How to check a variant valid or not in qml?

[Hit Tiger Tonight]

How to check a variant valid or not in qml?
The QVariant has an isValid() method to determine it is valid or not. But pass a QVaraint to qml method or property, the isValid() seems disappeared.
Is anyone know how to check the variant in qml?

[fromis_9]

Use 'undefined' instead.

var invalidValue = backend.GetInvalidQVar();
if (invalidValue == undefined) {
  console.log("Invalid");
}
else {
  console.log("Valid");
}