A reasonably fast, yet still readable implementation of UUID v4 in JS. Just about the fastest possible without ASM and extensive loop unrolling.

fast_v4_uuid.js

// RFC-4122 - UUID from Crypto Random Source (Version 4)
// https://www.ietf.org/rfc/rfc4122.txt
(function () {
  var crypto = self.crypto || self.msCrypto;
  var hex_values = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'];
  var template = 'xxxxxxxx-xxxx-4xxx-Rxxx-xxxxxxxxxxxx';
  var template_length = template.length;

  Math.uuid = function() {
    // Generate 16 random 8-bit values, which will be clamped down to 4-bits below
    var byte_array = new Uint8Array(16);
    crypto.getRandomValues(byte_array);

    // Use the above values to generate the UUID
    var uuid = "";
    var template_index = 0;
    var quad = -1;

    // Iterate over the UUID template
    for (template_index = 0; template_index < template_length; template_index++) {
      var char = template[template_index];

      // Copy '-' and the RFC-4122 version number from template into UUID
      if (char == '-' || char == '4') uuid += char;
      // Populate the random values into UUID
      else {
        // Alternate high and low order quads
        var hex = (++quad & 1 ? byte_array[quad >> 1] >> 4 : byte_array[quad >> 1] & 0xF);

        // Append the variant code 10xx, for the RFC-4122 variant of UUIDs
        if (char == 'R') uuid += hex_values[hex & 0x3 | 0x8];
        // Append the hex value for a normal quad
        else uuid += hex_values[hex];
      }
    }

    // Return the resulting UUID
    return uuid;
  };
})();