var start = document.getElementsByTagName('div')[0];
var stop = document.getElementsByTagName('div')[1];
var speed = document.getElementsByTagName('div')[2];
var timer = 0;
var num = 10;
//多次点击start按钮会产生多个定时器（加锁——操作系统学过）
var key = true;
start.onclick = function(){if(key){//当key为true时才产生定时器timer =setInterval(function(){window.scrollBy(0,num);},10)key = false;//开定时器后要把锁锁上}
}
stop.onclick = function(){clearInterval(timer);key = true;//结束定时器就要把开锁
}
speed.onclick = function(){num += 5;
}