Adds the page's slug to page nav items classes when using wp_nav_menu() in WordPress

nav_add_page_slug_class.php

<?php
// Adds the page's slug to page nav items classes when using wp_nav_menu()
add_filter('nav_menu_css_class', 'nav_add_page_slug_class', 10, 2);
function nav_add_page_slug_class($classes, $item) {
  if ($item->object === 'page') {
    $slug = get_post_field('post_name', $item->object_id);
    $classes[] = "page-$slug";
  }
  return $classes;
}
?>