code: function key$2(n, callback) {
      var self = this;
      var promise = self.ready().then(function () {
          var dbInfo = self._dbInfo;
          var result;
          try {
              result = localStorage.key(n);
          } catch (error) {
              result = null;
          }
  
          // Remove the prefix from the key, if a key is found.
          if (result) {
              result = result.substring(dbInfo.keyPrefix.length);
          }
  
          return result;
      });
  
      executeCallback(promise, callback);
      return promise;
  }
docs: Same as localStorage's key() method, except takes a callback.