code: function _LayoutCommon_getItemInfo(itemIndex) {
                      var result;
                      if (!this._itemInfo || typeof this._itemInfo !== "function") {
                          if (this._useDefaultItemInfo) {
                              result = this._defaultItemInfo(itemIndex);
                          } else {
                              throw new _ErrorFromName("WinJS.UI.GridLayout.ItemInfoIsInvalid", strings.itemInfoIsInvalid);
                          }
                      } else {
                          result = this._itemInfo(itemIndex);
                      }
                      return Promise.as(result).then(function (size) {
                          if (!size || +size.width !== size.width || +size.height !== size.height) {
                              throw new _ErrorFromName("WinJS.UI.GridLayout.ItemInfoIsInvalid", strings.itemInfoIsInvalid);
                          }
                          return size;
                      });
                  }
docs: itemIndex is optional