AJAX in Drupal Form

<?php

namespace Drupal\vishal_ajax_api\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\HtmlCommand;

/**
 * Implements an AjaxForm to test an Ajax sample.
 */
class AjaxForm extends FormBase {

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['user_mail'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Email'),
      '#description' => $this->t('Enter your email address'),
      '#prefix' => $this->t('<div id="user-mail"></div>'),
      '#ajax' => [
        'callback' => '::checkEmailValidation',
        'effect' => 'fade',
        'event' => 'change',
        'progress' => [
          'type' => 'throbber',
          'message' => NULL,
        ],
      ],
    ];

    return $form;
  }

  /**
   * Checks email validation.
   */
  public function checkEmailValidation(array $form, FormStateInterface $form_state) {
    $ajax_response = new AjaxResponse();

    // Check if email exists or not.
    if (user_load_by_name($form_state->getValue('user_mail')) || user_load_by_mail($form_state->getValue('user_mail'))) {
      $text = 'User or email exists.';
    }
    else {
      $text = 'User or email does not exists.';
    }

    $ajax_response->addCommand(new HtmlCommand('#user-mail', $text));
    return $ajax_response;
  }

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

  }

}