code: function (index) {
              var carousel   = this,
                  isCircular = carousel.get("isCircular"),
                  numItems   = carousel.get("numItems"),
                  numVisible = carousel.get("numVisible"),
                  sentinel   = numItems - 1;
  
              if (index < 0) {
                  index = isCircular ?
                          Math.ceil(numItems/numVisible)*numVisible + index : 0;
              } else if (index > sentinel) {
                  index = isCircular ? 0 : sentinel;
              }
  
              return index;
          }
docs: Return a valid item for a possibly out of bounds index considering
  the isCircular property.
  
  @method _getValidIndex
  @param index {Number} The index of the item to be returned
  @return {Object} Return a valid item index
  @protected