Deep Copy vs Shallow Copy in JavaScript: Key Differences and When to Use Them

Table of Contents

• What Are Deep Copy and Shallow Copy?
• Why Do Deep and Shallow Copies Matter?
• How to Create Shallow Copies in JavaScript
  • Method 1: Object.assign()
  • Method 2: Spread Operator (...)
• How to Create Deep Copies in JavaScript
  • Method 1: JSON.parse(JSON.stringify())
  • Method 2: Lodash cloneDeep
  • Method 3: Structured Clone API (Modern Browsers)
• When to Use Shallow vs Deep Copy
• Common Mistakes to Avoid
• Best Practices
• Real-World Use Cases
  • 1. React State Updates
  • 2. Resetting Form Data
  • 3. Caching API Responses
• Conclusion

What Are Deep Copy and Shallow Copy?

In JavaScript, shallow copy and deep copy refer to how objects and arrays are duplicated. The key distinction lies in how they handle nested data:

• Shallow Copy: Creates a new object but shares references to nested objects/arrays. Changes to nested properties affect the original and copied object.
• Deep Copy: Creates a fully independent copy of the original object, including all nested data. No shared references exist.

Understanding these concepts is vital for managing data integrity, especially in state-driven applications like React or Vue.

Why Do Deep and Shallow Copies Matter?

1. Data Integrity: Accidentally modifying shared references can lead to bugs.
2. State Management: Frameworks rely on immutable updates (e.g., React state).
3. Performance: Deep copies can be resource-intensive for large objects.

How to Create Shallow Copies in JavaScript

Method 1: Object.assign()

Method 2: Spread Operator (...)

Limitation: Both methods only copy top-level properties. Nested objects are still referenced.

How to Create Deep Copies in JavaScript

Method 1: JSON.parse(JSON.stringify())

Limitations:

• Fails for properties with undefined, Function, or Date objects.
• Inefficient for large objects.

Method 2: Lodash cloneDeep

Method 3: Structured Clone API (Modern Browsers)

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When to Use Shallow vs Deep Copy

Scenario	Shallow Copy	Deep Copy
Copying simple objects	✅	⚠️ Overkill
State updates in React	✅ (Immutable)	✅ If nested
Cloning objects with nested data	❌ Risky	✅ Mandatory
Handling functions/dates	❌	✅ (Use Lodash)

Common Mistakes to Avoid

1. Assuming Spread Operator Deep Copies:

2. Using JSON.stringify for Special Types: Loses function definitions and Date objects.

3. Unnecessary Deep Copies: Can slow down performance with large datasets.

Best Practices

1. Use Shallow Copies for Flat Objects: They’re faster and sufficient for simple data.
2. Leverage Libraries for Reliable Deep Copies: Lodash’s cloneDeep is widely trusted.
3. Modern Browsers: Prefer structuredClone() for native deep copying.
4. Immutable Patterns: Treat state as immutable in frameworks like React.

Real-World Use Cases

1. React State Updates

2. Resetting Form Data

3. Caching API Responses

Conclusion

Choosing between deep copy and shallow copy in JavaScript depends on your data structure and use case. Shallow copies are efficient for simple objects, while deep copies are essential for nested data to prevent unintended side effects.

Always validate your copying strategy to ensure data integrity and optimal performance.