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clientHeight){ obtn.style.display="block"; }else{ obtn.style.display="none"; } } obtn.onclick=function(){ timer=setInterval(function(){ var topH=document.documentElement.scrollTop||document.body.scrollTop; var stepLength=Math.ceil(topH/5); document.documentElement.scrollTop=document.body.scrollTop=topH-stepLength; if(topH==0){ clearInterval(timer); } },30); } }