Fast RFC-compliant UUID v4 Generator

uuid.js

const charset   = '0123456789abcdef'.split('');
const randomset = new Uint8Array(16);

export function uuid() {
  let rand = 0;
  for (let i = 0; i < 16; i++) {
    if (rand < 2) {
      rand += Math.random() * (1 << 24);
    }
    randomset[i] = rand & 0xFF;
    rand >>>= 8;
  }

  randomset[6] = (randomset[6] & 0x0F) | 0x40;
  randomset[8] = (randomset[8] & 0x3F) | 0x80;

  let output = '';
  for (let i = 0; i < 16; i++) {
    const rnd = randomset[i];
    output += charset[rnd & 15];
    output += charset[rnd >>> 4];
  }
  
  return output;
}

Another version with one uniform loop (slightly slower):

export function uuid() {
  let rand = 0;
  let output = '';
  for (let i = 0; i < 16; i++) {
    if (rand < 2) {
      rand += Math.random() * (1 << 24);
    }

    switch (i) {
      case 6: rand = (rand & 0x0F) | 0x40; break;
      case 8: rand = (rand & 0x3F) | 0x80; break;
      // forcing compiler build optimal jump table for us
      case 0: case 1: case 2:
      case 3: case 4: case 5:
      default: rand &= 0xFF; break;
    }

    output += charset[rand & 15];
    output += charset[rand >>> 4];

    rand >>>= 8;
  }

  return output;
}