title: JSON undefined value type
body: <p>I came across this JSON code. I noticed that it makes use of <code>undefined</code> value. Where can I find more information about this value type?</p>
  
  <pre><code>  tracks:[
       (         {
          codec:"h264",
          language:undefined,
          id:1,
          bitrate:785236,
          content:"video"
       }         ),
       (         {
          codec:"aac",
          language:undefined,
          id:2,
          bitrate:75969,
          content:"audio"
       }         )
    ],
  </code></pre>