How to Add Custom Classes to First and Last Items in WordPress Menu

functions.php

<?php

add_filter( 'wp_nav_menu_objects', 'afn_custom_menu_class' );

/**
 * Filters the first and last nav menu objects in your menus
 * to add custom classes.
 *
 * @since 1.0.0
 *
 * @param object $objects An array of nav menu objects
 * @return object $objects Amended array of nav menu objects with new class
 */
 
function afn_custom_menu_class( $objects ) {

    // Add "first-menu-item" class to the first menu object
    $objects[1]->classes[] = 'first-menu-item';

    // Add "last-menu-item" class to the last menu object
    $objects[count( $objects )]->classes[] = 'last-menu-item';

    // Return the menu objects
    return $objects;

}