不是很懂这段代码。。求解释

document.onkeyup=function(event){
        event=event||window.event;
        if(event.keyCode==40){    
            index++;
            if(index>=as.length){
                index=0;
            }
             for(var i=0;i<as.length;i++){
              as[i].style.background='#fff';//为什么这里就可以用sa[i].style.background了呢？
             }
            as[index].style.background='gray';
        }
        if(event.keyCode==38){    
            index--;
            if(index<0){
                index=as.length-1;
            }
             for(var i=0;i<as.length;i++){
              as[i].style.background='#fff';
             }
             as[index].style.background='gray';
        }
        if(event.keyCode==13){
            for(var i=0;i<as.length;i++){
              as[i].style.background='#fff';
             }
            title.innerHTML=as[index].innerHTML;
            index=-1;
            menu.style.display='none';
        }
   }