Drupal 7: How to use handlers to override ajax form submissions

Example:

function mymodule_form_alter(&$form, &$form_state, $form_id)
{
  if($form_id == 'comment_form')
  {
    $form['#submit'][] = 'my_module_comment_form_submit';
  }
}

With the above code, the function my_module_comment_form_submit() will be called after all other submit handlers have been run. In this function you can do something like this:

function my_module_comment_form_submit($form, &$form_state)
{
  // Only run our JS if there are no error messages. So first
  // we check to see if there are any error messages.
  $messages = drupal_get_messages(NULL, FALSE);
  if(!count($messages) || !isset($messages['error']) || !count($messages['error']))
  {
    // There are no error messages, so we send a custom ajax command
    $commands[] = array
    (
      // this is the JavaScript command we will run after the submit
      'command' => 'doSomethingOnCommentFormSubmit',
      'message' => t('You submitted the form'),
    );

    return array('#type' => 'ajax', '#commands' => $commands);
  }
}

With the above code, if there are no errors in the form submission, the AJAX command 'doSomethingOnCommentFormSubmit' will be called. Now we have to register that in a .js file:

Drupal.ajax.prototype.commands.doSomethingOnCommentFormSubmit = function(ajax, response)
{
  alert(response.message);
};

The above alerts the message that we passed from our submit handler.

For more information on using AJAX commands, see: http://www.jaypan.com/tutorial/calling-function-after-ajax-event-drupal-7