//页面加载完毕触发

window.onload = function(){

var time = null;

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

obtn.onclick = function(){

//设置定时器

time = setInterval(function(){

//获取滚动条与顶部的距离

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

document.documentElement.scrollTop = document.body.scrollTop -=200;

},30);

}

}