window.onload = function(){

var obtn = document.getElementById("btn");

var clientHeight = document.documentElement.clientHeight || document.body.clientHeight; ?//獲取屏幕高度

var timer = null;

var isTop = true;

//滾動條滾動時觸發

window.onscroll = function(){

var osTop = document.documentElement.scrollTop || document.body.scrollTop;

if(osTop >= clientHeight){

obtn.style.display = 'block';

}else{

obtn.style.display = 'none';

}

if(!isTop){

clearInterval(timer);

}

isTop = false;

}

obtn.onclick = function(){

//設置定時器

clearInterval(timer);

timer = setInterval(function(){

//滾動條距離頂部的距離

var osTop = document.documentElement.scrollTop || document.body.scrollTop;

var ispeed = Math.floor(-osTop / 30);

document.documentElement.scrollTop = document.body.scrollTop = osTop + ispeed;

isTop = true;

if(osTop == 0){

clearInterval(timer);

}

},30);

}

}