Drupal AJAX Forms

AJAX Callback Pattern

public function buildForm(array $form, FormStateInterface $form_state): array {
  $form['type'] = [
    '#type' => 'select',
    '#title' => $this->t('Type'),
    '#options' => ['a' => 'Type A', 'b' => 'Type B'],
    '#ajax' => [
      'callback' => '::updateOptions',
      'wrapper' => 'options-wrapper',
      'event' => 'change',
    ],
  ];