---
title: Object.assign()
short-title: assign()
slug: Web/JavaScript/Reference/Global_Objects/Object/assign
page-type: javascript-static-method
browser-compat: javascript.builtins.Object.assign
sidebar: 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()")}}

```js interactive-example
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

```js-nolint
Object.assign(target)
Object.assign(target, source1)
Object.assign(target, source1, source2)
Object.assign(target, source1, source2, /* …, */ sourceN)
```

### Parameters

- `target`
  - : The target object — what to apply the sources' properties to, which is returned after it is modified. If a primitive value is provided as the target, it will be converted to an object.
- `source1`, …, `sourceN`
  - : The source object(s) — objects containing the properties you want to apply.

### Return value

The target object.

### Exceptions

- {{jsxref("TypeError")}}
  - : Thrown in one of the following cases:
    - The `target` parameter is [`null`](/en-US/docs/Web/JavaScript/Reference/Operators/null) or {{jsxref("undefined")}}.
    - Assignment of a property on the target object fails; for example, because the property is non-writable on the target object, or because its setter throws an error.

## 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](/en-US/docs/Web/JavaScript/Reference/Functions/get) and [setters](/en-US/docs/Web/JavaScript/Reference/Functions/set). 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`](/en-US/docs/Web/JavaScript/Reference/Operators/null) or {{jsxref("undefined")}} sources.

## Examples

### Cloning an object

```js
const obj = { a: 1 };
const copy = Object.assign({}, obj);
console.log(copy); // { a: 1 }
```

### Warning for deep clone

For [deep cloning](/en-US/docs/Glossary/Deep_copy), 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.

```js
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

```js
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

```js
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

```js
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

```js
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

```js
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

```js
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

```js
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

- [Polyfill of `Object.assign` in `core-js`](https://github.com/zloirock/core-js#ecmascript-object)
- [es-shims polyfill of `Object.assign`](https://www.npmjs.com/package/object.assign)
- {{jsxref("Object.defineProperties()")}}
- [Enumerability and ownership of properties](/en-US/docs/Web/JavaScript/Guide/Enumerability_and_ownership_of_properties)
- [Spread in object literals](/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#spread_in_object_literals)