Master Object.assign() & Spread Operator for JS Object Handling

Using Object.assign() and the Spread Operator for Object Copying and Merging

JavaScript developers often need to copy or merge objects efficiently. Two of the most powerful and commonly used tools for these tasks are Object.assign() and the spread operator (...). While both serve similar purposes, understanding their nuances, performance implications, and best practices can significantly optimize your code and prevent subtle bugs.

In this comprehensive guide, we’ll dive deep into advanced usage patterns, explore edge cases, and provide practical examples tailored for experienced developers looking to master object copying and merging.

Key Takeaways

• Object.assign() and the spread operator both create shallow copies of objects.
• The spread operator offers cleaner syntax and better integration with destructuring.
• Deep cloning requires custom implementations or libraries; these methods are shallow.
• Property descriptors and non-enumerable properties are not preserved by either method.
• Order of merging affects property overwrites; last-in wins.
• Beware of prototype chain copying—neither method copies prototypes.

Understanding Object.assign()

Introduced in ES6, Object.assign() copies enumerable own properties from one or more source objects to a target object. It's commonly used for cloning and merging.

const target = { a: 1 };
const source = { b: 2, c: 3 };

const result = Object.assign(target, source);
console.log(result); // { a: 1, b: 2, c: 3 }

Key Characteristics:

• Mutates the target object.
• Copies only enumerable and own properties.
• Performs a shallow copy.

This means nested objects are copied by reference, not duplicated.

The Spread Operator for Objects

The spread operator (...) is a more recent addition (ES2018) that allows for simpler syntax when copying or merging objects.

const obj1 = { x: 1, y: 2 };
const obj2 = { y: 3, z: 4 };

const merged = { ...obj1, ...obj2 };
console.log(merged); // { x: 1, y: 3, z: 4 }

Highlights:

• Creates a new object without mutating originals.
• Syntax is concise and readable.
• Also performs shallow copying.

Shallow vs. Deep Copying: What You Need to Know

Both Object.assign() and the spread operator perform shallow copies. This means nested objects or arrays are copied by reference.

const original = { nested: { a: 1 } };
const copy = { ...original };

copy.nested.a = 2;
console.log(original.nested.a); // 2 — changed because nested object is shared

For true deep cloning, consider libraries like lodash (_.cloneDeep) or structured cloning (structuredClone in modern environments).

Merging Objects: Order and Property Overwrites

When merging multiple objects, property overwrites depend on the order of sources.

const a = { val: 1, common: 'a' };
const b = { val: 2, common: 'b' };

const mergedAssign = Object.assign({}, a, b);
console.log(mergedAssign); // { val: 2, common: 'b' }

const mergedSpread = { ...a, ...b };
console.log(mergedSpread); // { val: 2, common: 'b' }

The properties from later sources overwrite earlier ones.

Limitations: Non-enumerable and Symbol Properties

Neither Object.assign() nor the spread operator copies non-enumerable properties or copies property descriptors such as getters/setters.

const obj = {};
Object.defineProperty(obj, 'hidden', {
  value: 'secret',
  enumerable: false
});

const copy = Object.assign({}, obj);
console.log(copy.hidden); // undefined

Additionally, symbol-typed properties are copied by Object.assign() but not by the spread operator.

const sym = Symbol('sym');
const obj = { [sym]: 123 };

const assignCopy = Object.assign({}, obj);
console.log(assignCopy[sym]); // 123

const spreadCopy = { ...obj };
console.log(spreadCopy[sym]); // 123 (spread copies symbol properties as of ES2018)

Note: Spread operator behavior with symbols is consistent with Object.assign() in modern environments, but check compatibility for older browsers.

Handling Prototype Chains

Neither method copies an object's prototype chain — only own properties are copied.

const proto = { greet() { return 'hello'; } };
const obj = Object.create(proto);
obj.a = 1;

const copyAssign = Object.assign({}, obj);
console.log(copyAssign.greet); // undefined

const copySpread = { ...obj };
console.log(copySpread.greet); // undefined

To clone prototypes, use Object.create() or specialized cloning utilities.

Performance Considerations

• Object.assign() performs well in most cases but mutates the target.
• The spread operator creates a new object, which can be more memory-intensive but safer in immutable patterns.
• For large or deeply nested objects, neither is suitable for deep cloning.

Benchmark depending on your environment and use case.

Best Practices and Advanced Patterns

• Use the spread operator for immutability and cleaner syntax.
• Use Object.assign() when you need to mutate an existing object deliberately.
• For deep cloning, leverage structuredClone (where supported) or third-party libraries.
• When merging, order your sources carefully to avoid unintended property overwrites.
• Always consider property descriptors and prototype chains if they matter in your context.

Conclusion

Mastering Object.assign() and the spread operator empowers you to write cleaner, more efficient JavaScript when copying and merging objects. While both provide shallow copies, knowing their differences, limitations, and best use cases helps avoid bugs and improve code maintainability. For deep cloning or prototype preservation, additional tools and patterns are necessary. Keep experimenting with these methods to optimize your object handling strategies.

Frequently Asked Questions

1. What is the main difference between Object.assign() and the spread operator?

Object.assign() copies enumerable own properties from source objects to a target object and mutates the target, while the spread operator creates a new object copying own enumerable properties, offering cleaner syntax and immutability.

2. Do these methods perform deep cloning?

No, both perform shallow copies. Nested objects or arrays are copied by reference, not duplicated. Use libraries or structuredClone for deep cloning.

3. Are symbol properties copied?

Yes, both Object.assign() and the spread operator copy symbol-typed properties as of ES2018, but older environments may differ.

4. Do these methods copy non-enumerable properties or property descriptors?

No, neither copies non-enumerable properties or preserves property descriptors like getters/setters.

5. How does property overwrite work during merging?

Properties from later source objects overwrite those from earlier ones. The order of arguments or spread matters.

6. Do these methods copy the prototype chain?

No, they only copy own properties. To clone prototypes, use Object.create() or specialized cloning methods.