@J-Hilk
I managed the required behavior, somehow.

By blanking the key value:

stg.setValue("step_1", "")

then scanning the keys, which have non-empty value, and skipping the empty ones.

I'll file a bug, cause I think it's a needed method!
By the way, thank you all

@ars1614 Yes they are converted automatically.
http://doc.qt.io/qt-5/qtqml-cppintegration-data.html#qvariantlist-and-qvariantmap-to-javascript-array-and-object

Check the links I provided - there are some examples. Maybe Qt examples provide more code.