Shallow Copy vs Deep Copy

Introduction

Shallow copy and deep copy are fundamental concepts in programming, especially when working with objects, arrays, and memory references. Misunderstanding them often leads to bugs, unexpected data mutation, and performance issues. This article explains both concepts clearly with practical examples.

What Is a Shallow Copy?

A shallow copy creates a new object, but it copies references of nested objects instead of duplicating them.

This means:

• Top-level values are copied
• Nested objects still point to the same memory location

A detailed explanation is available on MDN Web Docs.

Example (JavaScript)

const original = { name: "Arsalan", address: { city: "Delhi" } };
const copy = { ...original };

copy.address.city = "Mumbai";
console.log(original.address.city); // Mumbai

const original = { name: "Arsalan", address: { city: "Delhi" } };
const copy = { ...original };

copy.address.city = "Mumbai";
console.log(original.address.city); // Mumbai

What Is a Deep Copy?

A deep copy creates a completely independent copy of an object, including all nested objects.

This means:

• No shared references
• Changes in the copy do not affect the original

A conceptual overview can be found on GeeksforGeeks.

Example (JavaScript)

const original = { name: "Arsalan", address: { city: "Delhi" } };
const copy = JSON.parse(JSON.stringify(original));

copy.address.city = "Mumbai";
console.log(original.address.city); // Delhi

const original = { name: "Arsalan", address: { city: "Delhi" } };
const copy = JSON.parse(JSON.stringify(original));

copy.address.city = "Mumbai";
console.log(original.address.city); // Delhi

Shallow vs Deep Copy: Comparison Table

Common Ways to Create Shallow Copy

JavaScript

• Spread operator ({ ...obj })
• Object.assign()
• Array.prototype.slice()

Common Ways to Create Deep Copy

JavaScript

• JSON.parse(JSON.stringify(obj)) (limitations apply)
• structuredClone() (modern browsers)
• Utility libraries like Lodash

Details on structuredClone() are explained on MDN.

Performance Considerations

• Shallow copy is faster and uses less memory
• Deep copy can be expensive for large or deeply nested objects
• Choose based on data size and mutation risk

Real-World Use Cases

When to Use Shallow Copy

• State updates with immutable patterns
• Objects without nested structures
• Performance-critical code

When to Use Deep Copy

• Configuration objects
• Data isolation between modules
• Avoiding side effects in complex systems

FAQs

Is deep copy always better than shallow copy?

No. Deep copy is safer but slower and memory-intensive.

Does spread operator create deep copy?

No. It creates a shallow copy only.

Is JSON-based deep copy reliable?

It fails for functions, undefined, Date, Map, Set, and circular references.

What is the best deep copy method in JavaScript?

structuredClone() is the safest modern solution when available.

Conclusion

Shallow copy and deep copy serve different purposes. Shallow copy offers speed and efficiency, while deep copy provides safety and isolation. Understanding the difference is critical for writing predictable and bug-free code.

Nasim Khan

I’m Nasim Khan, is a Software Engineer focused on building efficient and user-friendly web applications. SHe writes about web development, programming fundamentals, and modern technologies, sharing practical insights for developers and learners.