QJSEngine does not know how to handle Qt basic types

Question

I have a c++/qt application that I made scriptable via a QJSEngine my_eng in which I expose my QWidget my_obj via my_eng.globalObject().setProperty("my_obj", my_eng.newQObject(my_obj));

Now from within JS I can call all methods of my_obj. I have a method getRect() which returns a QRectF and when I call it I get a good representation of it: QRectF(10, 10, 200, 200)

Now, if I store this a in variable my_rect and try to access it via usual qt methods like my_rect.setWidth(11) I get a

TypeError: Property 'setWidth' of object QRectF(10, 10, 200, 200) is not a function

Does someone know how to make all qt type available in QJSEngine?