Run results for: I suppose you think we should write in assembly

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36 Edg/85.0.564.51

Browser: Chrome 85

Operating system: Windows

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
split and join	329209.8 Ops/sec
readable regex	263791.7 Ops/sec
slice and append mess	1249372.0 Ops/sec

Script Preparation code:

window.dates = [
  new Date(),
  new Date('1960-12-01'),
  new Date('3401-01-12')
].map((a) => (a.toISOString()));

window.test1 = (isoString) => {
	isoString = isoString.split('T')[0].split('-');
	return [isoString[1], isoString[2], isoString[0]].join('-');
};

let regex = /^(?<year>\d+)-(?<month>\d+)-(?<day>\d+)T.*$/;
let replace = '$<month>-$<day>-$<year>';
window.test2 = (isoString) => (isoString.replace(regex, replace));

window.test3 = (isoString) => {
	isoString = isoString.slice(0, 10);
	return isoString.slice(8, 10) + '-' + isoString.slice(5, 7) + '-' + isoString.slice(0, 4);
};

Tests:

split and join
for (let isoString of dates) test1(isoString);
readable regex
for (let isoString of dates) test2(isoString);
slice and append mess
for (let isoString of dates) test3(isoString);

Run results for: I suppose you think we should write in assembly

split and join

readable regex

slice and append mess