Source: modules/toolbar/js/views/MenuVisualView.js

/**
 * @file
 * A Backbone view for the collapsible menus.
 */

(function ($, Backbone, Drupal) {

  'use strict';

  Drupal.toolbar.MenuVisualView = Backbone.View.extend(/** @lends Drupal.toolbar.MenuVisualView# */{

    /**
     * Backbone View for collapsible menus.
     *
     * @constructs
     *
     * @augments Backbone.View
     */
    initialize: function () {
      this.listenTo(this.model, 'change:subtrees', this.render);
    },

    /**
     * @inheritdoc
     */
    render: function () {
      var subtrees = this.model.get('subtrees');
      // Add subtrees.
      for (var id in subtrees) {
        if (subtrees.hasOwnProperty(id)) {
          this.$el
            .find('#toolbar-link-' + id)
            .once('toolbar-subtrees')
            .after(subtrees[id]);
        }
      }
      // Render the main menu as a nested, collapsible accordion.
      if ('drupalToolbarMenu' in $.fn) {
        this.$el
          .children('.toolbar-menu')
          .drupalToolbarMenu();
      }
    }
  });

}(jQuery, Backbone, Drupal));