<a href="javascript:;" id="btn" title="回到頂部"></a>

#btn{position:fixed;display:none;}

window.onload = function(){
  var obtn = document.getElementById('btn');
  //獲取頁面可視區(qū)的高度
  var clientHeight = document.documentElement.clientHeight;
  var timer = null;
  var isTop = true;
  window.onscroll = function(){
    var osTop = document.documentElement.scrollTop || document.body.scrollTop;
    if (osTop >= clientHeight){
    //顯示按鈕
      obtn.style.display = 'block';
    }else {
    //隱藏按鈕
      obtn.style.display = 'none';
    }
    if (!isTop){
      clearInterval(timer);
    }
    isTop = false;
  };
  obtn.onclick = function(){    
    //設置定時器
    timer = setInterval(function(){
      //獲取滾動條距離頂部的高度
      var osTop = document.documentElement.scrollTop || document.body.scrollTop;
      var ispeed = Math.floor(-osTop / 6);
      document.documentElement.scrollTop = document.body.scrollTop = osTop +ispeed;
      
      isTop = true;
      if (osTop === 0){
        clearInterval(timer);
      }
    },30);
  };
};