Q:

What does QByteArray get converted to in QML?

If I have a
signal:
    void foo(QByteArray data);

And in QML I handle it:
 onFoo: {
     console.log(data.toString());
 }

What is the type of data in QML? What methods does it have? It doesn't seem to be a javascript string - it doesn't even have a .length, and no .charCodeAt(). But it also doesn't seem to be a QByteArray - no .at(). data[0] is undefined! It does have .toString().
How do I access its contents? E.g. if it is a four-byte uint32_t, how do I get that number into a javascript variable?

A:

As stated in here :

The QML engine provides automatic type conversion between QByteArray
  values and JavaScript ArrayBuffer objects.

This feature is available since Qt 5.8.

A:

According to http://doc.qt.io/qt-5/qtqml-cppintegration-data.html and the current source http://code.woboq.org/qt5/qtdeclarative/src/qml/compiler/qqmltypecompiler.cpp.html#567, there's no conversion between QByteArray towards QML. You'll probably get an opaque, unaccesible Object in JS. 
EDIT: The above was true up to Qt 5.8. 5.8 added a conversion between QByteArray and JS Array, see the other answer.
Needless to say: at the time of this writing, the only version you want to be using is >= 5.12.

