JavaScript实现网页背景图片自动轮播切换

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>背景切换</title>
</head>
<body>
<img src="img/1.jpg" width="500" id="imgId"/>
</body>
</html>
<script type="text/javascript">
 window.onload = function () {

 setInterval(step, 1000);
 }
 var num = 1;
 function step() {
 if (num < 5) {
  num++;
 } else {
  num = 1;
 }
 var dom = document.getElementById("imgId");
 //更改它images的src属性
 dom.src = 'img/' + num + '.jpg';
 }

</script>