QtCore

I guess it isn't available in QML

The Musescore main app is written in C++ and Qt framework, which supports QtCore module. To edit its code visit https://github.com/musescore/MuseScore/

It provides plugin scripting functionality to end users. Plugins use QML, it is not the C++Qt, see https://doc.qt.io/archives/qt-5.9/qmlapplications.html. QML has a "QtQML" module see https://doc.qt.io/archives/qt-5.9/qtqml-index.html . The List ListModel ListElement type may be useful, see https://doc.qt.io/archives/qt-5.9/reference-overview.html "All QML Basic Types", "All QML Modules", and "All QML Types" . See also snippet https://musescore.org/en/node/320673#s22 .

QML has JavaScript Host Environment https://doc.qt.io/archives/qt-5.9/qtqml-javascript-hostenvironment.html , ms3's support is ECMA-262 5th edition standard, the new constructors won't work:

var x=new Set()
var y=new Map()

but this works:

var x={a:123,b:456}
console.log(x.a) // 123
var thisstring='a'
console.log(x[thisstring]) // 123

edit: correct QML has "QtQML" module ; edit2 fix js missing quotation marks 'a'