JavascriptHow-ToBeginner · 3 min read

How to Deep Copy Object in JavaScript: Simple Guide

To deep copy an object in JavaScript, use structuredClone(object) for a full independent copy including nested objects. Alternatively, use JSON.parse(JSON.stringify(object)) for simple objects without functions or special types.

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Syntax

The most modern way to deep copy an object is using structuredClone(object). This creates a complete copy including nested objects.

Another common method is JSON.parse(JSON.stringify(object)), which converts the object to a string and back to a new object.

javascript

const copy1 = structuredClone(originalObject);

const copy2 = JSON.parse(JSON.stringify(originalObject));

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Example

This example shows how to deep copy an object with nested properties using both methods and how changes to the copy do not affect the original.

javascript

const original = {
  name: "Alice",
  details: {
    age: 25,
    hobbies: ["reading", "hiking"]
  }
};

// Using structuredClone (modern and recommended)
const copyStructured = structuredClone(original);
copyStructured.details.age = 30;

// Using JSON methods
const copyJSON = JSON.parse(JSON.stringify(original));
copyJSON.details.hobbies.push("coding");

console.log("Original:", original);
console.log("Copy with structuredClone:", copyStructured);
console.log("Copy with JSON methods:", copyJSON);

Output

Original: { name: 'Alice', details: { age: 25, hobbies: [ 'reading', 'hiking' ] } } Copy with structuredClone: { name: 'Alice', details: { age: 30, hobbies: [ 'reading', 'hiking' ] } } Copy with JSON methods: { name: 'Alice', details: { age: 25, hobbies: [ 'reading', 'hiking', 'coding' ] } }

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Common Pitfalls

Using JSON.parse(JSON.stringify(object)) fails if the object contains functions, undefined, Date, Map, Set, or other special types because they get lost or converted incorrectly.

Example of failure:

javascript

const obj = {
  date: new Date(),
  func: () => "hello",
  value: undefined
};

const copy = JSON.parse(JSON.stringify(obj));
console.log(copy); // { date: string } - func and undefined lost

Output

{ date: "2024-06-...T...Z" }

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Quick Reference

structuredClone(object): Best for full deep copy including special types (modern browsers and Node.js 17+).
JSON.parse(JSON.stringify(object)): Simple but limited to JSON-safe data.
Avoid shallow copy methods like Object.assign() or spread operator for nested objects.

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Key Takeaways

Use structuredClone(object) for a reliable deep copy including nested objects and special types.

JSON.parse(JSON.stringify(object)) works only for simple JSON-safe objects without functions or special types.

Shallow copy methods do not copy nested objects and can cause bugs.

Be aware that JSON methods lose functions, undefined, and special objects like Date or Map.

structuredClone is supported in modern environments; check compatibility if needed.