A simple and readable way to generate valid v4 UUIDs in JavaScript. CC0 https://creativecommons.org/publicdomain/zero/1.0/

randomUUID.js

function randomUUID() {
	return randhex(8) + "-" + randhex(4) + "-4" + randhex(3) + "-" + choice(["8", "9", "a", "b"]) + randhex(3) + "-" + randhex(12);
}
let scratchTarr = new Uint32Array(1);
function choice(arr) {
	crypto.getRandomValues(scratchTarr);
	return arr[Math.floor(scratchTarr[0]%arr.length)]
}
function randhex(count) {
	crypto.getRandomValues(scratchTarr);
	return ("00000000"+scratchTarr[0].toString(16)).slice(-count);
}