Object Spread vs Object.assign: Pros & Cons for Merging Options

The object spread operator { ...optionsDefault, ...options } shines for concise, immutable merging of default options in JavaScript, creating a fresh object without mutating originals or triggering setters—perfect for modern, readable code. Object.assign({}, optionsDefault, options) offers more flexibility for dynamic sources and better legacy support but feels wordier and can invoke getters/setters unexpectedly. Spread wins on brevity and avoiding side effects, while Object.assign handles prototypes and non-enumerable properties more reliably; both do shallow copies only.

Contents

• Object Merging in JavaScript
• Object.assign for Merging
• Object Spread Operator
• Key Differences: Spread vs Object.assign
• Best Use Cases for Default Options
• Performance and Browser Support
• Sources
• Conclusion

Object Merging in JavaScript

Ever needed to slap some default settings onto user-provided options without messing up the originals? That’s object merging in a nutshell—taking optionsDefault like { theme: 'dark', timeout: 5000 } and blending in options such as { timeout: 3000, apiKey: 'xyz' } to get { theme: 'dark', timeout: 3000, apiKey: 'xyz' }.

JavaScript gives you two go-to tools: the spread operator (ES2018+) and Object.assign (ES6). Why care? Mutable merges can lead to bugs in React components or Node configs, where state changes ripple unexpectedly. The spread syntax { ...optionsDefault, ...options } keeps things immutable by default. Object.assign lets you target an existing object but needs an empty {} upfront for the same effect.

Both handle the basics well for default options, but pick wrong and you’ll hit gotchas like overwritten properties or prototype pollution.

Object.assign for Merging

Object.assign is the workhorse here. Pass it a target (usually {}), then sources: Object.assign({}, optionsDefault, options). It copies enumerable own properties from right to left, so later args override earlier ones.

Pros:

• Dynamic: Sources can be variables or computed, like Object.assign({}, defaults, ...sourcesArray).
• Copies symbols and getters/setters—handy if your objects have fancy metadata.
• Polyfillable for ancient browsers.

Draws:

• Mutates the target if you forget {} (easy slip-up).
• Verbose, especially with multiple sources.
• Triggers setters on the target, which might fire side effects you don’t want.

In practice, for a logging function:

function log(message, options = {}) {
 const merged = Object.assign({}, logDefaults, options);
 console.log(message, merged.level);
}

It works, but feels clunky next to spread. As MDN notes, it’s great for preserving inheritance chains too.

Object Spread Operator

Enter { ...optionsDefault, ...options }—clean, no fuss. This “object spread” unpacks properties into a new literal, always immutable.

Pros:

• Super readable, like English: “defaults plus options.”
• No mutation risk; always spits out a new object.
• Ignores setters on sources (safer for computed props).
• Chains nicely: { ...a, ...b, overrides }.

Drawbacks:

• Static only—no dynamic source arrays without hacks.
• Skips non-enumerable properties and symbols.
• Newer (ES2018), so Babel/transpiling needed pre-2018 browsers.

Try this in a React hook:

const useConfig = (options) => {
 return { ...configDefaults, ...options };
};

Boom—defaults applied, no side effects. Stack Overflow devs love it for quick merges, and it’s Babel-friendly without polyfills.

But what if optionsDefault has getters? Spread won’t invoke them, which is often a feature, not a bug.

Key Differences: Spread vs Object.assign

Head-to-head time. Here’s a quick table breaking it down for default options merging:

Real-world? Spread for 90% of config merges—faster to write, less error-prone. Object.assign if you’re dealing with libraries expecting setter behavior or old IE. GeeksforGeeks nails this: spread prioritizes simplicity.

Both shallow, so nested objects? { nested: { foo: 1 } } copies the reference—mutate inside, and pain ensues. For deep, reach for lodash or custom recursion.

Best Use Cases for Default Options

Default options scream for these patterns. Spread? Everyday wins: React props, function args, API clients. It’s the default in hooks or reducers.

Object.assign shines when:

• Merging multiple dynamic objects (e.g., from promises).
• Preserving getters for validation.
• Legacy codebases needing polyfills.

Example mismatch: Using Object.assign in a pure function? Risky if someone passes a mutable target. Spread forces purity.

// Spread: Ideal for configs
const apiCall = (url, opts = {}) => fetch(url, { ...fetchDefaults, ...opts });

// Assign: Dynamic merge
const mergeConfigs = (...configs) => Object.assign({}, ...configs);

The Code Barbarian points out spread’s edge in performance for shallow cases, but assign for fidelity.

Pick spread unless you need the extras—keeps code lean.

Performance and Browser Support

Benchmarks? Spread edges out slightly in modern V8 (Chrome/Node)—faster allocation, less overhead. But Object.assign polyfills scale better for huge objects.

Spread’s your bet post-2018; assign for broader reach. Test in JSPerf for your workload—differences vanish under 1k props.

Sources

1. MDN Web Docs — Spread syntax documentation for object merging and immutability: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
2. Stack Overflow — Community comparison of object spread vs Object.assign pros/cons: https://stackoverflow.com/questions/32925460/object-spread-vs-object-assign
3. GeeksforGeeks — Detailed differences between Object.assign and spread operator: https://www.geeksforgeeks.org/javascript/difference-between-objectassign-and-spread-operator-in-javascript/
4. The Code Barbarian — Practical analysis of Object.assign vs object spread performance: https://thecodebarbarian.com/object-assign-vs-object-spread.html

Conclusion

For merging default options, grab the object spread operator—it’s concise, safe, and the modern JavaScript way to avoid mutation headaches. Object.assign steps in for dynamic needs or legacy quirks, but don’t default to it. Test your stack, prioritize readability, and you’ll merge like a pro. Future-proof with spread; your code (and sanity) will thank you.