Drupal → Отправить контактную форму с помощью AJAX

AJAXификация всех контактных форм с поддержкой валидации:

/**
 * Implements hook_form_BASE_FORM_ID_alter(): contact_message_form.
 */
function modulename_form_contact_message_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form_html_id = Html::getId($form_state->getFormObject()->getFormId());
  $form['#id'] = $form_html_id;

  $form['actions']['submit']['#ajax'] = [
    'callback' => 'modulename_form_contact_message_form_ajax',
    'event' => 'click',
  ];
}

/**
 * Contact form ajax submit callback.
 */
function modulename_form_contact_message_form_ajax(&$form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $form_selector = '#' . $form['#id'];

  // Validation fail
  if ($form_state->hasAnyErrors()) {
    // Update form
    $response->addCommand(new ReplaceCommand($form_selector, $form));

    // Show status message
    $response->addCommand(new PrependCommand($form_selector, ['#type' => 'status_messages']));
  }
  // Validation success
  else {
    // Clear system messages
    \Drupal::messenger()->deleteAll();

    // Show success message
    $contact_form = $form_state->getFormObject()->getEntity()->getContactForm(); /** @var ContactFormInterface $contact_form */
    $complete_message = $contact_form->getMessage() ?: t('Form is sent');
    $response->addCommand(new HtmlCommand($form_selector, '<div class="contact-form__success-message">' . $complete_message . '</div>'));
  }

  return $response;
}