怎么判断是否显示出返回顶点图标

xinsuiyidong888 2014-12-26 06:34:24

我想在页面中添加一个返回顶点如果这个页面完全显示出来就不出现返回顶点的图标
如果页面可以往下滑动就显示返回顶点的图标这个该怎么判断呢？
谢谢

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傲雪星枫 2014-12-27

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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>test</title>
<style>
body{margin:0; padding:0}
#to_top{width:30px; height:40px; padding:20px; font:14px/20px arial; text-align:center;  background:#06c; position:absolute; cursor:pointer; color:#fff}
</style>
<script>
window.onload = function(){
  var oTop = document.getElementById("to_top");
  var screenw = document.documentElement.clientWidth || document.body.clientWidth;
  var screenh = document.documentElement.clientHeight || document.body.clientHeight;
  oTop.style.left = screenw - oTop.offsetWidth +"px";
  oTop.style.top = screenh - oTop.offsetHeight + "px";
  oTop.onclick = function(){
    document.documentElement.scrollTop = document.body.scrollTop =0;
  }
} 

window.onscroll = function(){
  var oTop = document.getElementById("to_top");
  var screenw = document.documentElement.clientWidth || document.body.clientWidth;
  var screenh = document.documentElement.clientHeight || document.body.clientHeight;
  var scrolltop = document.documentElement.scrollTop || document.body.scrollTop;
  oTop.style.top = screenh - oTop.offsetHeight + scrolltop +"px";
  if(scrolltop>300){ // 滚动条超过某位置才显示返回顶部按钮
	oTop.style.visibility = 'visible';
  }else{
	oTop.style.visibility = 'hidden';
  }
}

window.onresize = function(){
  var oTop = document.getElementById("to_top");
  var screenw = document.documentElement.clientWidth || document.body.clientWidth;
  var screenh = document.documentElement.clientHeight || document.body.clientHeight;
  oTop.style.left = screenw - oTop.offsetWidth +"px";
  oTop.style.top = screenh - oTop.offsetHeight + "px";
}

</script>
</head>

<body style="height:2000px;">

<h1>返回顶部</h1>


<div id="to_top" style="visibility:hidden">返回顶部</div>
</body>
</html>