JavaScript实现灯泡开关点击切换特效

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <title></title>
  <link rel="stylesheet" href="">
  <script>
    function change(){
      var img = document.getElementById('img');
      var src = img.src;
      if(src.indexOf('on') >=0 ){
        img.src = 'off.jpg';
      }else{
        img.src = 'on.jpg';
      }
    }
  </script>
</head>
<body>
  <h1>开关灯泡</h1>
  <img id='img' src="on.jpg" onclick="change();" width='200' height="200">
</body>
</html>