code: function length(callback) {
          var self = this;
          var promise = new Promise(function(resolve, reject) {
              self.keys().then(function(keys) {
                  resolve(keys.length);
              })["catch"](reject);
          });
  
          executeCallback(promise, callback);
          return promise;
      }
docs: Supply the number of keys in the datastore to the callback function.