UUID v4 generator in JavaScript (RFC4122 compliant)

uuid.js

function uuid() {
  var uuid = "", i, random;
  for (i = 0; i < 32; i++) {
    random = Math.random() * 16 | 0;

    if (i == 8 || i == 12 || i == 16 || i == 20) {
      uuid += "-"
    }
    uuid += (i == 12 ? 4 : (i == 16 ? (random & 3 | 8) : random)).toString(16);
  }
  return uuid;
}

&3|8 === |8

'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/x|y/g, match => (Math.random()*16 | (match === 'y' ? 8 : 0)).toString(16))