Add a location class to WordPress menus

add-location-class-to-menus.php

<?php
/**
 * Add a class with the menu location to the WordPress menu wrapper
 *
 * @param array $args Menu arguments.
 * @return array
 */
function cameronjonesweb_add_location_class_to_menus( $args ) {
	if ( ! empty( $args['theme_location'] ) ) {
		$args['items_wrap'] = str_replace( '%2$s', '%2$s menu--' . sanitize_html_class( $args['theme_location'] ), $args['items_wrap'] );
	}
	return $args;
}

add_filter( 'wp_nav_menu_args', 'cameronjonesweb_add_location_class_to_menus' );