Bootstrap 4 Multi-Level Dropdown

dropdown.js

/*
 * Adapted from: https://stackoverflow.com/questions/44467377/bootstrap-4-multilevel-dropdown-inside-navigation
 */

'use strict';
( function( $ ) {
    $( '.dropdown-menu a.dropdown-toggle' ).on( 'click', function() {
        var $subMenu,
            $this      = $( this ),
            $ancestors = $this.parents( '.dropdown-menu' ),
            $parent    = $this.parent( '.dropdown' );

        if ( !$this.next().hasClass( 'show' ) ) {
            $ancestors.first().find( '.show' ).removeClass( 'show' );
            $parent.removeClass( 'show' );
        }

        $subMenu = $this.next( '.dropdown-menu' );
        $subMenu.toggleClass( 'show' );
        $parent.toggleClass( 'show' );

        $this.parents( 'li.menu-item.dropdown.show' ).on( 'hidden.bs.dropdown', function() {
            $( this ).removeClass( 'show' ).find( '.dropdown-menu .show' ).removeClass( 'show' );
        } );

        return false;
    } );
} )( jQuery );