为什么我这个不倒计时也不跳转呢
<body>
<div>
<h3>操作成功</h3>
<p><span id="s">5</span>秒后返回到主页<a href="" onclick="go_back()">返回</a></p>
</div>
<script type="text/javascript">
var a=5;
function countdown()
{
a=a-1;
document.getElementById("s").innerHTML=a;
if(a==0)
window.location.href="http://www.imooc.com/";
}
var i=setInterval=("countdown()",1000);
function go_back()
{
window.history.back();
}
</script>
</body>
