Shallow vs deep object copying

JavaScriptJunior~1 min read

In short

Copy type	What it does	Nested objects
Shallow copy	Copies only the top level of properties	References to the original objects are kept
Deep copy	Copies the entire tree (including nested objects)	Creates new copies of all nested structures

Shallow copy example

javascript

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

const clone = { ...user }; // shallow copy

clone.address.city = 'Lviv';

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

Why this happened:

{ ...user } copies only the first level of properties;
address is an object, and the copy keeps the same reference to it;
changing clone.address.city affected the original.

Deep copy example

javascript

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

const clone = structuredClone(user); // deep copy

clone.address.city = 'Lviv';

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

Here a fully independent copy is created: all nested objects are copied anew too.

Illustration (a simple analogy)

Imagine:

javascript

user ──► address ──► { city: 'Kyiv' }
clone ─┘   ↑
           └── (the same reference)

In a shallow copy, both variables "point" to the same inner object.

With a deep copy:

javascript

user ──► address1 ─► { city: 'Kyiv' }
clone ─► address2 ─► { city: 'Lviv' }

These are two fully independent objects.

How to make a deep copy

Method	Deep copy	Notes
`structuredClone(obj)`	Yes	Modern, preserves types
`JSON.parse(JSON.stringify(obj))`	Yes	Loses functions, `Date`, `undefined`
`_.cloneDeep(obj)` (lodash)	Yes	The most reliable and cross-platform

Summary

Property	Shallow copy	Deep copy
Copies only the top level	Yes	No
Copies nested objects	No (references)	Yes
Changes to nested data affect the original	Yes	No
Examples	`{ ...obj }`, `Object.assign()`	`structuredClone()`, `_.cloneDeep()`

Short Answer

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Shallow vs deep object copying

#In short

#Shallow copy example

#Deep copy example

#Illustration (a simple analogy)

#How to make a deep copy

#Summary

Short Answer

In short

Shallow copy example

Deep copy example

Illustration (a simple analogy)

How to make a deep copy

Summary