怎么清除定时器，大神们求救！

oTop.onclick = function(){
var a=setInterval(function (){
var top = document.documentElement.scrollTop || document.body.scrollTop;
var iSpeed = (top - 0) * 0.9;
document.documentElement.scrollTop = document.body.scrollTop = iSpeed;
}, 30);
}
好急，求救！！！

声明一个全部变量的a，然后在定义一个function clear(){ clearInterval(a); } 找一个按钮执行这个clear()，就可以清除定时器

 oTop.onclick = function(){
var a=setInterval(function (){
var top = document.documentElement.scrollTop || document.body.scrollTop;
if(top<=0){
   clearInterval(a);
}else{
    var iSpeed = (top - 0) * 0.9;
    document.documentElement.scrollTop = document.body.scrollTop = iSpeed;
}
}, 30);
}

           document.documentElement.scrollTop = document.body.scrollTop = iSpeed;
           if (iSpeed <= 0) clearInterval(a);