Add Remove Fields Dynamically using Form API and Ajax

Example: Add/Remove 'test' Field depending on 'switch' field

1. on build()    

$form['test'] = [

      '#type' => 'container',

      '#attributes' => ['id' => 'edit-fields-test'],

    ];

    if ($form_state->getValue('switch') == 2) {

      $form['test'][0] = $this->getReturnTripFields($form, $form_state);

      $form['test'][0]['#prefix'] = '<div>';

      $form['test'][0]['#suffix'] = '</div>';

    }

    $form['switch'] = [

      '#type' => 'radios',

      '#title' => 'Trip Type',

      '#options' => [1 => 'NO', 2 => 'YES'],

      '#ajax' => array(

        'callback' => '::returnAjax',

        'wrapper' => 'edit-fields-test',

        'method' => 'replace',

      ),

    ];

2. AjaxMethode

  function returnAjax($form, FormStateInterface $form_state) {

    $response = new AjaxResponse();

    if ($form_state->getValue('switch') == 2) {

      $content = [ '#markup' => ' YES ', ];

    }

    else {

      $content = [ '#markup' => ' ', ];

    }

    $response->addCommand(new HtmlCommand('#edit-fields-test', $content));

    return $response;

  }