JavaScript延迟跳转上一页代码

<script language=javascript>
var secs=3;//3秒
for(i=1;i<=secs;i++) 
{ window.setTimeout("update(" + i + ")", i * 1000);} 
function update(num) 
{ 
if(num == secs) 
{ history.go(-1); } 
else 
{ } 
}
</script>