How to change menu URLs to hashes?

Here’s how I solved it so far.

class themename_walker_nav_menu extends Walker_Nav_Menu {
     function start_el( &$output, $item, $depth, $args ) {
        $slug = get_post( $item->object_id )->post_name;
        $item->url = "#{$slug}";
        return parent::start_el( $output, $item, $depth, $args  );
    }
}