code: function (groupKey, itemKey, itemIndex) {
                      this._lastFocusedItemKey = itemKey;
                      this._lastFocusedItemIndex = itemIndex;
                      itemIndex = "" + itemIndex;
                      this._itemToIndex[itemKey] = itemIndex;
                      this._groupToItem[groupKey] = itemKey;
                  }
docs: We store indices as strings in the cache so index=0 does not evaluate to false as when we check for the existance of an index in the cache. The index is converted back into a number when calling getIndexForGroup