Run results for: Object.assign vs for-in performance

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36

Browser: Chrome 74

Operating system: Mac OS X 10.14.5

Device Platform: Desktop

Date tested: 5 years ago

Test name	Executions per second
For-in assignment	989456064.0 Ops/sec
Object.assign mutable assignment	979745216.0 Ops/sec
Object.assign immutable assignment	994985472.0 Ops/sec

Script Preparation code:

const itemA = { name: 'itemA', things: ['one', 2, 'three', Symbol('four')] };
const itemB = { name: 'itemB', otherThings: { fieldOne: 1, fieldTwo: 'two' } };

Tests:

For-in assignment
// Object.assign() is at least 4x slower than a "for in" loop const fastObjectAssign = (target, source) => { if (!source || !target) return target; for (let key in source) target[key] = source[key]; return target; };
Object.assign mutable assignment
const directAssign = (target, source) => { return Object.assign(target, source); };
Object.assign immutable assignment
const directAssign = (target, source) => { return Object.assign({}, target, source); };

Run results for: Object.assign vs for-in performance

For-in assignment

Object.assign mutable assignment

Object.assign immutable assignment