Can no longer assign properties to rect in QML

import QtQuick 2.3

Rectangle {
    id: root
    width: 100
    height: 40

    Component.onCompleted: {
        var result = root.mapToItem(null, 2, 2, 4, 4);
        result.left = result.x
        result.right = result.x + result.width
        console.log(result, JSON.stringify(result));
        console.log(result.left, result.right)
    }
}

qml: [object Object] {"x":2,"y":2,"width":4,"height":4,"left":2,"right":6}
qml: 2 6

qml: QRectF(2, 2, 4, 4) {}
qml: undefined undefined