code: function syncUI(customSelect, defaultValue, isMultiple) {
      var _this2 = this;
  
      console.log(defaultValue, isMultiple);
      var menuItems = this.getMenuItems(customSelect);
      var tglBtn = this.getToggleBtn(customSelect);
      var hasSelected = false;
      [].forEach.call(menuItems, function (menuItem) {
        if (defaultValue !== '') {
          var value = _this2.getItemValue(menuItem);
          if (isMultiple) {
            for (var k = 0; k < defaultValue.length; k++) {
              if (defaultValue[k] == value) {
                _this2.setItemActive(menuItem);
                hasSelected = true;
                _this2.getOptionByValue(customSelect, value).selected = true;
                break;
              }
            }
          } else if (value == defaultValue) {
            _this2.setItemActive(menuItem);
            hasSelected = true;
            _this2.getOptionByValue(customSelect, value).selected = true;
            if (tglBtn.innerHTML === '') {
              tglBtn.innerHTML = menuItem.innerHTML;
            }
          }
        } else if (menuItem.hasAttribute('aria-selected')) {
          _this2.setItemActive(menuItem);
          hasSelected = true;
          if (tglBtn.innerHTML === '') {
            tglBtn.innerHTML = menuItem.innerHTML;
          }
        } else if (menuItem.classList.contains(_this2.Config.classNameActive)) {
          _this2.setItemInactive(menuItem);
        }
      });
  
      if (!hasSelected) {
        this.setItemActive(menuItems[0]);
        this.getHiddenSelect(customSelect).options[0].setAttribute('selected', '');
        if (tglBtn.innerHTML === '') {
          tglBtn.innerHTML = menuItems[0].innerHTML;
        }
      }
    }
docs: Synchronizes default value and selected options with UI
  If dropdown item has aria-selected but has no active class, add it
  If dropdown item has no aria-selected but has active class, remove it
  If no dropdown item selected, select 1st item and hidden option
  If customselect has value attribute, sets selected option according to it in highest priority
  
  @param {HTMLElement} customSelect
  @param {String|Array} defaultValue
  @param {boolean} isMultiple