class EntityReferenceAjaxController

class EntityReferenceAjaxController extends ControllerBase {
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected RendererInterface $renderer;
    
    /**
     * Constructs a new EntityReferenceAjaxController.
     *
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     */
    public function __construct(RendererInterface $renderer) {
        $this->renderer = $renderer;
    }
    
    /**
     * Render the desired field into an AjaxResponse.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity to get the field from.
     * @param string $field_name
     *   The field to display.
     * @param string $view_mode
     *   The view mode to display the field with.
     * @param string $language
     *   The language to get the field in.
     * @param int $start
     *   Currently only used in the EntityReferenceAjaxFormatter.
     * @param ?string $printed
     *   Used when sorting and order isn't guaranteed to ensure no duplicates.
     *
     * @return \Drupal\Core\Ajax\AjaxResponse
     *   The ajax response.
     *
     * @throws \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
     *   If the field doesn't exist or not a Content Entity.
     */
    public function viewField(EntityInterface $entity, string $field_name, string $view_mode, string $language, int $start, ?string $printed) {
        // Ensure that this is a valid Content Entity.
        if (!$entity instanceof ContentEntityInterface) {
            throw new BadRequestHttpException('Requested Entity is not a Content Entity.');
        }
        // Check that this field exists.
        
        /** @var \Drupal\Core\Field\FieldItemListInterface $field */
        if (!($field = $entity->getTranslation($language)
            ->get($field_name))) {
            throw new BadRequestHttpException('Requested Field does not exist.');
        }
        $response = new AjaxResponse();
        $field_elements = $field->view($view_mode);
        $response->addCommand(new ReplaceCommand("#ajax-field-{$entity->getEntityTypeId()}-{$entity->id()}-{$field_name}", $this->renderer
            ->render($field_elements)));
        return $response;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('renderer'));
    }

}