Be Careful with Object.assign in Javascript

🔖 code  javascript 

Immutability is important say the React docs. And of course this is correct, especially in the context of React, Vue.js and the like that depend on immutability to work correctly. It’s also a core facet of functional programming which is becoming more and more popular by the hour. But can you over do it?

Object.assign for the win?

One of the most popular tools for writing immutable code in Javascript is Object.assign().

Instead of mutating an object:

x = { baz: 'boo'} = 'bar'
// x is now:
{foo: 'bar', baz: 'boo'}

We can use Object.assign to create a new object as a copy of an existing object but with new values(s):

x = { baz: 'boo'}
y = Object.assign({}, {foo: 'bar'}, x)
//y is now:
{foo: 'bar', baz: 'boo'}
//x is still:
{ baz: 'boo'}

So why not just use Object.assign or the spread operator all the time thus removing mutability, side effects, and all that stuff that functional programming teaches us is bad? Well, because performance can be abysmal.

Take the following test suite using benchmark.js:

var Benchmark = require('benchmark')

const suite = new Benchmark.Suite;

const obj = { foo: 1, bar: 2 };
let mutObj = { foo: 1, bar: 2};

  add('Object spread', function() {
    ({ baz: 3, ...obj});
  add('Object.assign()', function() {
    Object.assign({}, { baz: 3 }, obj);
  add('Mutation', function() {
    mutObj.baz = 3
  on('cycle', function(event) {
  on('complete', function() {
    console.log('Fastest is ' + this.filter('fastest').map('name'));

The results are telling:

Object spread x 18,041,542 ops/sec ±0.81% (85 runs sampled)
Object.assign() x 12,785,551 ops/sec ±0.87% (89 runs sampled)
Mutation x 780,033,935 ops/sec ±1.86% (84 runs sampled)
Fastest is Mutation

We can see here that mutating an object is 65x faster than using Object.assign. Which makes sense because Object.assign is creating an entire new object.

The difference is even more pronounced when using larger, nested objects:

const obj = {
  foo: 1,
  bar: 2,
  lorem: 'ipsum, dolor, amet...',
  nested: {
    bird: 'yes',
    mammal: 'no',
    platypus: 'maybe',

Object spread x 7,612,732 ops/sec ±1.14% (85 runs sampled)
Object.assign() x 7,264,250 ops/sec ±1.16% (87 runs sampled)
Mutation x 769,863,543 ops/sec ±1.50% (82 runs sampled)
Fastest is Mutation

Again, it makes intuitive sense that using Object.assign would be slower.

So is it a big deal? Probably not, as you’ll usually be using these slower, immutable patterns to work with React/Vue data in which the performance impact is not only negligible but necessary.

A real world example

I was recently asked to take a look at some very poorly performing pages in a Vue.js app. When I took a look I found some code that looked like this:

trackpoints[i] = new Object()
track.trackpoints.forEach(t => {
    const temp = trackpoints[i]
    const key = someFunction(t)
    trackpoints[i] = Object.assign({}, temp, {
        [key]: [,, t.baz]
return trackpoints

Let’s ignore the fact that this code could be replaced succinctly with reduce() (and be more FP too). The problem is that track.trackpoints consists of 10s to 100s of thousands of objects. While the above code is technically immutable, it is also creating a new Object per loop. Once the usage of Object.assign() was removed, the performance issues went away.

To me this is a good lesson of why it’s not a good idea to be too dogmatic in programming. Programming languages are just tools to do a job and to a certain extent the way you write your code is as well.