Run results for: Reduce and Spread vs. Foreach and Mutate (fewer, but larger, objects)

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36

Browser: Chrome 83

Operating system: Mac OS X 10.15.4

Device Platform: Desktop

Date tested: 4 years ago

Test name	Executions per second
Reduce and Spread	2186000.5 Ops/sec
ForEach and Mutate (for .. in loop)	4260658.0 Ops/sec
Mutate with Object.assign	2471662.8 Ops/sec
Foreach and Mutate (Object.assign)	494034.0 Ops/sec

Script Preparation code:

var objectsArray = Array(10).fill(() => {
  return Object.fromEntries(Array(100).fill(() => {
    const key = Math.random().toString(36).substring(2, 5);
    const value = Math.random().toString(36).substring(2, 5);
    return [key, value];
  }));
});

Tests:

Reduce and Spread
var combined = objectsArray.reduce((memo, obj) => ({...memo, ...obj}), {});
ForEach and Mutate (for .. in loop)
var combined = {} objectsArray.forEach(obj => { for (const key in obj) { combined[key] = obj[key]; } });
Mutate with Object.assign
var combined = Object.assign(...objectsArray)
Foreach and Mutate (Object.assign)
var combined = {} objectsArray.forEach(obj => Object.assign(combined, obj));

Run results for: Reduce and Spread vs. Foreach and Mutate (fewer, but larger, objects)

Reduce and Spread

ForEach and Mutate (for .. in loop)

Mutate with Object.assign

Foreach and Mutate (Object.assign)