Javascript滑鼠座標

<script type="text/javascript">
function mousePosition(ev){
        if(ev.pageX || ev.pageY){
                return {x:ev.pageX, y:ev.pageY};
        }
        return {
                x:ev.clientX + document.body.scrollLeft - document.body.clientLeft,
                y:ev.clientY + document.body.scrollTop - document.body.clientTop
        };
}
document.onmousemove = mouseMove;
function mouseMove(ev){
    ev = ev || window.event;
    var mousePos = mousePosition(ev);

        document.getElementById('mouseXPosition').value = mousePos.x;
        document.getElementById('mouseYPosition').value = mousePos.y;
}
</script>
<fieldset>
        <input id=mouseXPosition>
        <input id=mouseYPosition>
</fieldset>