Copying through Object.assign

JavaScriptJunior~1 min read

Short answer

Copying with Object.assign() creates a shallow copy. This means nested objects and arrays are copied by reference, not by value.

Example

javascript

const user = {
  name: 'Bohdan',
  address: {
    city: 'Kyiv',
    zip: 12345
  }
};

const clone = Object.assign({}, user);
clone.address.city = 'Lviv';

console.log(user.address.city); // 'Lviv'

Why:

Object.assign() copies only the first level of properties;
the address property is an object, so a reference is copied;
changing the nested object (city) is reflected in the original.

How this looks in memory

javascript

user.address ─────┐
                   │  (the same object)
clone.address ────┘

Both objects (user and clone) reference the same address. So a change inside address affects both.

But! Primitive properties are copied normally

javascript

const user = { name: 'Bohdan', age: 25 };
const clone = Object.assign({}, user);

clone.name = 'Oleh';
console.log(user.name); // "Bohdan" (independent)

Only nested objects or arrays stay shared.

How to avoid this problem

If you want to make a deep copy, use:

`structuredClone()` (a built-in modern way)

javascript

const clone = structuredClone(user);
clone.address.city = 'Lviv';
console.log(user.address.city); // "Kyiv"

`JSON.parse(JSON.stringify())` (universal, but with limitations)

javascript

const clone = JSON.parse(JSON.stringify(user));

`_.cloneDeep()` from Lodash (reliable and compatible with all types)

javascript

import _ from 'lodash';
const clone = _.cloneDeep(user);

SUMMARY

What `Object.assign()` copies	Behavior
Primitive values (`string`, `number`, `boolean`)	copied by value
Nested objects and arrays	copied by reference
Functions	copied by reference
Deep structures	not copied independently

Short Answer

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