Why the JavaScript Spread Operator Can Cause Hidden Bugs and How structuredClone Fixes Them

In JavaScript the spread operator ( ...) is commonly used to merge objects, but it performs a shallow copy, which can unintentionally mutate the source object when nested structures are present.

Spread operator vs Object.assign

Both the spread operator and Object.assign() copy only the top‑level properties. When a property value is itself an object or array, only the reference is copied, leading to potential data‑pollution bugs.

Object.assign() suffers from the same shallow‑copy issue.

Introducing structuredClone()

The Web platform provides structuredClone(), a native API that uses the structured‑clone algorithm to create a deep copy of any value, eliminating reference sharing.

const source = {
  user: 'Alice',
  profile: {
    age: 25,
    hobbies: ['coding', 'reading']
  }
};

// Deep copy then merge
const safeMerged = { ...structuredClone(source), user: 'Bob' };
safeMerged.profile.age = 30;

// Verify original object remains unchanged
console.log(source.profile.age); // 25 (✅ safe)

For complex data structures containing nested objects or arrays, using structuredClone() is recommended to protect the original data from accidental mutation.