jQuery实现平滑动画回到顶部

<div id="btn3" class="bt">回到顶部</div>

    //总距离，总时间，过渡时间
  $('#btn3').click(function () {
    var $page = $('html,body')
    var distance = $('html').scrollTop() + $('body').scrollTop()
    var totaltime = 500
    var time = 50
    var itemdistance = distance / (totaltime / time)

    var intervalId = setInterval(function () {
      distance -= itemdistance
      if (distance<=0){
        distance=0
        clearInterval(intervalId)
      }
      $page.scrollTop(distance)
    },time)
  })