class JsAjaxTestForm

class JsAjaxTestForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'js_ajax_test_form';
    }
    
    /**
     * Form for testing the addition of various types of elements via Ajax.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['#attached']['library'][] = 'js_ajax_test/ajax';
        $form['custom']['#prefix'] = '<div id="js_ajax_test_form_wrapper">';
        $form['custom']['#suffix'] = '</div>';
        // Button to test for the waitForButton() assertion.
        $form['test_button'] = [
            '#type' => 'submit',
            '#value' => $this->t('Add button'),
            '#button_type' => 'primary',
            '#ajax' => [
                'callback' => [
                    static::class,
                    'addButton',
                ],
                'progress' => [
                    'type' => 'throbber',
                    'message' => NULL,
                ],
                'wrapper' => 'js_ajax_test_form_wrapper',
            ],
        ];
        return $form;
    }
    
    /**
     * Ajax callback for the "Add button" button.
     */
    public static function addButton(array $form, FormStateInterface $form_state) {
        return (new AjaxResponse())->addCommand(new JsAjaxTestCommand());
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // An empty implementation, as we never submit the actual form.
    }

}