Drupal 7   JS code through the AJAX
$form['myitem'] = array(
'#type'=>'textfield',
'#ajax' => array(
  'callback' => 'my_callback',
  'wrapper' => 'new-id',
  'method' => 'html',
  'effect' => 'fade',
),
);
$form['myitem2'] = array(
'#type'=>'markup',
'#value'=>"<div id='new-id'> I'm #new-id </div>"
);


function my_callback(&$form, &$form_state){
  $commands = array();
  $commands[] = ajax_command_invoke('#new-id', 'html', array('<div id="new-id2">#new.id2 here!</div>'));
  $commands[] = ajax_command_invoke('#new-id2', 'addClass', array('error'));
  return array('#type' => 'ajax', '#commands' => $commands);
}