Run results for: epoch milliseconds to ISO string, precalculated numbers

Run details:

User agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.72 Safari/537.36

Browser: Chrome 89

Operating system: Linux

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
template strings and padding	776826.1 Ops/sec
toISOString()	1431274.0 Ops/sec

Script Preparation code:

var seed = Date.now();

var twoDigits = [];

for(let i = 0; i<100; i++) {
	twoDigits.push(String(i).padStart(2, '0'));
}

var threeDigits = [];

for(let i = 0; i<1000; i++) {
	threeDigits.push(String(i).padStart(3, '0'));
}

function toISO1(x) {
    Number.isFinite(x);

    const time = new Date(x);
    
    const year = String(time.getUTCFullYear());
    const mo = twoDigits[time.getUTCMonth()+1];
    const da = twoDigits[time.getUTCDate()];
    const ho = twoDigits[time.getUTCHours()];
    const mi = twoDigits[time.getUTCMinutes()];
    const se = twoDigits[time.getUTCSeconds()];
    const mil = threeDigits[time.getUTCMilliseconds()];
    
    return `${year}-${mo}-${da}T${ho}:${mi}:${se}.${mil}Z`;
}

function toISO2(x) {
	return new Date(x).toISOString();
}

Tests:

template strings and padding
var value = toISO1(seed);
toISOString()
var value = toISO2(seed);

Run results for: epoch milliseconds to ISO string, precalculated numbers

A function that converts an epoch time into an ISO string

template strings and padding

toISOString()