Run results for: epoch milliseconds to ISO string

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:86.0) Gecko/20100101 Firefox/86.0

Browser: Firefox 86

Operating system: Mac OS X 10.15

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
template strings and padding	977165.0 Ops/sec
toISOString()	1347133.1 Ops/sec

Script Preparation code:

var seed = Date.now();

function toISO1(x) {
    Number.isFinite(x);

    const time = new Date(x);
    
    const year = String(time.getUTCFullYear()).padStart(2, '0');
    const mo = String(time.getUTCMonth()+1).padStart(2, '0');
    const da = String(time.getUTCDate()).padStart(2, '0');
    const ho = String(time.getUTCHours()).padStart(2, '0');
    const mi = String(time.getUTCMinutes()).padStart(2, '0');
    const se = String(time.getUTCSeconds()).padStart(2, '0');
    const mil = String(time.getUTCMilliseconds()).padStart(3, '0');
    
    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

A function that converts an epoch time into an ISO string

template strings and padding

toISOString()