uuid-v4.js

//     32            16                 16                                    16                           48
//  097d43fb        e23d               4633                                  9608                     e930f252d32e
// time_low(8) - time_mid(4) - time_hi_and_version(4) - clock_seq_hi_and_reserved(2)/clock_seq_low(2) - node(12)
//                             leftmost bits are 0100      leftmost bits are 10
var uuid = function () {
  var a = new Uint8Array(16); // 16 8-bit octets that are represented by 2-digit hex numbers
  crypto.getRandomValues(a);

  a[6] = a[6] & 0x0f | 0x40;
  a[8] = a[8] & 0x3f | 0x80;

  var f = function () {
    var array = this;
    return [].map.call(arguments, function (index) {
      return ('00' + array[index].toString(16)).slice(-2);
    }).join('');
  }.bind(a);

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