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',
        ]);
    }

}