If you do that in JS you get "undefined" and "null", respectively. Our C++-based conversion methods should do the same. The documentation for QJSValue also suggests that QJSValue::toFoo() should behave like qjsvalue_cast<Foo>(x). So far QJSValue::toString() produced "undefined" and "null" while qjsvalue_cast<String>(x) produced an empty string. [ChangeLog][QtQml][Important Behavior Changes] qjsvalue_cast<QString>(x) now returns "undefined" for undefined JS values, and "null" for null JS values. This is in line with what QJSValue::toString() does, and also what JavaScript itself would produce when stringifying such values. Previously, qjsvalue_cast would return an empty string for both, undefined and null JS values. Change-Id: Ib93f4157f092ed769dca946541ffbcfbd7317d4c Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>