Page callback: Returns an element suitable for use by ajax_render().
Additionally ensures that ajax_render() incorporates JavaScript settings generated during the page request by invoking drupal_add_js() with a dummy setting.
function ajax_test_render() {
drupal_add_js(array(
'ajax' => 'test',
), 'setting');
return array(
'#type' => 'ajax',
'#commands' => array(),
);
}