code: function selectIndex(element, index) {
    const count = element.items.length;
  
    const boundedIndex = (element.selectionWraps) ?
      // JavaScript mod doesn't handle negative numbers the way we want to wrap.
      // See http://stackoverflow.com/a/18618250/76472
      ((index % count) + count) % count :
  
      // Keep index within bounds of array.
      Math.max(Math.min(index, count - 1), 0);
  
    const previousIndex = element.selectedIndex;
    if (previousIndex !== boundedIndex) {
      element.selectedIndex = boundedIndex;
      return true;
    } else {
      return false;
    }
  }
docs: Ensure the given index is within bounds, and select it if it's not already selected.