var clock=document.getElementById("txt")和clock()之间有一定联系吗，当i=clock.innerHTML时，时间没动，i=clock.innerHTML和clock.innerHTML=i有什么区别？
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>制作一个跳转提示页面</title>
</head>
<body>
<p>操作成功</p>
<p><span id="txt">5</span>秒后回到主页 <a href="javascript:GoBack();">返回</a></p>
<script type="text/javascript">
var i=5;
function clock(){
var clock=document.getElementById("txt");
i=i-1;
clock.innerHTML=i;
if(i==1){
location.href="http://www.imooc.com";
}
setTimeout("clock()",1000);
}
setTimeout("clock()",1000);
function GoBack(){
window.history.back();
}
</script>
</body>
</html>
