Run results for: Array.prototype.concat vs splice vs loop for joining lots of arrays

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.75 Safari/537.36

Browser: Chrome 73

Operating system: Mac OS X 10.14.0

Device Platform: Desktop

Date tested: 5 years ago

Test name	Executions per second
immutable with concat	467.1 Ops/sec
mutation with splice + spread	4085.0 Ops/sec
mutation with splice into preallocated array	4000.0 Ops/sec
mutation with loop-assign	220526.8 Ops/sec

Tests:

immutable with concat
var a = [] var b = [ "hello", "jello" ] for (var i = 0; i < 1000; i++) { a = a.concat(b) }
mutation with splice + spread
var a = [] var b = [ "hello", "jello" ] for (var i = 0; i < 1000; i++) { a.splice(a.length, 0, ...b) }
mutation with splice into preallocated array
var a = new Array(2000) var b = [ "hello", "jello" ] for (var i = 0; i < 1000; i++) { a.splice(i * b.length, b.length, ...b) }
mutation with loop-assign
var a = new Array(2000) var b = [ "hello", "jello" ] for (var i = 0; i < 1000; i++) { for (var j = 0; j < b.length; j++) { a[i+j] = b[j] } }