How to add custom nav_menu_css_class to certain menu only?

Question

I want to add a custom CSS class to menu items. So far, I got it working by using the filter hook nav_menu_css_class. However, I want to add this class to items of a specific menu only. So, how must a condition look like to affect a specific menu (e.g. 'custom-menu' in theme_location 'footer_menu') only?

function add_footer_nav_item_class( $classes, $item ) {
  // I am looking for a condition here
  if ($item belongs to 'custom-menu'){
    $classes[] = "footer--menu-item";
    return $classes;
  }
}
add_filter( 'nav_menu_css_class' , 'add_footer_nav_item_class' , 10, 2 );

Answer 1

You just need the third argument $args, of which one of it's properties is theme_location:

function wpse_218377_nav_menu_css_class( $classes, $item, $args ) {
    if ( ! empty( $args->theme_location ) && $args->theme_location === 'custom-menu' ) {
        $classes[] = 'footer--menu-item';
    }

    // ALWAYS return, not from inside the if
    return $classes;
}

add_filter( 'nav_menu_css_class' , 'wpse_218377_nav_menu_css_class' , 10, 3 );