怎样获取一个标记与屏幕左边的距离啊？

Question

<div style="height:300px;">
</div>
fdsafdsa<span style='background-color:#ff0000;' onclick="alert(getLeft(this))">
adfasdfasdf
</span>
<script>
function getLeft(obj)
{
    var left = obj.offsetLeft;
    var top  = obj.offsetTop;
    obj = obj.offsetParent;
    while(obj.tagName != "BODY")
    {
        left += obj.offsetLeft;
        top  += obj.offsetTop;
        obj   = obj.offsetParent;
    }
    return left;
}
</script>