code: function (key, countBefore, countAfter, hints) {
                      var that = this;
                      return this._fetchItems(
                          // getGroup
                          function () {
                              return that._keyMap[key];
                          },
  
                          // mayExist
                          function () {
                              var lastGroup = that._lastGroup;
                              if (!lastGroup) {
                                  return true;
                              }
                              if (+lastGroup.index !== lastGroup.index) {
                                  return true;
                              }
                          },
  
                          // fetchInitialBatch
                          function () {
                              hints = hints || {};
                              var itemPromise = (
                                  typeof hints.groupMemberKey === "string" && that._listBinding.fromKey ?
                                      that._listBinding.fromKey(hints.groupMemberKey) :
                                  typeof hints.groupMemberIndex === "number" && that._listBinding.fromIndex ?
                                      that._listBinding.fromIndex(hints.groupMemberIndex) :
                                  hints.groupMemberDescription !== undefined && that._listBinding.fromDescription ?
                                      that._listBinding.fromDescription(hints.groupMemberDescription) :
                                      that._listBinding.first()
                              );
  
                              var fetchBefore = Math.floor(0.5 * (that._batchSize - 1));
                              that._fetchBatch(itemPromise, fetchBefore, that._batchSize - 1 - fetchBefore);
                          },
  
                          countBefore, countAfter
                      );
                  }
docs: itemsFromStart: not implemented itemsFromEnd: implemented in constructor