关于JS展开与收缩课程最后编程题的更改

<script type="text/javascript">
function getDOM(id) {
return document.getElementById(id);
}
var timer=null;
newTimer=null,
pn=getDOM("pn");
window.onload=function () {
startMove(600);
newTimer=setTimeout(function () {
startMove(0);
},5000)
}

    //个人方法--------------
    function startMove(iTarget) {
        if (timer){
            clearInterval(timer);
        }
        timer=setInterval(function () {
            var speed=0;
            pn.offsetWidth<iTarget?speed=10:speed=-10;
            if (pn.offsetWidth==iTarget){
                clearInterval(timer);
                if (pn.style.width==0+'px'){
                    pn.style.display='none';
                }
            }else {
                pn.style.display='block';
                pn.style.width=pn.offsetWidth+speed+'px';
            }
        },30)
    }