In ECMAScript, default function arguments are placed in their own environment record, which then acts as the parent scope for the function body. That means that

console.log((function() {
        let a = 1;
        function f(fn = function(){ return a; }){ const a = 2; return fn() }
        return f();
    })())

should print 1. However, in QML, it will print 2 because fn ends up in the scope of f's body, and thus uses the a from there instead of from the outer scope.