js区分鼠标单击和双击事件

<input type="button" onclick="aa()" ondblclick="bb()" value="点我">
<script language="javascript">
var isdb;
function aa(){
isdb=false;
window.setTimeout(cc, 500)
function cc(){
if(isdb!=false)return;
alert("单击")
}
}
function bb(){
isdb=true;
alert("双击")
}
</script>