AutocompleteController.php

<?php

namespace Drupal\form_test;

use Symfony\Component\HttpFoundation\JsonResponse;

/**
 * Defines a controller class with methods for autocompletion.
 */
class AutocompleteController {
  
  /**
   * Returns some autocompletion content.
   *
   * @return \Symfony\Component\HttpFoundation\JsonResponse
   *   A JSON response.
   */
  public function autocomplete1() {
    return new JsonResponse([
      'key' => 'value',
    ]);
  }

}