protected function js_vars()
    {
        $args = array(
            'class'  => get_class($this),
            'screen' => array(
                'id'   => $this->screen->id,
                'base' => $this->screen->base,
            )
        );

        printf("<script type='text/javascript'>list_args = %s;</script>\n", json_encode($args));
    }