Drupal 8 Ajax form example when select box changed

TestAjaxFormSelect.php

<?php

namespace Drupal\my_module\Form;

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

/**
 * Class TestAjaxFormSelect.
 */
class TestAjaxFormSelect extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'test_ajax_form_select';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    // Get the form values and raw input (unvalidated values).
    $values = $form_state->getValues();

    // Define a wrapper id to populate new content into.
    $ajax_wrapper = 'my-ajax-wrapper';

    // Sector.
    $form['my_select'] = [
      '#type' => 'select',
      '#empty_value' => '',
      '#empty_option' => '- Select a value -',
      '#default_value' => (isset($values['my_select']) ? $values['my_select'] : ''),
      '#options' => [
        1 => 'One',
        2 => 'Two',
        3 => 'Three'
      ],
      '#ajax' => [
        'callback' => [$this, 'mySelectChange'],
        'event' => 'change',
        'wrapper' => $ajax_wrapper,
      ],
    ];

    // Build a wrapper for the ajax response.
    $form['my_ajax_container'] = [
      '#type' => 'container',
      '#attributes' => [
        'id' => $ajax_wrapper,
      ]
    ];

    // ONLY LOADED IN AJAX RESPONSE OR IF FORM STATE VALUES POPULATED.
    if (!empty($values) && !empty($values['my_select'])) {
      $form['my_ajax_container']['my_response'] = [
        '#markup' => 'The current select value is ' . $values['my_select'],
      ];
    }

    return $form;
  }

  /**
   * The callback function for when the `my_select` element is changed.
   *
   * What this returns will be replace the wrapper provided.
   */
  public function mySelectChange(array $form, FormStateInterface $form_state) {
    // Return the element that will replace the wrapper (we return itself).
    return $form['my_ajax_container'];
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Required by FormInterface.
  }

}