fast-uuidv4.js

let map;

function generateUUID () {
	const buf = crypto.getRandomValues(new Uint8Array(16));
	buf[6] = (buf[6] & 0x0f) | 0x40;
	buf[8] = (buf[8] & 0x3f) | 0x80;

	if (!map) {
		map = [];

		for (let i = 0; i < 256; ++i) {
			map.push((i + 256).toString(16).slice(1));
		}
	}

	return (
		map[buf[0]] +
		map[buf[1]] +
		map[buf[2]] +
		map[buf[3]] +
		'-' +
		map[buf[4]] +
		map[buf[5]] +
		'-' +
		map[buf[6]] +
		map[buf[7]] +
		'-' +
		map[buf[8]] +
		map[buf[9]] +
		'-' +
		map[buf[10]] +
		map[buf[11]] +
		map[buf[12]] +
		map[buf[13]] +
		map[buf[14]] +
		map[buf[15]]
	);
}

const uuidv4 = 'randomUUID' in crypto
	? () => crypto.randomUUID()
	: generateUUID