求大神帮忙 动态设置背景图片问题

Question

我想让页面背景每个一秒就换一张图片而且不间断，以下是我写的 不知循环哪里错了 求指导
<script type="text/javascript">
        var a=0;
        var url = new Array(3);
        url[0] = "url(images/show_fengjingta_236621_4.jpg)";
        url[1] = "url(images/shulinfengjingzhuomianbizhixiazai_361581_4.jpg)";
        url[2] = "url(images/shulinfengjingzhuomianbizhixiazai_399624_4.jpg)";
    function change() {
        //        document.getElementById("bg").style.backgroundImage = "url(images/show_fengjingta_236621_4.jpg)";
        
        if (a < 3) {
            document.getElementById("bg").style.backgroundImage = url[a];
            a++;
        }
        else {
            a = 0;
        }              
        }
    
    setTimeout(change, 1000)
    </script>

Answer 1

setTimeout(change, 1000)只会执行一次改成setInterval(change, 1000)

Answer 2

<script type="text/javascript">
    var a=0;
    var url = new Array(3);
    url[0] = "url(images/show_fengjingta_236621_4.jpg)";
    url[1] = "url(images/shulinfengjingzhuomianbizhixiazai_361581_4.jpg)";
    url[2] = "url(images/shulinfengjingzhuomianbizhixiazai_399624_4.jpg)";
    var id=document.getElementById("bg");
    setInterval(function(){
        id.style.backgroundImage = url[a%3];
        a++;
    },1000)
</script>