drupal/core/modules/menu/menu.admin.js

(function ($) {

"use strict";

Drupal.behaviors.menuChangeParentItems = {
  attach: function (context, settings) {
    // Update list of available parent menu items.
    $('#edit-menu').on('change', 'input', Drupal.menuUpdateParentList);
  }
};

/**
 * Function to set the options of the menu parent item dropdown.
 */
Drupal.menuUpdateParentList = function () {
  var $menu = $('#edit-menu');
  var values = [];

  $menu.find('input:checked').each(function () {
    // Get the names of all checked menus.
    values.push(Drupal.checkPlain($.trim($(this).val())));
  });

  $.ajax({
    url: location.protocol + '//' + location.host + Drupal.url('admin/structure/menu/parents'),
    type: 'POST',
    data: {'menus[]' : values},
    dataType: 'json',
    success: function (options) {
      var $select = $('#edit-menu-parent');
      // Save key of last selected element.
      var selected = $select.val();
      // Remove all exisiting options from dropdown.
      $select.children().remove();
      // Add new options to dropdown. Keep a count of options for testing later.
      var totalOptions = 0;
      for (var machineName in options) {
        if (options.hasOwnProperty(machineName)) {
          $select.append(
            $('<option ' + (machineName === selected ? ' selected="selected"' : '') + '></option>').val(machineName).text(options[machineName])
          );
          totalOptions++;
        }
      }

      // Hide the parent options if there are no options for it.
      $select.closest('div').toggle(totalOptions > 0).attr('hidden', totalOptions === 0);
    }
  });
};

})(jQuery);