code: function getSelectedValue(customSelect) {
      var hiddenSelect = this.UI.getHiddenSelect(customSelect);
      if (this.isMultiple(customSelect)) {
        var selectedOptions = [];
        [].forEach.call(hiddenSelect.options, function (option) {
          if (option.selected) {
            selectedOptions.push(option.value);
          }
        });
        return selectedOptions;
      } else {
        return hiddenSelect.options[hiddenSelect.selectedIndex].value;
      }
    }
docs: Get selected value
  If select is multiple then returns array
  
  @param customSelect
  @returns {String|Array}