Simple Drupal Ajax Form

AjaxExampleForm.php

<?php

declare(strict_types=1);

namespace Drupal\my_module\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a simple ajax form.
 */
final class AjaxExampleForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId(): string {
    return 'my_module_ajax_example';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state): array {
    $form['choice'] = [
      '#type' => 'select',
      '#title' => 'First field',
      '#options' => [
        '1' => 'Mandatory',
        '2' => 'Optional',
      ],
      '#description' => 'Determines whether the second field is mandatory or optional.',
      '#required' => TRUE,
      '#ajax' => [
        'callback' => '::changeRequired',
        'wrapper' => 'second-field',
      ],
    ];

    $isOptional = ($form_state->getValue('choice') === '2');
    $form['second'] = [
      '#type' => 'textfield',
      '#title' => 'Second field',
      '#description' => $isOptional ? 'This field is optional.' : 'This field is mandatory.',
      '#required' => !$isOptional,
      '#prefix' => '<div id="second-field">',
      '#suffix' => '</div>',
    ];

    return $form;
  }

  // Ajax Callback.
  public function changeRequired(array &$form, FormStateInterface $form_state) {
    return $form['second'];
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {}

}