Can't push() to QML property Array

I would expect this to work:

    property var myArray: ["a", "b", "c"]
     property var myPoint: { x: 100, y: 200 };



function addThreeToArray() {
    for(var i = 0; i < 3; i++) {
        myArray.push("X");
        print("Array length: " + myArray.length + "; items: " + myArray.toString());
    }
    ... similar for myPoint ...
}

and print
Array length: 4; items: a,b,c,x
Array length: 5; items: a,b,c,x,x
Array length: 6; items: a,b,c,x,x,x

but it doesn't, it prints
Array length: 3; items: a,b,c
Array length: 3; items: a,b,c
Array length: 3; items: a,b,c

To get what I want, I have to move the code to a separate .js file (changing the
property definition to: var myArray = ["a", "b", "c"]; ) and then import it into my QML file.
Then it works as expected. Don't think I should have to.