code: function(key) {
  
              // try to fetch expiration time for key
              var expireTime = parseInt(this._storage.getItem(this._expiration_key_prefix + key));
  
              // if got expiration time return how much left to live
              if (expireTime && !isNaN(expireTime)) {
  
                  return expireTime - this.getTimestamp();
              }
  
              // if don't have expiration time return null
              return null;
          }
docs: Get item time left to live.
  @param key: Item key to get (string).
  @return: Time left to expire (in seconds), or null if don't have expiration date.