docs: function () {
              var cache = this._cache;
              var expirationTime = ((new Date()).getTime() - this._expiresAfter * 1000);
  
              for (var url in cache) {
                  if (cache.hasOwnProperty(url) && cache[url].age < expirationTime) {
                      delete cache[url];
                  }
              }
          }