_selectItem(index) {
    for (let i = 0; i < this.getListLength(); i += 1) {
      this.removeClass(i, selectedClassName);
      this.removeClass(i, disabledClassName);
      if (i > index) {
        this.addClass(i, disabledClassName);
      }
    }
    this.addClass(index, selectedClassName);
  }
Change item's state to selected state
@param {number} index - index of selected item