js回到顶部

<body style="height: 3000px;">
    <li id="btn-top">回到顶部</li>
    <style>
        #btn-top{
            position: fixed;
            bottom: 0px;
            right: 0px;
            display: none;
        }
    </style>
    <script>
          window.onscroll=function(){
              var top=document.getElementById("btn-top");
               var scrollTop=document.documentElement.scrollTop||document.body.scrollTop;//获取滚动条高度
               //console.log(scrollTop);
              if(scrollTop>1000){
                  top.style.display='block'
                
            }else{
                top.style.display='none'
            }
             
          }
          var top=document.getElementById("btn-top");
          top.onclick=function(){
             
               timer=setInterval(function(){
                   var scrollTop=document.documentElement.scrollTop||document.body.scrollTop;//获取滚动条高度
                  var ispeed=Math.floor(-scrollTop/6);//速度
            
                   if(scrollTop==0){
                       clearInterval(timer);
                   }
                   document.documentElement.scrollTop=document.body.scrollTop=scrollTop+ispeed;
               },30)
               
           };  
    </script>