// 页面加载完后触发
  window.onload = function(){
    var gotop = document.getElementById('gotop');
    var clientHeight = document.documentElement.clientHeight;
    var timer = null;
    var isTop = true;

    // 滚动条滚动时触发
    window.onscroll = function(){
      var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //前者兼容ie，后者兼容chrome
      if (scrollTop >= clientHeight) {
        gotop.className = 'gotop show';
      }else{
        gotop.className = 'gotop';
      }
      if (!isTop) {
        clearInterval(timer);
      }
      isTop = false;
    }

    gotop.onclick = function(e){
      e = event || window.event;
      if (e.preventDefault) {
        e.preventDefault();
      }else{
        e.returnValue = false;
      }

      timer = setInterval(function(){
        var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //前者兼容ie，后者兼容chrome
        var ispeed = Math.floor(-scrollTop / 5);
        document.documentElement.scrollTop = document.body.scrollTop = scrollTop + ispeed;
        console.log(scrollTop)

        isTop = true;

        if (scrollTop == 0) {
          clearInterval(timer);
        };
      }, 30);
    }
  }