What does Object.assign() do?

JavaScriptJunior~1 min read

Object.assign() is a built-in JavaScript method used to copy properties from one or more objects into another object.

It lets you merge objects and create shallow copies.

Syntax

javascript

Object.assign(target, ...sources)

Parameter	Description
`target`	The object that properties are copied into
`sources`	One or more source objects that properties are taken from

Returns target itself.

A simple example

javascript

const user = { name: 'Alice' };
const details = { age: 25, city: 'Kyiv' };

Object.assign(user, details);

console.log(user);
// { name: 'Alice', age: 25, city: 'Kyiv' }

All the properties from details are copied into user. If keys match, the values are overwritten.

Example with property overwriting

javascript

const a = { name: 'Alice', age: 25 };
const b = { age: 30, city: 'Kyiv' };

const result = Object.assign(a, b);

console.log(result); // { name: 'Alice', age: 30, city: 'Kyiv' }
console.log(a === result); // true (a is changed)

Important: Object.assign() mutates the first argument (target). If you don't want to mutate the object, pass an empty object as the first parameter:

javascript

const merged = Object.assign({}, a, b);

Cloning an object

javascript

const user = { name: 'Alice', city: 'Kyiv' };

const clone = Object.assign({}, user);

console.log(clone); // { name: 'Alice', city: 'Kyiv' }
console.log(clone === user); // false (different objects)

This creates a shallow copy: nested objects are copied by reference, not duplicated.

Example with a nested object (shallow copy)

javascript

const user = {
  name: 'Alice',
  address: { city: 'Kyiv' }
};

const clone = Object.assign({}, user);

clone.address.city = 'Lviv';

console.log(user.address.city); // "Lviv" - it's the same reference!

That is, Object.assign() does a shallow copy, not a deep one. For a deep copy, use structuredClone() or _.cloneDeep() instead.

Example of merging several sources

javascript

const a = { x: 1 };
const b = { y: 2 };
const c = { z: 3 };

const merged = Object.assign({}, a, b, c);
console.log(merged); // { x: 1, y: 2, z: 3 }

Lets you merge several objects into one.

You can copy values from primitives

javascript

const target = {};
Object.assign(target, { a: 1 }, 'abc', null);

console.log(target);
// { '0': 'a', '1': 'b', '2': 'c', a: 1 }

The string "abc" behaves like an array of characters, each letter becomes a property '0', '1', '2'.

Merging with existing objects (a common pattern)

javascript

const settings = { darkMode: false, lang: 'en' };
const userSettings = { darkMode: true };

const final = Object.assign({}, settings, userSettings);
console.log(final); // { darkMode: true, lang: 'en' }

Often used to "merge default settings" with user ones.

Summary

What it does	Example	Behavior
Copies properties from one object into another	`Object.assign(a, b)`	Mutates `a`
Creates a shallow copy	`Object.assign({}, obj)`	Does not mutate the source
Merges several objects	`Object.assign({}, a, b, c)`	Later ones overwrite earlier ones
Does not do a deep copy	-	Nested objects are copied by reference

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