how to execute a "property" of type function is not documented

I have a very simple example (see snapshot). I defined a property
that "points" to a function.
In a Component.onCompleted handler, I can display the object type of that
property which identifies it as a "function".

But how do I execute (i.e. call/invoke) that function? I have looked everywhere
in the documentation. I have even tried methods documented in Javascript
documentation (neither of the two work in qml). The JS engine is suppose to
be EMCA compliant (i.e. the QML runtime implements the ECMAScript Language
Specification standard).

Is there a hidden way of doing this?