JSString *backend(JSObject *obj) {
  Value val = JS::GetReservedSlot(obj, Slots::Backend);
  return val.isString() ? val.toString() : nullptr;
}
JSString *backend(JSObject *obj) {
  Value val = JS::GetReservedSlot(obj, Slots::Backend);
  return val.isString() ? val.toString() : nullptr;
}