

(a property key). It performs the

following steps when called:

1. Return !

OrdinaryGetOwnProperty

(

The abstract operation OrdinaryGetOwnProperty takes arguments

(an Object) and

(a property key). It performs

the following steps when called:

) is

true

The loop in step

guarantees that there will be no circularities in any prototype chain that only

includes objects that use the

definitions for [[GetPrototypeOf]] and

[[SetPrototypeOf]].

10.1.3 [[IsExtensible]] ( )

10.1.3.1 OrdinaryIsExtensible (

)

10.1.4 [[PreventExtensions]] ( )

10.1.4.1 OrdinaryPreventExtensions (

)

10.1.5 [[GetOwnProperty]] (

)

10.1.5.1 OrdinaryGetOwnProperty (

)

223

2. If

does not have an own property with key

, return

undefined

3. Let

be a newly created

with no fields.

4. Let

's own property whose key is

5. If

is a

, then

a. Set

.[[Value]] to the value of

's [[Value]] attribute.

b. Set

.[[Writable]] to the value of

's [[Writable]] attribute.

6. Else,

is an

b. Set

.[[Get]] to the value of

's [[Get]] attribute.

c. Set

.[[Set]] to the value of

's [[Set]] attribute.

7. Set

.[[Enumerable]] to the value of

's [[Enumerable]] attribute.

8. Set

.[[Configurable]] to the value of

's [[Configurable]] attribute.

9. Return

The [[DefineOwnProperty]] internal method of an

takes arguments

(a property key) and

Desc

). It performs the following steps when called:

OrdinaryDefineOwnProperty

Desc

The abstract operation OrdinaryDefineOwnProperty takes arguments

(an Object),

(a property key), and

Desc

ValidateAndApplyPropertyDescriptor

). It performs the following steps when called:

1. Let

current

be ?

.[[GetOwnProperty]](

2. Let

extensible

be ?

IsExtensible

(

3. Return

The abstract operation IsCompatiblePropertyDescriptor takes arguments

extensible

Desc

current

ValidateAndApplyPropertyDescriptor

1. Return

undefined

Extensible

Desc

Current

The abstract operation ValidateAndApplyPropertyDescriptor takes arguments

(an Object or

undefined

property key),

extensible

(a Boolean),

Desc

), and

current

). It performs the

following steps when called:

NOTE

: If

is not

undefined

, then

(

) is

true

2. If

current

undefined

, then

a. If

extensible

false

, return

false

undefined

is passed as

, only validation is performed and no object updates are performed.

10.1.6 [[DefineOwnProperty]] (

Desc

)

10.1.6.1 OrdinaryDefineOwnProperty (

Desc

)

10.1.6.2 IsCompatiblePropertyDescriptor (

Extensible

Desc

Current

)

10.1.6.3 ValidateAndApplyPropertyDescriptor (

extensible

Desc

current

)

224

extensible

true

c. If

IsGenericDescriptor

Desc

) is

true

(

Desc

) is

true

, then

i. If

is not

undefined

, create an own

named

of object

whose [[Value]],

[[Writable]], [[Enumerable]], and [[Configurable]] attribute values are described by

Desc

. If the

value of an attribute field of

Desc

is absent, the attribute of the newly created property is set to its

d. Else,

(

Desc

) is

true

ii. If

is not

undefined

, create an own

is absent, the attribute of the newly created property is set to its

named

of object

whose [[Get]], [[Set]],

[[Enumerable]], and [[Configurable]] attribute values are described by

Desc

. If the value of an

attribute field of

Desc

default

value

e. Return

true

3. If every field in

Desc

is absent, return

true

4. If

current

.[[Configurable]] is

false

, then

a. If

Desc

.[[Configurable]] is present and its value is

true

, return

false

b. If

Desc

.[[Enumerable]] is present and !

(

Desc

.[[Enumerable]],

current

.[[Enumerable]]) is

false

return

false

5. If !

IsGenericDescriptor

Desc

) is

true

, then

a. NOTE: No further validation is required.

6. Else if !

(

current

), !

(

Desc

)) is

false

, then

a. If

current

.[[Configurable]] is

false

, return

false

b. If

current

) is

true

, then

i. If

is not

undefined

, convert the property named

of object

from a

to an

. Preserve the existing values of the converted property's [[Configurable]] and

[[Enumerable]] attributes and set the rest of the property's attributes to their

default values

c. Else,

i. If

is not

undefined

, convert the property named

of object

from an

to a

. Preserve the existing values of the converted property's [[Configurable]] and

[[Enumerable]] attributes and set the rest of the property's attributes to their

default values

7. Else if

(

current

) and

(

Desc

) are both

true

, then

a. If

current

.[[Configurable]] is

false

and

current

.[[Writable]] is

false

, then

i. If

Desc

.[[Writable]] is present and

Desc

.[[Writable]] is

true

, return

false

ii. If

Desc

.[[Value]] is present and

(

Desc

.[[Value]],

current

.[[Value]]) is

false

, return

false

iii. Return

true

8. Else,

: !

IsAccessorDescriptor

(

current

) and !

IsAccessorDescriptor

(

Desc

) are both

true

b. If

current

.[[Configurable]] is

false

, then

i. If

Desc

.[[Set]] is present and

Desc

.[[Set]],

current

.[[Set]]) is

false

, return

false

ii. If

Desc

.[[Get]] is present and

Desc

.[[Get]],

current

.[[Get]]) is

false

, return

false

iii. Return

true

9. If

is not

undefined

, then

a. For each field of

Desc

that is present, set the corresponding attribute of the property named

of object

to the value of the field.

10. Return

true

10.1.7 [[HasProperty]] (

)

225

The [[HasProperty]] internal method of an

takes argument

(a property key). It performs the

following steps when called:

OrdinaryHasProperty

(

The abstract operation OrdinaryHasProperty takes arguments

(an Object) and

(a property key). It performs the

following steps when called:

The [[Get]] internal method of an

) is

true

2. Let

hasOwn

be ?

.[[GetOwnProperty]](

3. If

hasOwn

is not

undefined

, return

true

4. Let

parent

be ?

.[[GetPrototypeOf]]().

5. If

parent

is not

null

, then

a. Return ?

parent

.[[HasProperty]](

6. Return

false

OrdinaryGet

The abstract operation OrdinaryGet takes arguments

Receiver

(an Object),

(a property key), and

) is

true

2. Let

desc

be ?

.[[GetOwnProperty]](

3. If

desc

undefined

, then

a. Let

parent

be ?

.[[GetPrototypeOf]]().

b. If

parent

null

, return

undefined

c. Return ?

parent

.[[Get]](

Receiver

4. If

desc

) is

true

, return

desc

.[[Value]].

IsAccessorDescriptor

(

desc

) is

true

6. Let

getter

desc

.[[Get]].

7. If

getter

undefined

, return

undefined

8. Return ?

The [[Set]] internal method of an

(

getter

Receiver

ECMAScript language value