AFAIK Qt also has support for writing apps in JS (isn't that what Qml is?).
Yes and no. You need a C++ skeleton to integrate the Qml files. Also Qml/JS is slow and doesn't support all the features of Qt/C++. Practically you use Qml for the GUI, but C++ for the logic.
Or you can create components in C++ callable from Qml, but this requires some extra code ( and work ).