为什么我的timer 是undefined呢？

<script type="text/javascript">

window.onload=function(){

var timer=null;

var list=document.getElementById('notice-tit').getElementsByTagName('li');

var divs=document.getElementById('notice-con').getElementsByTagName('div');

for(var i=0;i<list.length;i++){

list[i].id=i;

list[i].onmouseover=function(){

var that=this;

if(timer){

clearTimeout(timer);

timer=null;

}

alert(timer);

var timer=window.setTimeout(function(){

for(var j=0;j<list.length;j++){

list[j].className='';

divs[j].className='hide';

}

   list[that.id].className='select';

   divs[that.id].className='';},1000);

}

}

}

</script>