application development - QML: QChar Booleans are undefined

function myFunction(x)
{
    console.log(x.isLetter)
}

myFunction("s")

When I run this in a .qml file, it tells me that "isLetter" is undefined. Why?

http://qt-project.org/doc/qt-5/qchar.html#isLetter

You can't directly use Qt methods on JavaScript var, to do what you want prefer a pure js implementation:

x.match(/[a-z]/i);