Deep Copy vs Shallow Copy in JavaScript

📝 Definitions

• Shallow copy
  A new object is created, but its nested objects/arrays are still referenced (not copied).
• Deep copy
  A new object is created, and all nested objects/arrays are recursively copied, so they are fully independent.

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📦 Shallow Copy Examples

const obj = { user: "Ali", settings: { theme: "dark" } };

// Shallow copies
const copy1 = { ...obj };
const copy2 = Object.assign({}, obj);

copy1.user = "Sara";
console.log(obj.user); // "Ali" (independent primitive)

copy1.settings.theme = "light";
console.log(obj.settings.theme); // "light" (nested object was shared!)

👉 Only the top-level properties are copied. Nested references still point to the same objects.

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📦 Deep Copy Examples

Using structuredClone (modern JS)

const obj = { user: "Ali", settings: { theme: "dark" } };
const deepCopy = structuredClone(obj);

deepCopy.settings.theme = "light";
console.log(obj.settings.theme); // "dark" ✅ independent

Using JSON trick (not recommended for all cases)

const deepCopy = JSON.parse(JSON.stringify(obj));

⚠️ This fails with Date, Map, Set, undefined, functions, circular references.

Using libraries

• Lodash: _.cloneDeep(obj)
• Immer: handles immutable updates with deep cloning.

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🛑 Pitfalls of Shallow Copy

• Mutating nested structures leaks changes:

  const state = { user: { name: "Ali" } };
  const next = { ...state };
  next.user.name = "Sara";
  
  console.log(state.user.name); // "Sara" ❌ unexpected mutation
  

• In React/Redux, this can cause stale renders because React thinks the state is unchanged (reference is same).

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✅ When to Use

• Shallow copy is fine when:

  • Object has only primitives.
  • You don’t need to mutate nested structures.
  • Performance is critical and you control object shape.

• Deep copy is needed when:

  • You must ensure no shared references.
  • Working with nested state in frameworks (e.g., Redux).
  • Serializing complex objects (careful with special types).

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🔍 Performance Considerations

• Deep copy is slower and heavier because it traverses entire object trees.
• Shallow copy is cheap, just reassigns top-level references.

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✅ Summary

• Shallow copy: Copies top-level properties only; nested objects are shared.
• Deep copy: Recursively copies everything; no shared references.
• Use shallow copy for flat/simple objects, and deep copy when you need complete independence.

✨ Rule of thumb: If you change a nested value and don’t want it to affect the original → you need a deep copy.