The Object.assign()
method copies all enumerable own properties from one or more source objects to a target object. It returns the target object.
Syntax
Object.assign(target, ...sources)
Parameters
target
- The target object — what to apply the sourcesβ properties to, which is returned after it is modified.
sources
- The source object(s) — objects containing the properties you want to apply.
Return value
The target object.
Description
Properties in the target object are overwritten by properties in the sources if they have the same key. Later sources' properties overwrite earlier ones.
The Object.assign()
method only copies enumerable and own properties from a source object to a target object. It uses [[Get]]
on the source and [[Set]]
on the target, so it will invoke getters and setters. Therefore it assigns properties, versus copying or defining new properties. This may make it unsuitable for merging new properties into a prototype if the merge sources contain getters.
For copying property definitions (including their enumerability) into prototypes, use Object.getOwnPropertyDescriptor()
and Object.defineProperty()
instead.
Both String
and Symbol
properties are copied.
In case of an error, for example if a property is non-writable, a TypeError
is raised, and the target
object is changed if any properties are added before the error is raised.
Polyfill
This polyfill doesn't support symbol properties, since ES5 doesn't have symbols anyway:
if (typeof Object.assign !== 'function') { // Must be writable: true, enumerable: false, configurable: true Object.defineProperty(Object, "assign", { value: function assign(target, varArgs) { // .length of function is 2 'use strict'; if (target === null || target === undefined) { throw new TypeError('Cannot convert undefined or null to object'); } var to = Object(target); for (var index = 1; index < arguments.length; index++) { var nextSource = arguments[index]; if (nextSource !== null && nextSource !== undefined) { for (var nextKey in nextSource) { // Avoid bugs when hasOwnProperty is shadowed if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) { to[nextKey] = nextSource[nextKey]; } } } } return to; }, writable: true, configurable: true }); }
Examples
Cloning an object
const obj = { a: 1 }; const copy = Object.assign({}, obj); console.log(copy); // { a: 1 }
Warning for Deep Clone
For deep cloning, we need to use alternatives, because Object.assign()
copies property values.
If the source value is a reference to an object, it only copies the reference value.
function test() { 'use strict'; let obj1 = { a: 0 , b: { c: 0}}; let obj2 = Object.assign({}, obj1); console.log(JSON.stringify(obj2)); // { "a": 0, "b": { "c": 0}} obj1.a = 1; console.log(JSON.stringify(obj1)); // { "a": 1, "b": { "c": 0}} console.log(JSON.stringify(obj2)); // { "a": 0, "b": { "c": 0}} obj2.a = 2; console.log(JSON.stringify(obj1)); // { "a": 1, "b": { "c": 0}} console.log(JSON.stringify(obj2)); // { "a": 2, "b": { "c": 0}} obj2.b.c = 3; console.log(JSON.stringify(obj1)); // { "a": 1, "b": { "c": 3}} console.log(JSON.stringify(obj2)); // { "a": 2, "b": { "c": 3}} // Deep Clone obj1 = { a: 0 , b: { c: 0}}; let obj3 = JSON.parse(JSON.stringify(obj1)); obj1.a = 4; obj1.b.c = 4; console.log(JSON.stringify(obj3)); // { "a": 0, "b": { "c": 0}} } test();
Merging objects
const o1 = { a: 1 }; const o2 = { b: 2 }; const o3 = { c: 3 }; const obj = Object.assign(o1, o2, o3); console.log(obj); // { a: 1, b: 2, c: 3 } console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.
Merging objects with same properties
const o1 = { a: 1, b: 1, c: 1 }; const o2 = { b: 2, c: 2 }; const o3 = { c: 3 }; const obj = Object.assign({}, o1, o2, o3); console.log(obj); // { a: 1, b: 2, c: 3 }
The properties are overwritten by other objects that have the same properties later in the parameters order.
Copying symbol-typed properties
const o1 = { a: 1 }; const o2 = { [Symbol('foo')]: 2 }; const obj = Object.assign({}, o1, o2); console.log(obj); // { a : 1, [Symbol("foo")]: 2 } (cf. bug 1207182 on Firefox) Object.getOwnPropertySymbols(obj); // [Symbol(foo)]
Properties on the prototype chain and non-enumerable properties cannot be copied
const obj = Object.create({ foo: 1 }, { // foo is on obj's prototype chain. bar: { value: 2 // bar is a non-enumerable property. }, baz: { value: 3, enumerable: true // baz is an own enumerable property. } }); const copy = Object.assign({}, obj); console.log(copy); // { baz: 3 }
Primitives will be wrapped to objects
const v1 = 'abc'; const v2 = true; const v3 = 10; const v4 = Symbol('foo'); const obj = Object.assign({}, v1, null, v2, undefined, v3, v4); // Primitives will be wrapped, null and undefined will be ignored. // Note, only string wrappers can have own enumerable properties. console.log(obj); // { "0": "a", "1": "b", "2": "c" }
Exceptions will interrupt the ongoing copying task
const target = Object.defineProperty({}, 'foo', { value: 1, writable: false }); // target.foo is a read-only property Object.assign(target, { bar: 2 }, { foo2: 3, foo: 3, foo3: 3 }, { baz: 4 }); // TypeError: "foo" is read-only // The Exception is thrown when assigning target.foo console.log(target.bar); // 2, the first source was copied successfully. console.log(target.foo2); // 3, the first property of the second source was copied successfully. console.log(target.foo); // 1, exception is thrown here. console.log(target.foo3); // undefined, assign method has finished, foo3 will not be copied. console.log(target.baz); // undefined, the third source will not be copied either.
Copying accessors
const obj = { foo: 1, get bar() { return 2; } }; let copy = Object.assign({}, obj); console.log(copy); // { foo: 1, bar: 2 } // The value of copy.bar is obj.bar's getter's return value. // This is an assign function that copies full descriptors function completeAssign(target, ...sources) { sources.forEach(source => { let descriptors = Object.keys(source).reduce((descriptors, key) => { descriptors[key] = Object.getOwnPropertyDescriptor(source, key); return descriptors; }, {}); // By default, Object.assign copies enumerable Symbols, too Object.getOwnPropertySymbols(source).forEach(sym => { let descriptor = Object.getOwnPropertyDescriptor(source, sym); if (descriptor.enumerable) { descriptors[sym] = descriptor; } }); Object.defineProperties(target, descriptors); }); return target; } copy = completeAssign({}, obj); console.log(copy); // { foo:1, get bar() { return 2 } }
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Object.assign' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
assign | Chrome Full support 45 | Edge Full support 12 | Firefox Full support 34 | IE No support No | Opera Full support 32 | Safari Full support 9 | WebView Android Full support 45 | Chrome Android Full support 45 | Firefox Android Full support 34 | Opera Android Full support 32 | Safari iOS Full support 9 | Samsung Internet Android Full support 5.0 | nodejs Full support 4.0.0 |
Legend
- Full support
- Full support
- No support
- No support