title: How to get the element inside an iframe element in Javascript?
body: <p>I have a element as below inside the document, I could get the iframe element by <code>document.getElementById('iframe_id')</code>, but how to get the element inside this iframe? I tried <code>iframeElement.contentWindow</code>, and the returned <code>DOMWindow</code> has no properties. Also tried <code>iframeElement.docuemnt</code> and <code>iframeElement.contentDocument</code>, both of them are undefined.  How can I get it?  I am using the latest Chrome in my experiment.</p>
  
  <p>Here is the iframe element</p>
  
  <pre><code>&lt;iframe id='iframe_id'&gt;
  &lt;html&gt;
  &lt;body&gt;many content here&lt;/body&gt;
  &lt;/html&gt;
  &lt;/iframe&gt;
  </code></pre>