Run results for: groupBy spread vs mutating

Run details:

User agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36

Browser: Chrome 68

Operating system: Linux

Device Platform: Desktop

Date tested: 6 years ago

Test name	Executions per second
Using spread operators	333.2 Ops/sec
Mutating	12495.1 Ops/sec

Script Preparation code:

let entries = []
let keys = ['A', 'B', 'C', 'D', 'E'];

for (var i = 0; i < 1000; i++) {
  entries.push({ key: keys[i % 5], property: 'something'});
}

function getEntries() {
  return entries;
}

Tests:

Using spread operators
getEntries().reduce((groups, entry) => { const group = groups[entry.key] || []; return { ...groups, [entry.key]: [...group, entry] }; }, {});
Mutating
getEntries().reduce(function(groups, entry) { (groups[entry.key] = groups[entry.key] || []).push(entry); return groups; }, {});

Run results for: groupBy spread vs mutating

Using spread operators

Mutating