JS加载解析Markdown文档过程详解

# Hello World! 
asdfa 
asd 
*斜体文本* 
**粗体文本** 
***斜粗体文本*** 
分隔线（如下） 
***
* * *
****
- - -
-----------
GOOGLE.COM 
~~删除线~~ 
<u>下划线</u>

<!doctype html>
<html>
<head>
 <meta charset="utf-8"/>
 <title>md文档内容转成html显示</title>
</head>
<body>
 <div id="content" style="width: 500px;height: 500px;overflow: auto"></div>
 <div style="margin-top: 30px">
   <form name="form" action="" method="post">
    <select name="q">
      <option value="md/gs.md">公式</option>
      <option value="md/test.md">第一个Markdown文档</option>
    </select>
    <input type="button" value="显示" onclick="showMarkdown()">
  </form>
 </div>
 <script type="text/javascript" src='https://cdn.jsdelivr.net/npm/marked/marked.min.js'></script>
 <script type="text/javascript">
   function showMarkdown() {
     var f = form;
     var xmlhttp;
     if (window.XMLHttpRequest) {
       xmlhttp = new XMLHttpRequest();
     } else {
       xmlhttp = new ActiveXObject('Microsoft.XMLHttp');
     }

     xmlhttp.onreadystatechange = function() {
       if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
         document.getElementById('content').innerHTML = marked(xmlhttp.responseText);
       }
     }

     // 向服务器发送请求
     xmlhttp.open('GET', f.q.value, true);
     xmlhttp.send();
   }
 </script>
</body>
</html>