启用setInterval()定时器更换背景

html代码
<input id="btn1" type="button" value="开启换背景"/>
<input id="btn2" type="button" value="停止换背景"/>
<div class="box"></div>

.box{width:500px;height:500px;border:1px solid #f5f5f5}

<script>
    window.onload=function(){
        var aBtn1=document.getElementById("btn1");
        var aBtn2=document.getElementById("btn2");
        var oBox=document.getElementsByClassName("box")[0];
        var arrUrl=["img/pic1.jpg","img/pic2.jpg","img/pic3.jpg"];
        var num=0;
        var timer=null;
        function fn(){
            oBox.style.background=‘url("‘+arrUrl[num]+‘")‘;
            num++;
            num%=arrUrl.length;
        }
        aBtn1.onclick=function(){
            clearInterval(timer);//快速点击按钮时，先清除定时器，然后再执行就不会使定时器崩溃
            timer =  setInterval(fn,1000);
        };
        aBtn2.onclick=function(){
            clearInterval(timer);
        }
    };
</script>