如何获取DOM中当前获取焦点的元素

<script type="text/javascript">
function msg(e) // e = event
{
var target; //initialisation
if ("activeElement" in document)
{
target = document.activeElement; //Si IE
}
else {
target = e ? e.explicitOriginalTarget : null; // Si Firefox
}
alert(target.id);
}
</script>