求助高手一个简单的问题

没见有问题啊，代码如下，请向右拖动scroll<html>
<body>
<p>dfdddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddsaf</p>
</body></html>
<script  language="javascript">
function  a()
{
   alert(document.body.scrollLeft);
}
window.onscroll=a;
</script>