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Object.assign()

{{JSRef}} 

The Object.assign() static method copies all {{jsxref("Object/propertyIsEnumerable", "enumerable", "", 1)}}  {{jsxref("Object/hasOwn", "own properties", "", 1)}}  from one or more source objects to a target object. It returns the modified target object.

{{InteractiveExample("JavaScript Demo: Object.assign()")}} 

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);

console.log(target);
// Expected output: Object { a: 1, b: 4, c: 5 }

console.log(returnedTarget === target);
// Expected output: true

Syntax

Object.assign(target)
Object.assign(target, source1)
Object.assign(target, source1, source2)
Object.assign(target, source1, source2, /* …, */ sourceN)

Parameters

Return value

The target object.

Exceptions

Description

Properties in the target object are overwritten by properties in the sources if they have the same {{jsxref("Object/keys", "key", "", 1)}} . 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 {{jsxref("Object.getOwnPropertyDescriptor()")}}  and {{jsxref("Object.defineProperty()")}}  instead.

Both {{jsxref("String")}}  and {{jsxref("Symbol")}}  properties are copied.

In case of an error, for example if a property is non-writable, a {{jsxref("TypeError")}}  is raised, and the target object is changed if any properties are added before the error is raised.

Note: Object.assign() does not throw on null or {{jsxref("undefined")}}  sources.

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 like {{DOMxRef("Window.structuredClone", "structuredClone()")}} , because Object.assign() copies property values.

If the source value is a reference to an object, it only copies the reference value.

const obj1 = { a: 0, b: { c: 0 } };
const obj2 = Object.assign({}, obj1);
console.log(obj2); // { a: 0, b: { c: 0 } }

obj1.a = 1;
console.log(obj1); // { a: 1, b: { c: 0 } }
console.log(obj2); // { a: 0, b: { c: 0 } }

obj2.a = 2;
console.log(obj1); // { a: 1, b: { c: 0 } }
console.log(obj2); // { a: 2, b: { c: 0 } }

obj2.b.c = 3;
console.log(obj1); // { a: 1, b: { c: 3 } }
console.log(obj2); // { a: 2, b: { c: 3 } }

// Deep Clone
const obj3 = { a: 0, b: { c: 0 } };
const obj4 = structuredClone(obj3);
obj3.a = 4;
obj3.b.c = 4;
console.log(obj4); // { a: 0, b: { c: 0 } }

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 is on obj's prototype chain.
  { foo: 1 },
  {
    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" }

// Primitives as the target are also wrapped to objects
const number = Object.assign(3, { a: 1 });
console.log(number); // Number {3, a: 1}
console.log(typeof number); // object
console.log(number.a); // 1

// null and undefined as targets throw TypeError
try {
  Object.assign(null, { a: 1 });
} catch (e) {
  console.log(e.message); // "Cannot convert undefined or null to object"
}

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) => {
    const 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) => {
      const 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

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

In this article

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