很多的网站上都有返回顶部功能，判断滚动参数动态显示与隐藏，比较适合初学者
<!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"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>实现动态的显示回到顶部与否</title> <style> /*返回顶部*/ .back_top {width:45px;height:45px;background:url(back_top_new.png) 0 0;position:fixed;left:50%;margin-left:500px;bottom:105px;display:none;} .back_top:hover{background:url(images/back_top_new.png) 0 -46px;} </style> </head> <body> <a title="返回顶部" href="javascript:void(0)"></a> <script src="js/jquery-1.11.0.min.js"></script> <script> var mousewheelevt=(/Firefox/i.test(navigator.userAgent))"DOMMouseScroll" : "mousewheel"; //FF doesn't recognize mousewheel as of FF3.x if (document.attachEvent) //if IE (and Opera depending on user setting) document.attachEvent("on"+mousewheelevt, function(e){ var t=document.body.scrollTop+document.documentElement.scrollTop; if(t==0){ $(".back_top").css("display","none"); } else{ $(".back_top").css("display","block"); } }); else if (document.addEventListener) //WC3 browsers document.addEventListener(mousewheelevt, function(e){ var t=document.body.scrollTop+document.documentElement.scrollTop; if(t==0){ $(".back_top").css("display","none"); } else{ $(".back_top").css("display","block"); } }, false); </script> </body> </html>
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