Simple and fast generation of RFC4122 UUIDs in Browser

uuid.js

/* Simple and fast generation of RFC4122 UUIDS. */

(function(root, factory) {
  'use strict';
  root.uuid = factory();
})(window, function() {

  var byteToHex = [];
  for (var i=0; i<256; ++i) {
    byteToHex[i] = (i + 0x100).toString(16).substr(1);
  }

  /* It's not a cryptographically strong random number generation.
   * Unique ID creation requires a high quality random # generator. In the
   * browser this is a little complicated due to unknown quality of Math.random().
   * It's fast, but is of unspecified quality, and is better to use crypto libs.
   */
  var rng = function() {
    var rnds = new Array(16);
    for (var i=0, r; i<16; i++) {
      if ((i & 0x03) === 0) r = Math.random() * 0x100000000;
      rnds[i] = r >>> ((i & 0x03) << 3) & 0xff;
    }
    return rnds;
  };

  function bytesToUuid(buf) {
    var i = 0;
      bth = byteToHex;
    return  bth[buf[i++]] + bth[buf[i++]] + 
            bth[buf[i++]] + bth[buf[i++]] + '-' + 
            bth[buf[i++]] + bth[buf[i++]] + '-' + 
            bth[buf[i++]] + bth[buf[i++]] + '-' + 
            bth[buf[i++]] + bth[buf[i++]] + '-' + 
            bth[buf[i++]] + bth[buf[i++]] + 
            bth[buf[i++]] + bth[buf[i++]] + 
            bth[buf[i++]] + bth[buf[i++]];
  }

  function uuid() {
    var rnds = rng();
    rnds[6] = (rnds[6] & 0x0f) | 0x40;
    rnds[8] = (rnds[8] & 0x3f) | 0x80;
    return bytesToUuid(rnds);
  }

  return uuid;
});