add extra class in wordpress menu.php

 /** 
 * Add li class in menu
 */
function add_additional_class_on_li($classes, $item, $args) {
    if($args->add_li_class) {
        $classes[] = $args->add_li_class;
    }
    return $classes;
}
add_filter('nav_menu_css_class', 'add_additional_class_on_li', 1, 3);

/**
* Add li > a class menu
*/
function add_additional_class_on_a( $atts, $item, $args ) {
   $class = 'nav-link'; // or something based on $item
   $atts['class'] = $class;
   return $atts;
}
add_filter( 'nav_menu_link_attributes', 'add_additional_class_on_a', 1, 3 );