Object initializer

Objects can be initialized using new Object(), Object.create(), or using the literal notation (initializer notation). An object initializer is a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}).


var o = {};
var o = {a: 'foo', b: 42, c: {}};

var a = 'foo', b = 42, c = {};
var o = {a: a, b: b, c: c};

var o = {
  property: function ([parameters]) {},
  get property() {},
  set property(value) {}

New notations in ECMAScript 2015

Please see the compatibility table for support for these notations. In non-supporting environments, these notations will lead to syntax errors.

// Shorthand property names (ES2015)
var a = 'foo', b = 42, c = {};
var o = {a, b, c};

// Shorthand method names (ES2015)
var o = {
  property([parameters]) {}

// Computed property names (ES2015)
var prop = 'foo';
var o = {
  [prop]: 'hey',
  ['b' + 'ar']: 'there'


An object initializer is an expression that describes the initialization of an Object. Objects consist of properties, which are used to describe an object. Values of object properties can either contain primitive (en-US) data types or other objects.


An empty object with no properties can be created like this:

var object = {};

However, the advantage of the literal or initializer notation is, that you are able to quickly create objects with properties inside the curly braces. You simply notate a list of key: value pairs delimited by comma. The following code creates an object with three properties and the keys are "foo", "age" and "baz". The values of these keys are a string "bar", a number 42 and the third property has another object as its value.

var object = {
  foo: 'bar',
  age: 42,
  baz: {myProp: 12}


Once you have created an object, you might want to read or change them. Object properties can be accessed by using the dot notation or the bracket notation. See property accessors for detailed information.

object.foo; // "bar"
object['age']; // 42

object.foo = 'baz';


We have already learned how to notate properties using the initializer syntax. Oftentimes, there are variables in your code that you would like to put into an object. You will see code like this:

var a = 'foo',
    b = 42,
    c = {};

var o = {
  a: a,
  b: b,
  c: c

With ECMAScript 2015, there is a shorter notation available to achieve the same:

var a = 'foo',
    b = 42,
    c = {};

// Shorthand property names (ES2015)
var o = {a, b, c};

// In other words,
console.log((o.a === {a}.a)); // true

Duplicate property names

When using the same name for your properties, the second property will overwrite the first.

var a = {x: 1, x: 2};
console.log(a); // {x: 2}

In ECMAScript 5 strict mode code, duplicate property names were considered a SyntaxError.  With the introduction of computed property names making duplication possible at runtime, ECMAScript 2015 has removed this restriction.

function haveES2015DuplicatePropertySemantics() {
  'use strict';
  try {
    ({prop: 1, prop: 2});

    // No error thrown, duplicate property names allowed in strict mode
    return true;
  } catch(e) {
    // Error thrown, duplicates prohibited in strict mode
    return false;


A property of an object can also refer to a function or a getter or setter method.

var o = {
  property: function ([parameters]) {},
  get property() {},
  set property(value) {}

In ECMAScript 2015, a shorthand notation is available, so that the keyword "function" is no longer necessary.

// Shorthand method names (ES2015)
var o = {
  property([parameters]) {},
  *generator() {}

In ECMAScript 2015 There is a way to concisely define properties whose values are generator functions:

var o = {
  *generator() {

Which is equivalent to this ES5-like notation (but note that ECMAScript 5 has no generators):

var o = {
  generator: function* () {

For more information and examples about methods, see method definitions.

Computed property names

Starting with ECMAScript 2015, the object initializer syntax also supports computed property names. That allows you to put an expression in brackets [], that will be computed as the property name. This is symmetrical to the bracket notation of the property accessor syntax, which you might have used to read and set properties already. Now you can use the same syntax in object literals, too:

// Computed property names (ES2015)
var i = 0;
var a = {
  ['foo' + ++i]: i,
  ['foo' + ++i]: i,
  ['foo' + ++i]: i

console.log(a.foo1); // 1
console.log(a.foo2); // 2
console.log(a.foo3); // 3

var param = 'size';
var config = {
  [param]: 12,
  ['mobile' + param.charAt(0).toUpperCase() + param.slice(1)]: 4

console.log(config); // {size: 12, mobileSize: 4}

Spread properties

The Rest/Spread Properties for ECMAScript proposal (stage 3) adds spread properties to object literals. It copies own enumerable properties from a provided object onto a new object.

Shallow-cloning (excluding prototype) or merging objects is now possible using a shorter syntax than Object.assign().

var obj1 = { foo: 'bar', x: 42 };
var obj2 = { foo: 'baz', y: 13 };

var clonedObj = { ...obj1 };
// Object { foo: "bar", x: 42 }

var mergedObj = { ...obj1, ...obj2 };
// Object { foo: "baz", x: 42, y: 13 }

Note that Object.assign() triggers setters whereas the spread operator doesn't.

Prototype mutation

A property definition of the form __proto__: value or "__proto__": value does not create a property with the name __proto__.  Instead, if the provided value is an object or null, it changes the [[Prototype]] of the created object to that value.  (If the value is not an object or null, the object is not changed.)

var obj1 = {};
assert(Object.getPrototypeOf(obj1) === Object.prototype);

var obj2 = {__proto__: null};
assert(Object.getPrototypeOf(obj2) === null);

var protoObj = {};
var obj3 = {'__proto__': protoObj};
assert(Object.getPrototypeOf(obj3) === protoObj);

var obj4 = {__proto__: 'not an object or null'};
assert(Object.getPrototypeOf(obj4) === Object.prototype);

Only a single prototype mutation is permitted in an object literal: multiple prototype mutations are a syntax error.

Property definitions that do not use "colon" notation are not prototype mutations: they are property definitions that behave identically to similar definitions using any other name.

var __proto__ = 'variable';

var obj1 = {__proto__};
assert(Object.getPrototypeOf(obj1) === Object.prototype);
assert(obj1.__proto__ === 'variable');

var obj2 = {__proto__() { return 'hello'; }};
assert(obj2.__proto__() === 'hello');

var obj3 = {['__prot' + 'o__']: 17};
assert(obj3.__proto__ === 17);

Object literal notation vs JSON

Object literal notation與JavaScript Object Notation (JSON)是不一樣的東西. 儘管它們看起來很相似,但還是有以下的不同:

  • JSON只允許屬性透過"property": value格式定義. 屬性名稱必須使用雙引號包起來,且不能是速記。
  • JSON的數值僅能為string, numbers, arrays, true, false, null, 或另一個JSON物件.
  • A function value (see "Methods" above) can not be assigned to a value in JSON.
  • Objects like Date will be a string after JSON.parse().
  • JSON.parse() will reject computed property names and an error will be thrown.


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard initial definition.
ECMAScript 5.1 (ECMA-262)
The definition of 'Object Initializer' in that specification.
Standard getter and setter added.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object Initializer' in that specification.
Standard Shorthand method/property names and computed property names added.
ECMAScript (ECMA-262)
The definition of 'Object Initializer' in that specification.
Living Standard  
Rest/Spread Properties for ECMAScript Draft Stage 3 draft.


We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help! (en-US)
Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1.0 (Yes) 1.0 (1.7 or earlier) 1 1 1
Computed property names (Yes) (Yes) 34 (34) No support 34 7.1
Shorthand property names (Yes) (Yes) 33 (33) No support 34 9
Shorthand method names 42.0 (Yes) 34 (34) No support 34 9
Spread properties 60 No support 55 (55) No support No support No support
Feature Android Android Webview Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support (Yes) (Yes) (Yes) 1.0 (1.0) 1 1 1 1.0
Computed property names No support (Yes) (Yes) 34.0 (34) No support 34 7.1 No support
Shorthand property names No support (Yes) (Yes) 33.0 (33) No support 34 9 No support
Shorthand method names No support 42.0 (Yes) 34.0 (34) No support 34 9 42.0
Spread properties No support No support No support 55.0 (55) No support No support No support No support