<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
<title>Javascript 返回顶部</title>
<style type="text/css">
#btn {width:40px; height:40px; position:fixed; display:none;right:65px; bottom:10px; background:url(images/top_bg.png) no-repeat left top;}
#btn:hover {background:url(images/top_bg.png) no-repeat 0 -39px;}
.bg {width:1190px; margin:0 auto;}
</style>
</head>
<body>
<a href="javascript:;" id="btn" title="回到顶部"></a>
<div>
<img src="images/tb_bg.jpg" alt="" />
</div>
<script>
window.onload=function(){
var time=null;
var btn=document.getElementById("btn")
btn.onclick=function(){
time=setTimeout(app,300)
function app(){
var scroll=document.body.scrollTop||document.documentElement.scrollTop;
document.body.scrollTop=0;
}
}
var clientHeight=document.documentElement.clientHeight;
var scroll=document.documentElement.scrollTop||document.body.scrollTop;
window.onscroll=function(){
if(scroll>=clientHeight/2){
btn.style.display="blcok";
}
else{btn.style.display="none"}
}
}
</script>
</body>
</html>滚动没有触发事件，btn没有出现，不知道哪里错了
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