timer前面加上了var，取消定时器就失效了，这是为什么？ 求大神解答，
<script type="text/javascript">
window.onload=function(){
var box1= document.getElementById("box1");
box1.onmouseover=function(){
mvfn1();
}
};
var timer = null ;
function mvfn1(){
clearInterval(timer);
var box1= document.getElementById("box1");
var timer=setInterval(function(){
if (box1.offsetLeft>=0) {
clearInterval(timer);
} else{
box1.style.left=box1.offsetLeft+1+"px";
}
},30);
};
</script>
