Since MousePosition[] returns $Failed in WolframCloud I was wondering if I could use CloudSystem`JavaScriptExecute to emulate MousePosition[].

But how does JavaScriptExecute actually work?

Suppose I want to first add a javascript function, like

function mouse_position()
{
    var e = window.event;

    var posX = e.clientX;
    var posY = e.clientY;

    document.Form1.posx.value = posX;
    document.Form1.posy.value = posY;

    var t = setTimeout("mouse_position()",100);

}

and then execute it. How to do it? Or did I overlook any documentation of JavaScriptExecute?