javascript判断触发事件event的鼠标按钮

<html>
<head>
<script type="text/javascript">
function whichButton(event)
{
var btnNum = event.button;
if (btnNum==2)
{
alert("您点击了鼠标左键！")
}
else if(btnNum==0)
{
alert("您点击了鼠标右键！")
}
else if(btnNum==1)
{
alert("您点击了鼠标中键！");
}
else
{
alert("您点击了" + btnNum+ "号键，我不能确定它的名称。");
}
}
</script>
</head>

<body onmousedown="whichButton(event)">
<p>请在文档中点击鼠标。一个消息框会提示出您点击了哪个鼠标按键。</p>
</body>

</html>