js图片随机切换

使用js做到随机切换图片
<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title></title>
</head>
<body>
<img src="1.jpg" width="280px" height="280px" id="tpid" style="display: block; margin: auto;"><br><br>
<button onclick="aa()"style="display: block; margin: auto;">开始</button>
<button onclick="bb()"style="display: block; margin:20px auto;">四秒后停止</button>
 <script>
 var sj;
 var tpm;
 var intervalId;
 function bb(){setTimeout(function() {clearInterval(intervalId);},4000)}
 function aa(){intervalId=setInterval(function() {sj=parseInt(Math.random()*3+1);
  tpm=sj+".jpg";
  document.getElementById("tpid").src=tpm;},50)}
 </script>
</body>
</html>
效果如下：