有關oBox[i].timer=null;的 問題？

<script>
?? ?window.onload=function(){
?? ?<!--?? ?var timer;-->
?? ??? ?var oBox=document.getElementsByTagName("div");
?? ??? ?for(var i=0;i<=oBox.length;i++){
?? ??? ?oBox[i].timer=null;
?? ??? ?oBox[i].onmouseover=function(){startMove(this,300);};
?? ??? ?oBox[i].onmouseout=function(){startMove(this,0);};
?? ??? ?}
?? ??? ??? ?function startMove(obj,target){
?? ??? ??? ??? ?clearInterval(obj.timer);?? ?
?? ??? ??? ??? ?obj.timer=setInterval(function(){
?? ??? ??? ??? ??? ?var speed=(target-obj.offsetLeft)/10;
?? ??? ??? ??? ??? ?speed=speed>0 ? Math.ceil(speed):Math.floor(speed);?? ?
?? ??? ??? ??? ??? ?if (obj.offsetLeft==target){
?? ??? ??? ??? ??? ??? ?clearInterval(obj.timer);}
?? ??? ??? ??? ??? ?else{
?? ??? ??? ??? ??? ??? ?obj.style.left=obj.offsetLeft+speed+"px";}
?? ??? ??? ??? ??? ?},30);
?? ??? ??? ??? ?};
?? ??? ?};
?? ??? ?
</script>