code: function (val) {
              var carousel = this, numItems = carousel.get("numItems");
  
              if (JS.isNumber(val)) {
                  if (numItems === 0 && val == numItems) {
                      return true;
                  } else {
                      return (val >= 0 && val < numItems);
                  }
              }
  
              return false;
          }
docs: Validate the firstVisible value.
  
  @method _validateFirstVisible
  @param val {Number} The first visible value
  @return {Boolean} The status of the validation
  @protected