//頁面加載后觸發window.onload = function(){  var btn = document.getElementById('btn');  var timer = null;  var isTop = true;  //獲取頁面可視區高度  var clientHeight = document.documentElement.clientHeight;    //滾動條滾動時觸發  window.onscroll = function() {  //顯示回到頂部按鈕    var osTop = document.documentElement.scrollTop || document.body.scrollTop;    if (osTop >= clientHeight) {      btn.style.display = "block";    } else {      btn.style.display = "none";    };  //回到頂部過程中用戶滾動滾動條，停止定時器    if (!isTop) {      clearInterval(timer);    };    isTop = false;  };  btn.onclick = function() {    //設置定時器    timer = setInterval(function(){      //獲取滾動條距離頂部高度      var osTop = document.documentElement.scrollTop || document.body.scrollTop;      var ispeed = Math.floor(-osTop / 7);            document.documentElement.scrollTop = document.body.scrollTop = osTop+ispeed;      //到達頂部，清除定時器      if (osTop == 0) {        clearInterval(timer);      };      isTop = true;          },30);  };};