W3cubDocs

/JavaScript

Object

The Object class represents one of JavaScript's data types. It is used to store various keyed collections and more complex entities. Objects can be created using the Object() constructor or the object initializer / literal syntax.

Description

Nearly all objects in JavaScript are instances of Object; a typical object inherits properties (including methods) from Object.prototype, although these properties may be shadowed (a.k.a. overridden). However, an Object may be deliberately created for which this is not true (e.g. by Object.create(null)), or it may be altered so that this is no longer true (e.g. with Object.setPrototypeOf).

Changes to the Object prototype object are seen by all objects through prototype chaining, unless the properties and methods subject to those changes are overridden further along the prototype chain. This provides a very powerful although potentially dangerous mechanism to override or extend object behavior.

The Object constructor creates an object wrapper for the given value.

  • If the value is null or undefined, it will create and return an empty object.
  • Otherwise, it will return an object of a Type that corresponds to the given value.
  • If the value is an object already, it will return the value.

When called in a non-constructor context, Object behaves identically to new Object().

See also the object initializer / literal syntax.

Deleting a property from an object

There isn't any method in an Object itself to delete its own properties (such as Map.prototype.delete()). To do so, one must use the delete operator.

Constructor

Object()
Creates a new Object object. It is a wrapper for the given value.

Static methods

Object.assign()
Copies the values of all enumerable own properties from one or more source objects to a target object.
Object.create()
Creates a new object with the specified prototype object and properties.
Object.defineProperty()
Adds the named property described by a given descriptor to an object.
Object.defineProperties()
Adds the named properties described by the given descriptors to an object.
Object.entries()
Returns an array containing all of the [key, value] pairs of a given object's own enumerable string properties.
Object.freeze()
Freezes an object. Other code cannot delete or change its properties.
Object.fromEntries()
Returns a new object from an iterable of [key, value] pairs. (This is the reverse of Object.entries).
Object.getOwnPropertyDescriptor()
Returns a property descriptor for a named property on an object.
Object.getOwnPropertyDescriptors()
Returns an object containing all own property descriptors for an object.
Object.getOwnPropertyNames()
Returns an array containing the names of all of the given object's own enumerable and non-enumerable properties.
Object.getOwnPropertySymbols()
Returns an array of all symbol properties found directly upon a given object.
Object.getPrototypeOf()
Returns the prototype (internal [[Prototype]] property) of the specified object.
Object.is()
Compares if two values are the same value. Equates all NaN values (which differs from both Abstract Equality Comparison and Strict Equality Comparison).
Object.isExtensible()
Determines if extending of an object is allowed.
Object.isFrozen()
Determines if an object was frozen.
Object.isSealed()
Determines if an object is sealed.
Object.keys()
Returns an array containing the names of all of the given object's own enumerable string properties.
Object.preventExtensions()
Prevents any extensions of an object.
Object.seal()
Prevents other code from deleting properties of an object.
Object.setPrototypeOf()
Sets the object's prototype (its internal [[Prototype]] property).
Object.values()
Returns an array containing the values that correspond to all of a given object's own enumerable string properties.

Instance properties

Object.prototype.constructor
Specifies the function that creates an object's prototype.
Object.prototype.__proto__
Points to the object which was used as prototype when the object was instantiated.
Object.prototype.__noSuchMethod__
Allows a function to be defined that will be executed when an undefined object member is called as a method.

Instance methods

Object.prototype.__defineGetter__()
Associates a function with a property that, when accessed, executes that function and returns its return value.
Object.prototype.__defineSetter__()
Associates a function with a property that, when set, executes that function which modifies the property.
Object.prototype.__lookupGetter__()
Returns the function associated with the specified property by the __defineGetter__() method.
Object.prototype.__lookupSetter__()
Returns the function associated with the specified property by the __defineSetter__() method.
Object.prototype.hasOwnProperty()
Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.
Object.prototype.isPrototypeOf()
Returns a boolean indicating whether the object this method is called upon is in the prototype chain of the specified object.
Object.prototype.propertyIsEnumerable()
Returns a boolean indicating if the internal ECMAScript [[Enumerable]] attribute is set.
Object.prototype.toLocaleString()
Calls toString().
Object.prototype.toString()
Returns a string representation of the object.
Object.prototype.unwatch()
Removes a watchpoint from a property of the object.
Object.prototype.valueOf()
Returns the primitive value of the specified object.
Object.prototype.watch()
Adds a watchpoint to a property of the object.

Examples

Using Object given undefined and null types

The following examples store an empty Object object in o:

let o = new Object()
let o = new Object(undefined)
let o = new Object(null)

Using Object to create Boolean objects

The following examples store Boolean objects in o:

// equivalent to o = new Boolean(true)
let o = new Object(true)
// equivalent to o = new Boolean(false)
let o = new Object(Boolean())

Object prototypes

When altering the behavior of existing Object.prototype methods, consider injecting code by wrapping your extension before or after the existing logic. For example, this (untested) code will pre-conditionally execute custom logic before the built-in logic or someone else's extension is executed.

When a function is called, the arguments to the call are held in the array-like "variable" arguments. For example, in the call myFn(a, b, c), the arguments within myFn's body will contain 3 array-like elements corresponding to (a, b, c).

When modifying prototypes with hooks, pass this and the arguments (the call state) to the current behavior by calling apply() on the function. This pattern can be used for any prototype, such as Node.prototype, Function.prototype, etc.

var current = Object.prototype.valueOf;

// Since my property "-prop-value" is cross-cutting and isn't always
// on the same prototype chain, I want to modify Object.prototype: 
Object.prototype.valueOf = function() {
  if (this.hasOwnProperty('-prop-value')) {
    return this['-prop-value'];
  } else {
    // It doesn't look like one of my objects, so let's fall back on 
    // the default behavior by reproducing the current behavior as best we can.
    // The apply behaves like "super" in some other languages.
    // Even though valueOf() doesn't take arguments, some other hook may.
    return current.apply(this, arguments);
  }
}

Since JavaScript doesn't exactly have sub-class objects, prototype is a useful workaround to make a “base class” object of certain functions that act as objects. For example:

var Person = function(name) {
  this.name = name;
  this.canTalk = true;
};

Person.prototype.greet = function() {
  if (this.canTalk) {
    console.log('Hi, I am ' + this.name);
  }
};

var Employee = function(name, title) {
  Person.call(this, name);
  this.title = title;
};

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee; //If you don't set Object.prototype.constructor to Employee, 
                                           //it will take prototype.constructor of Person (parent). 
                                           //To avoid that, we set the prototype.constructor to Employee (child).


Employee.prototype.greet = function() {
  if (this.canTalk) {
    console.log('Hi, I am ' + this.name + ', the ' + this.title);
  }
};

var Customer = function(name) {
  Person.call(this, name);
};

Customer.prototype = Object.create(Person.prototype);
Customer.prototype.constructor = Customer; //If you don't set Object.prototype.constructor to Customer, 
                                           //it will take prototype.constructor of Person (parent). 
                                           //To avoid that, we set the prototype.constructor to Customer (child).


var Mime = function(name) {
  Person.call(this, name);
  this.canTalk = false;
};

Mime.prototype = Object.create(Person.prototype);
Mime.prototype.constructor = Mime; //If you don't set Object.prototype.constructor to Mime,
                                   //it will take prototype.constructor of Person (parent).
                                   //To avoid that, we set the prototype.constructor to Mime (child).


var bob = new Employee('Bob', 'Builder');
var joe = new Customer('Joe');
var rg = new Employee('Red Green', 'Handyman');
var mike = new Customer('Mike');
var mime = new Mime('Mime');

bob.greet();
// Hi, I am Bob, the Builder

joe.greet();
// Hi, I am Joe

rg.greet();
// Hi, I am Red Green, the Handyman

mike.greet();
// Hi, I am Mike

mime.greet();

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Object 1 12 1 3 3 1
Object() constructor 1 12 1 3 3 1
assign 45 12 34 No 32 9
constructor 1 12 1 4 4 1
create 5 12 4 9 11.6 5
__defineGetter__ 1 12 1
1
Starting with Firefox 48, this method can no longer be called at the global scope without any object. A TypeError will be thrown otherwise. Previously, the global object was used in these cases automatically, but this is no longer the case.
11 9.5 3
defineProperties 5 12 4 9 11.6 5
defineProperty 5 12 4 9
9
8
In Internet Explorer 8, this was only supported on DOM objects and with some non-standard behaviors. This was later fixed in Internet Explorer 9.
11.6 5.1
5.1
Also supported in Safari 5, but not on DOM objects.
__defineSetter__ 1 12 1
1
Starting with Firefox 48, this method can no longer be called at the global scope without any object. A TypeError will be thrown otherwise. Previously, the global object was used in these cases automatically, but this is no longer the case.
11 9.5 3
entries 54 14 47 No 41 10.1
freeze 6 12 4 9 12 5.1
fromEntries 73 79 63 No 60 12.1
getOwnPropertyDescriptor 5 12 4 9
9
8
In Internet Explorer 8, this was only supported on DOM objects and with some non-standard behaviors. This was later fixed in Internet Explorer 9.
12 5
getOwnPropertyDescriptors 54 15 50 No 41 10
getOwnPropertyNames 5 12 4 9 12 5
getOwnPropertySymbols 38 12 36 No 25 9
getPrototypeOf 5 12 3.5 9 12.1 5
hasOwnProperty 1 12 1 5.5 5 3
is 30 12 22 No 17 9
isExtensible 6 12 4 9 12 5.1
isFrozen 6 12 4 9 12 5.1
isPrototypeOf 1 12 1 9 4 3
isSealed 6 12 4 9 12 5.1
keys 5 12 4 9 12 5
__lookupGetter__ 1 12 1 11 9.5 3
__lookupSetter__ 1 12 1 11 9.5 3
preventExtensions 6 12 4 9 12 5.1
propertyIsEnumerable 1 12 1 5.5 4 3
__proto__ 1 12 1 11 10.5 3
seal 6 12 4 9 12 5.1
setPrototypeOf 34 12 31 11 21 9
toLocaleString 1 12 1 5.5 4 1
toSource No No 1 — 74
1 — 74
Starting in Firefox 74, toSource() is no longer available for use by web content. It is still allowed for internal and privileged code.
No No No
toString() 1 12 1 3 3 1
valueOf 1 12 1 4 3 1
values 54 14 47 No 41 10.1
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
Object 1 18 4 10.1 1 1.0
Object() constructor 1 18 4 10.1 1 1.0
assign 45 45 34 32 9 5.0
constructor 1 18 4 10.1 1 1.0
create 1 18 4 12 5 1.0
__defineGetter__ 1 18 4 10.1 1 1.0
defineProperties 1 18 4 12 5 1.0
defineProperty 1 18 4 12 6
6
Also supported in Safari for iOS 4.2, but not on DOM objects.
1.0
__defineSetter__ 1 18 4 10.1 1 1.0
entries 54 54 47 41 10.3 6.0
freeze 1 18 4 12 6 1.0
fromEntries 73 73 63 No 12.2 No
getOwnPropertyDescriptor 1 18 4 12 5 1.0
getOwnPropertyDescriptors 54 54 50 41 10 6.0
getOwnPropertyNames 1 18 4 12 5 1.0
getOwnPropertySymbols 38 38 36 25 9 3.0
getPrototypeOf 1 18 4 12.1 5 1.0
hasOwnProperty 1 18 4 10.1 1 1.0
is ≤37 30 22 18 9 2.0
isExtensible 1 18 4 12 6 1.0
isFrozen 1 18 4 12 6 1.0
isPrototypeOf 1 18 4 10.1 1 1.0
isSealed 1 18 4 12 6 1.0
keys 1 18 4 12 5 1.0
__lookupGetter__ 1 18 4 10.1 1 1.0
__lookupSetter__ 1 18 4 10.1 1 1.0
preventExtensions 1 18 4 12 6 1.0
propertyIsEnumerable 1 18 4 10.1 1 1.0
__proto__ 1 18 4 11 1 1.0
seal 1 18 4 12 6 1.0
setPrototypeOf 37 34 31 21 9 2.0
toLocaleString 1 18 4 10.1 1 1.0
toSource No No 4 No No No
toString() 1 18 4 10.1 1 1.0
valueOf 1 18 4 10.1 1 1.0
values 54 54 47 41 10.3 6.0
Server
Node.js
Object Yes
Object() constructor Yes
assign 4.0.0
constructor Yes
create Yes
__defineGetter__ Yes
defineProperties Yes
defineProperty Yes
__defineSetter__ Yes
entries 7.0.0
7.0.0
6.5.0
Disabled
Disabled From version 6.5.0: this feature is behind the --harmony runtime flag.
freeze Yes
fromEntries 12.0.0
getOwnPropertyDescriptor Yes
getOwnPropertyDescriptors 7.0.0
7.0.0
6.5.0
Disabled
Disabled From version 6.5.0: this feature is behind the --harmony runtime flag.
getOwnPropertyNames Yes
getOwnPropertySymbols 0.12
getPrototypeOf Yes
hasOwnProperty Yes
is 0.10
isExtensible Yes
isFrozen Yes
isPrototypeOf Yes
isSealed Yes
keys Yes
__lookupGetter__ Yes
__lookupSetter__ Yes
preventExtensions Yes
propertyIsEnumerable Yes
__proto__ Yes
seal Yes
setPrototypeOf 0.12
toLocaleString Yes
toSource No
toString() Yes
valueOf Yes
values 7.0.0
7.0.0
6.5.0
Disabled
Disabled From version 6.5.0: this feature is behind the --harmony runtime flag.

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object