Simple Menu Javascript Example

menu.js

/**
 * @file
 * A JavaScript file containing menu functionality (small/large screen)
 *
 */

(function (Drupal) {

  Drupal.behaviors.mainMenu = {
    attach: function (context) {

      var menu = context.getElementById('menu');
      var expand_menu = menu.getElementsByClassName('expand-sub');

      for (var i = 0; i < expand_menu.length; i++) {
        expand_menu[i].addEventListener("click", function(e) {
          var menu_item = e.currentTarget;
          var sub_menu = menu_item.nextElementSibling;

          menu_item.classList.toggle('menu--open');
          sub_menu.classList.toggle('expand-sub--open');
        });
      }

    }
  };

})(Drupal);