Json

<script>
      var USERNAME="admi";
      var PASSWORD="chanud";

      var person = {
        firstName:"tester2",
        lastName:"drupal",
        userName:"tester2drupal",
        eMail:"teter2@drupalvip.com",
        pass1:"teter2drupal",
        passConfirm:"teter2drupal",
        role:"Affiliate" 
      };
      
      $.ajax({
        type: "POST",
        url: 'https://mydomain/api/route',
        dataType: 'json',
        headers: {
         "Authorization": "Basic " + btoa(USERNAME + ":" + PASSWORD)
        },
        data: person,
        //data: JSON.stringify(person),
        success: function (data) {
          $("response").html(data.msg);
        },                
      });
    </script>

use Symfony\Component\HttpFoundation\JsonResponse;

/**
 * Class JsonApiArticlesController
 * @package Drupal\mymodule\Controller
 */
class JsonApiArticlesController {

  /**
   * @return JsonResponse
   */
  public function index() {
    return new JsonResponse([ 'data' => $this->getData(), 'method' => 'GET', 'status'=> 200]);
  }

  /**
   * @return array
   */
  public function getData() {

    $result=[];
    $query = \Drupal::entityQuery('node')
      ->condition('type', 'article')
      ->sort('title', 'DESC');
    $nodes_ids = $query->execute();
    if ($nodes_ids) {
      foreach ($nodes_ids as $node_id) {
        $node = \Drupal\node\Entity\Node::load($node_id);
        $result[] = [
          "id" => $node->id(),
          "title" => $node->getTitle(),
        ];
      }
    }
    return $result;
  }
}

use Symfony\Component\HttpFoundation\JsonResponse;

// if you know the data to send when creating the response
$response = new JsonResponse(['data' => 123]);

// if you don't know the data to send or if you want to customize the encoding options
$response = new JsonResponse();
// ...
// configure any custom encoding options (if needed, it must be called before "setData()")
//$response->setEncodingOptions(JsonResponse::DEFAULT_ENCODING_OPTIONS | \JSON_PRESERVE_ZERO_FRACTION);
$response->setData(['data' => 123]);

// if the data to send is already encoded in JSON
$response = JsonResponse::fromJsonString('{ "data": 123 }');

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\CloseModalDialogCommand;
use Drupal\Core\Ajax\RedirectCommand;

/**
 * Implements hook_form_alter().
 */
function modal_form_example_form_node_article_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form['actions']['submit']['#submit'][] = '_modal_form_example_ajax_submit';
  $form['actions']['submit']['#attributes']['class'][] = 'use-ajax-submit';
  // in some cases this was needed:
  // $form['#attached']['library'][] = 'core/jquery.form';
  // $form['#attached']['library'][] = 'core/drupal.ajax';
}

/**
 * Close the Modal and redirect the user to the homepage.
 *
 * @param array $form
 *   The form that will be altered.
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 *   FormState Object.
 */
function _modal_form_example_ajax_submit(array $form, FormStateInterface &$form_state) {
  $response = new AjaxResponse();
  $response->addCommand(new CloseModalDialogCommand());
  $form_state->setResponse($response);
}