exports.randomString = function (len, charSet) {
      charSet = charSet || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
      var randomString = '';
      for (var i = 0; i < len; i++) {
          var randomPoz = Math.floor(Math.random() * charSet.length);
          randomString += charSet.substring(randomPoz,randomPoz+1);
      }
      return randomString;
    };