Отправка JSON с сервера на клиент?

var xhr = new XMLHttpRequest();

    xhr.open('GET', '/api/customers', true);
    xhr.onload  = function (data) {
        console.log(JSON.parse(data.target['response'])) // Object 
        document.body.innerText = JSON.parse(data.target['response']).customer
    };
    xhr.send();

$app->get('/api/customers', function(Request $request, Response $response){
    $sql = "SELECT * FROM customers";

    try{
        // Get DB Object
        $db = new db();
        // Connect
        $db = $db->connect();

        $stmt = $db->query($sql);
        $customers = $stmt->fetchAll(PDO::FETCH_OBJ);
        $db = null;
        return $response->withJson($customers);
    } catch(PDOException $e){
        echo '{"error": {"text": '.$e->getMessage().'}';
    }
});

$app->any('/panel/post', 'App\Controllers\AdminController:all_post') //основной шаблон, вывод всех постов
public function all_post(Request $request, Response $response, $args)
    {
        $stmt = $this->pdo->query("SELECT * FROM `post` ORDER BY `post`.`id` DESC");
        $posts = $stmt->fetchAll(\PDO::FETCH_ASSOC);

        return $this->view->render($response, 'admins/xhrHTML.twig');
    }

$app->any('/panel/post/one', 'App\Controllers\AdminController:post_one')
    public function xhrHttp(Request $request, Response $response)
    {
        $post =  [
            'file' => 'file.txt'
        ];

        return $response->withJson($attr);
    }

$app->get('/api', function(Request $request, Response $response) {
    // тут просто выводит шаблон
}
$app->get('/api/customers', function(Request $request, Response $response) {
    //  а тут выводит выдачи всех покупателей
}