session_start();

if (isset($_SESSION['user'])) {
  $status = 'ERROR';
    $validator = new Validator();
    $validator->add('id:Product_Id', 'required');
    $validator->add('id:Product_Id', 'integer');

    if ($validator->validate($_POST)) {
      try {
        Closet::create([
          'user_id' => $_SESSION['user']['id'],
          'product_id' => $_POST['id'],
        ]);
        $status = 'OK';
      } catch (QueryException $e) {
        Log::logError($e->getMessage());
      }
    }

    header('Type-Content: application/json');
    echo json_encode(['status' => $status]);
}
else {
  header('Location: pantalladelogin.php');
}

<a onclick="addProduct(1)">
  <i class="material-icons">add</i>
</a>

function addProduct (id) {
  const formdata = new FormData()
  formdata.append('id', id)

  fetch("/addproduct.php", {
    method: 'post',
    body: formdata
  }).then(function(response) {
    console.log(response)
    return response.text()
  }).then(function(data) {
    console.log(data)
  });
}

Response { type: "basic", url: "http://localhost/pantalladelogin.php", redirected: true, status: 200, ok: true, statusText: "OK", headers: Headers, bodyUsed: false }

var sesion = <?=json_encode($_SESSION['identificadorSesion'])?>;