div可随意拖动

  function scrollToTop() {
    const scrollTopTimer = setInterval(function() {
      const top = document.body.scrollTop || document.documentElement.scrollTop;
      const speed = top / 30;
      document.documentElement.scrollTop -= speed;
      if (top == 0) {
        clearInterval(scrollTopTimer);
      }
    }, 5);
  }