Switch/case taking return value of function directly randomly fails

import QtQuick 2.0

Rectangle {
    width: 180
    height: 100

    Timer {
        running: true
        repeat: true
        interval: 100
        onTriggered: {
            r();
        }
    }

    function f() {
        return Math.random() >= 0.5 ? 1 : 0;
    }
    function r() {
        switch (f()) {
        case 0:
        case 1:
            print("0 or 1")
            return;
        default:
            console.log("fail")
        }
    }
}