ECMAScript® 2024 Language Specification

Draft ECMA-262 / July 26, 2023

20.1 Object Objects

20.1.1 The Object Constructor

The Object constructor:

  • is %Object%.
  • is the initial value of the "Object" property of the global object.
  • creates a new ordinary object when called as a constructor.
  • performs a type conversion when called as a function rather than as a constructor.
  • may be used as the value of an extends clause of a class definition.

20.1.1.1 Object ( [ value ] )

This function performs the following steps when called:

  1. If NewTarget is neither undefined nor the active function object, then
    1. Return ? OrdinaryCreateFromConstructor(NewTarget, "%Object.prototype%").
  2. If value is either undefined or null, return OrdinaryObjectCreate(%Object.prototype%).
  3. Return ! ToObject(value).

20.1.2 Properties of the Object Constructor

The Object constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has a "length" property whose value is 1𝔽.
  • has the following additional properties:

20.1.2.1 Object.assign ( target, ...sources )

This function copies the values of all of the enumerable own properties from one or more source objects to a target object.

It performs the following steps when called:

  1. Let to be ? ToObject(target).
  2. If only one argument was passed, return to.
  3. For each element nextSource of sources, do
    1. If nextSource is neither undefined nor null, then
      1. Let from be ! ToObject(nextSource).
      2. Let keys be ? from.[[OwnPropertyKeys]]().
      3. For each element nextKey of keys, do
        1. Let desc be ? from.[[GetOwnProperty]](nextKey).
        2. If desc is not undefined and desc.[[Enumerable]] is true, then
          1. Let propValue be ? Get(from, nextKey).
          2. Perform ? Set(to, nextKey, propValue, true).
  4. Return to.

The "length" property of this function is 2𝔽.

20.1.2.2 Object.create ( O, Properties )

This function creates a new object with a specified prototype.

It performs the following steps when called:

  1. If O is not an Object and O is not null, throw a TypeError exception.
  2. Let obj be OrdinaryObjectCreate(O).
  3. If Properties is not undefined, then
    1. Return ? ObjectDefineProperties(obj, Properties).
  4. Return obj.

20.1.2.3 Object.defineProperties ( O, Properties )

This function adds own properties and/or updates the attributes of existing own properties of an object.

It performs the following steps when called:

  1. If O is not an Object, throw a TypeError exception.
  2. Return ? ObjectDefineProperties(O, Properties).

20.1.2.3.1 ObjectDefineProperties ( O, Properties )

The abstract operation ObjectDefineProperties takes arguments O (an Object) and Properties (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let props be ? ToObject(Properties).
  2. Let keys be ? props.[[OwnPropertyKeys]]().
  3. Let descriptors be a new empty List.
  4. For each element nextKey of keys, do
    1. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
    2. If propDesc is not undefined and propDesc.[[Enumerable]] is true, then
      1. Let descObj be ? Get(props, nextKey).
      2. Let desc be ? ToPropertyDescriptor(descObj).
      3. Append the Record { [[Key]]: nextKey, [[Descriptor]]: desc } to descriptors.
  5. For each element property of descriptors, do
    1. Perform ? DefinePropertyOrThrow(O, property.[[Key]], property.[[Descriptor]]).
  6. Return O.

20.1.2.4 Object.defineProperty ( O, P, Attributes )

This function adds an own property and/or updates the attributes of an existing own property of an object.

It performs the following steps when called:

  1. If O is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? ToPropertyDescriptor(Attributes).
  4. Perform ? DefinePropertyOrThrow(O, key, desc).
  5. Return O.

20.1.2.5 Object.entries ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let entryList be ? EnumerableOwnProperties(obj, key+value).
  3. Return CreateArrayFromList(entryList).

20.1.2.6 Object.freeze ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? SetIntegrityLevel(O, frozen).
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.7 Object.fromEntries ( iterable )

This function performs the following steps when called:

  1. Perform ? RequireObjectCoercible(iterable).
  2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  3. Assert: obj is an extensible ordinary object with no own properties.
  4. Let closure be a new Abstract Closure with parameters (key, value) that captures obj and performs the following steps when called:
    1. Let propertyKey be ? ToPropertyKey(key).
    2. Perform ! CreateDataPropertyOrThrow(obj, propertyKey, value).
    3. Return undefined.
  5. Let adder be CreateBuiltinFunction(closure, 2, "", « »).
  6. Return ? AddEntriesFromIterable(obj, iterable, adder).
 Note
The function created for adder is never directly accessible to ECMAScript code.

20.1.2.8 Object.getOwnPropertyDescriptor ( O, P )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? obj.[[GetOwnProperty]](key).
  4. Return FromPropertyDescriptor(desc).

20.1.2.9 Object.getOwnPropertyDescriptors ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let ownKeys be ? obj.[[OwnPropertyKeys]]().
  3. Let descriptors be OrdinaryObjectCreate(%Object.prototype%).
  4. For each element key of ownKeys, do
    1. Let desc be ? obj.[[GetOwnProperty]](key).
    2. Let descriptor be FromPropertyDescriptor(desc).
    3. If descriptor is not undefined, perform ! CreateDataPropertyOrThrow(descriptors, key, descriptor).
  5. Return descriptors.

20.1.2.10 Object.getOwnPropertyNames ( O )

This function performs the following steps when called:

  1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, string)).

20.1.2.11 Object.getOwnPropertySymbols ( O )

This function performs the following steps when called:

  1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, symbol)).

20.1.2.11.1 GetOwnPropertyKeys ( O, type )

The abstract operation GetOwnPropertyKeys takes arguments O (an ECMAScript language value) and type (string or symbol) and returns either a normal completion containing a List of property keys or a throw completion. It performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let keys be ? obj.[[OwnPropertyKeys]]().
  3. Let nameList be a new empty List.
  4. For each element nextKey of keys, do
    1. If nextKey is a Symbol and type is symbol, or if nextKey is a String and type is string, then
      1. Append nextKey to nameList.
  5. Return nameList.

20.1.2.12 Object.getPrototypeOf ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Return ? obj.[[GetPrototypeOf]]().

20.1.2.13 Object.hasOwn ( O, P )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let key be ? ToPropertyKey(P).
  3. Return ? HasOwnProperty(obj, key).

20.1.2.14 Object.is ( value1, value2 )

This function performs the following steps when called:

  1. Return SameValue(value1, value2).

20.1.2.15 Object.isExtensible ( O )

This function performs the following steps when called:

  1. If O is not an Object, return false.
  2. Return ? IsExtensible(O).

20.1.2.16 Object.isFrozen ( O )

This function performs the following steps when called:

  1. If O is not an Object, return true.
  2. Return ? TestIntegrityLevel(O, frozen).

20.1.2.17 Object.isSealed ( O )

This function performs the following steps when called:

  1. If O is not an Object, return true.
  2. Return ? TestIntegrityLevel(O, sealed).

20.1.2.18 Object.keys ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let keyList be ? EnumerableOwnProperties(obj, key).
  3. Return CreateArrayFromList(keyList).

20.1.2.19 Object.preventExtensions ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? O.[[PreventExtensions]]().
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.20 Object.prototype

The initial value of Object.prototype is the Object prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.21 Object.seal ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? SetIntegrityLevel(O, sealed).
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.22 Object.setPrototypeOf ( O, proto )

This function performs the following steps when called:

  1. Set O to ? RequireObjectCoercible(O).
  2. If proto is not an Object and proto is not null, throw a TypeError exception.
  3. If O is not an Object, return O.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status is false, throw a TypeError exception.
  6. Return O.

20.1.2.23 Object.values ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let valueList be ? EnumerableOwnProperties(obj, value).
  3. Return CreateArrayFromList(valueList).

20.1.3 Properties of the Object Prototype Object

The Object prototype object:

  • is %Object.prototype%.
  • has an [[Extensible]] internal slot whose value is true.
  • has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in 10.4.7.1. (Thus, it is an immutable prototype exotic object.)
  • has a [[Prototype]] internal slot whose value is null.

20.1.3.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is %Object%.

20.1.3.2 Object.prototype.hasOwnProperty ( V )

This method performs the following steps when called:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(this value).
  3. Return ? HasOwnProperty(O, P).
 Note

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

20.1.3.3 Object.prototype.isPrototypeOf ( V )

This method performs the following steps when called:

  1. If V is not an Object, return false.
  2. Let O be ? ToObject(this value).
  3. Repeat,
    1. Set V to ? V.[[GetPrototypeOf]]().
    2. If V is null, return false.
    3. If SameValue(O, V) is true, return true.
 Note

The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

20.1.3.4 Object.prototype.propertyIsEnumerable ( V )

This method performs the following steps when called:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(this value).
  3. Let desc be ? O.[[GetOwnProperty]](P).
  4. If desc is undefined, return false.
  5. Return desc.[[Enumerable]].
 Note 1

This method does not consider objects in the prototype chain.

 Note 2

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

20.1.3.5 Object.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Return ? Invoke(O, "toString").

The optional parameters to this method are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString methods. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.

Note 1

This method provides a generic toLocaleString implementation for objects that have no locale-sensitive toString behaviour. Array, Number, Date, and %TypedArray% provide their own locale-sensitive toLocaleString methods.

 Note 2

ECMA-402 intentionally does not provide an alternative to this default implementation.

20.1.3.6 Object.prototype.toString ( )

This method performs the following steps when called:

  1. If the this value is undefined, return "[object Undefined]".
  2. If the this value is null, return "[object Null]".
  3. Let O be ! ToObject(this value).
  4. Let isArray be ? IsArray(O).
  5. If isArray is true, let builtinTag be "Array".
  6. Else if O has a [[ParameterMap]] internal slot, let builtinTag be "Arguments".
  7. Else if O has a [[Call]] internal method, let builtinTag be "Function".
  8. Else if O has an [[ErrorData]] internal slot, let builtinTag be "Error".
  9. Else if O has a [[BooleanData]] internal slot, let builtinTag be "Boolean".
  10. Else if O has a [[NumberData]] internal slot, let builtinTag be "Number".
  11. Else if O has a [[StringData]] internal slot, let builtinTag be "String".
  12. Else if O has a [[DateValue]] internal slot, let builtinTag be "Date".
  13. Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be "RegExp".
  14. Else, let builtinTag be "Object".
  15. Let tag be ? Get(O, @@toStringTag).
  16. If tag is not a String, set tag to builtinTag.
  17. Return the string-concatenation of "[object ", tag, and "]".
 Note

Historically, this method was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.

20.1.3.7 Object.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? ToObject(this value).
Normative Optional, Legacy

20.1.3.8 Object.prototype.__proto__

Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:

20.1.3.8.1 get Object.prototype.__proto__

The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Return ? O.[[GetPrototypeOf]]().

20.1.3.8.2 set Object.prototype.__proto__

The value of the [[Set]] attribute is a built-in function that takes an argument proto. It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If proto is not an Object and proto is not null, return undefined.
  3. If O is not an Object, return undefined.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status is false, throw a TypeError exception.
  6. Return undefined.
Normative Optional, Legacy

20.1.3.9 Legacy Object.prototype Accessor Methods

20.1.3.9.1 Object.prototype.__defineGetter__ ( P, getter )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. If IsCallable(getter) is false, throw a TypeError exception.
  3. Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]: true, [[Configurable]]: true }.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Return undefined.

20.1.3.9.2 Object.prototype.__defineSetter__ ( P, setter )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. If IsCallable(setter) is false, throw a TypeError exception.
  3. Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]: true, [[Configurable]]: true }.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Return undefined.

20.1.3.9.3 Object.prototype.__lookupGetter__ ( P )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is not undefined, then
      1. If IsAccessorDescriptor(desc) is true, return desc.[[Get]].
      2. Return undefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O is null, return undefined.

20.1.3.9.4 Object.prototype.__lookupSetter__ ( P )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is not undefined, then
      1. If IsAccessorDescriptor(desc) is true, return desc.[[Set]].
      2. Return undefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O is null, return undefined.

20.1.4 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.