Javascript clientside GUID

guid.js

 /**
   * @function _guid
   * @description Creates GUID for user based on several different browser variables
   * It will never be RFC4122 compliant but it is robust
   * @returns {Number}
   * @private
   */
  var guid = function() {

      var nav = window.navigator;
      var screen = window.screen;
      var guid = nav.mimeTypes.length;
      guid += nav.userAgent.replace(/\D+/g, '');
      guid += nav.plugins.length;
      guid += screen.height || '';
      guid += screen.width || '';
      guid += screen.pixelDepth || '';

      return guid;
  };

It returned same guid for 2 different PC in my company.