title: How to get video src attribute
body: <p>I need to get the src attribute of a video tag, but something is wrong:</p>
  
  <pre><code>&lt;video id="vdisk" controls poster="placeh/02.jpg"&gt;
      &lt;source src="placeh/01.mp4" type="video/mp4"&gt;
  &lt;/video&gt;
  </code></pre>
  
  <h3>JavaScript</h3>
  
  <pre><code>var a = $("#vdisk").attr("src");
  console.log(a); // undefined
  </code></pre>