Simple "spec-driven" v4 UUID implementation

uuid4.js

// Version 4 UUIDs have the form xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx where
// x is any hexadecimal digit and y is one of 8, 9, A, or B (e.g.,
// f47ac10b-58cc-4372-a567-0e02b2c3d479).
// See: http://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_.28random.29
function uuid() {
  var reservedBitValues = ['8','9','a','b'];
  return Array.prototype.map.call("xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx", function mapUUID(c) {
    if (c == "x") {
      return Math.round(Math.random()*16-0.5).toString(16);
    }
    else if (c == "y") {
      return reservedBitValues[Math.round(Math.random()*reservedBitValues.length-0.5)]
    }
    else {
      return c;
    }
  }).join('');
}