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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)" class="back_top"></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.xif (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 browsersdocument.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> JavaScript技术：返回顶部按钮响应滚动且动态显示与隐藏，转载需保留来源！
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