Run results for: Mutable object reducer vs immutable object reducer

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.142 Safari/537.36

Browser: Chrome 75

Operating system: Mac OS X 10.14.6

Device Platform: Desktop

Date tested: 5 years ago

Test name	Executions per second
Immutable reducer via spread operator	156399.7 Ops/sec
Mutable reducer via property assign	1791683.0 Ops/sec

Script Preparation code:

var arr = [
  { key: 'one',       val: 1 },
  { key: 'two',       val: 2 },
  { key: 'three',     val: 3 },
  { key: 'four',      val: 4 },
  { key: 'five',      val: 5 },
  { key: 'six',       val: 6 },
  { key: 'seven',     val: 7 },
  { key: 'eight',     val: 8 },
  { key: 'nine',      val: 9 },
  { key: 'ten',       val: 10 },
  { key: 'eleven',    val: 11 },
  { key: 'twelve',    val: 12 },
  { key: 'thirteen',  val: 13 },
  { key: 'fourteen',  val: 14 },
  { key: 'fifteen',   val: 15 },
  { key: 'sixteen',   val: 16 },
  { key: 'seventeen', val: 17 },
  { key: 'eighteen',  val: 18 },
  { key: 'nineteen',  val: 19 },
  { key: 'twenty',    val: 20 }
];

Tests:

Immutable reducer via spread operator
const reducer = (obj, pair) => ({...obj, [pair.key]: pair.val }); arr.reduce(reducer, {});
Mutable reducer via property assign
const reducer = (obj, pair) => { obj[pair.key] = pair.val; return obj; }; arr.reduce(reducer, {});

Run results for: Mutable object reducer vs immutable object reducer

Immutable reducer via spread operator

Mutable reducer via property assign