WordPress Custom Menu Walker for Bootstrap 4

functions.php

/**
 * WordPress Custom Menu Walker for Bootstrap 4.
 */
add_filter ( 'nav_menu_css_class', 'navitemclass', 10, 4 );

function navitemclass( $classes, $item, $args, $depth ){
  $classes[] = 'nav-item';
  return $classes;
}

add_filter('walker_nav_menu_start_el', 'navlinkclass', 10, 4);

function navlinkclass($item_output, $item, $depth, $args) {
    $menu_locations = get_nav_menu_locations();

    if ( has_term($menu_locations['top-menu'], 'nav_menu', $item) ) {
       $item_output = preg_replace('/<a /', '<a class="nav-link" ', $item_output, 1);
    if ($item->menu_order == 1){
     $item_output = preg_replace('/<a /', '<a class="nav-link active" ', $item_output, 1);
    }
    }

    return $item_output;
}