원생 js 스크롤 막대를 천천히 위로 되돌리는 효과 실현

 timer=setInterval(function(){
           var scrollTop=document.documentElement.scrollTop||document.body.scrollTop;
           var ispeed=Math.floor(-scrollTop/6);
           console.log(ispeed)
           if(scrollTop==0){
               clearInterval(timer);
           }
           document.documentElement.scrollTop=document.body.scrollTop=scrollTop+ispeed;
       },30)

     var speed = 200; // 
      $('body,html').animate({ scrollTop: 0 }, speed);
      return false;