//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);};};