What is the Difference Between Deep Copy and Shallow Copy in JavaScript?

When working with objects and arrays in JavaScript, understanding copying methods is essential to avoid unexpected bugs. Let's explore the key differences between shallow and deep copies.

Shallow Copy: Surface-Level Duplication

A shallow copy creates a new object/array but only copies the top-level elements. Nested objects still reference the original.

How to Create:

// Spread operator
const shallowCopy = {...originalObj};

// Object.assign()
const shallowCopy = Object.assign({}, originalObj);

// Array.slice()
const shallowArrCopy = originalArr.slice();

Characteristics:

✔ Faster performance
✔ Memory efficient
✖ Nested objects remain linked
✖ Changes to nested elements affect both copies

Deep Copy: Complete Independence

A deep copy creates a completely new version where all levels are duplicated, breaking all references to the original.

How to Create:

// JSON methods (simplest way)
const deepCopy = JSON.parse(JSON.stringify(originalObj));

// Lodash cloneDeep
import { cloneDeep } from 'lodash';
const deepCopy = cloneDeep(originalObj);

// Structured cloning (modern browsers)
const deepCopy = structuredClone(originalObj);

Characteristics:

✔ Fully independent copies
✔ Safe for nested modifications
✖ Slower performance
✖ More memory intensive

Key Differences Table

When to Use Each?

• Use shallow copy when:

  • Working with flat objects/arrays
  • Performance is critical
  • You want to maintain references

• Use deep copy when:

  • Modifying nested structures
  • Working with state management
  • You need complete isolation

Common Pitfalls

1. Unexpected mutations: Modifying a shallow-copied nested object affects the original
2. JSON limitations: JSON.stringify() can't handle functions, Dates, or circular references
3. Overhead: Deep copying large objects unnecessarily impacts performance