Copying an object

JavaScriptJunior~1 min read

1. Shallow copy

Creates a new object, but nested objects are copied by reference.

Way 1: Spread (`...`)

javascript

const user = { name: 'Tim', age: 25 };
const clone = { ...user };

console.log(clone); // { name: 'Tim', age: 25 }
console.log(clone === user); // false

Fast, convenient, and modern. But nested structures (objects, arrays) remain references to the original:

javascript

const user = { name: 'Tim', address: { city: 'Kyiv' } };
const clone = { ...user };

clone.address.city = 'Lviv';
console.log(user.address.city); // "Lviv" (affects the original)

Way 2: `Object.assign()`

javascript

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

console.log(clone); // { name: 'Tim', age: 25 }

Same as spread, also a shallow copy.

Way 3: Via a loop (manual way)

javascript

const clone = {};
for (const key in user) {
  if (user.hasOwnProperty(key)) {
    clone[key] = user[key];
  }
}

Old-fashioned, but works even in old browsers.

2. Deep copy

Creates a fully independent copy - even nested objects are not linked to the original.

Way 1: `structuredClone()` (modern and safe)

javascript

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

const clone = structuredClone(user);

clone.address.city = 'Lviv';

console.log(user.address.city); // "Kyiv"

Supported in all modern browsers and Node.js 17+. It copies:

nested objects, arrays, dates, Map, Set, and so on;
it does not copy functions and prototypes (they are lost).

Way 2: Via `JSON.parse(JSON.stringify())`

javascript

const user = { name: 'Tim', address: { city: 'Kyiv' } };
const clone = JSON.parse(JSON.stringify(user));

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

Works almost everywhere, but:

functions, undefined, Symbol, Date, Map, Set, and BigInt are lost;
it can be slow on large structures.

Way 3: Libraries (lodash, Ramda, etc.)

javascript

import _ from 'lodash';

const clone = _.cloneDeep(user);

The most reliable way for complex structures - it preserves types, nesting, and runs fast.

SUMMARY

Copy type	Way	Copies nested objects?	Preserves types (`Date`, `Map`, `Set`)?
Shallow	`{ ...obj }`, `Object.assign()`	No	Yes
Deep	`structuredClone()`	Yes	Yes
Deep (old way)	`JSON.parse(JSON.stringify())`	Yes	No
Deep (reliable)	`_.cloneDeep()`	Yes	Yes

Short Answer

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