title: How to understand "return obj === void 0" in the source of underscore?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/1291942/what-does-javascriptvoid0-mean">What does &ldquo;javascript:void(0)&rdquo; mean?</a><br>
    <a href="https://stackoverflow.com/questions/7452341/what-does-void-0-mean">What does <code>void 0</code> mean?</a>  </p>
  </blockquote>
  
  
  
  <p>In the file <a href="http://underscorejs.org/underscore.js" rel="nofollow noreferrer">http://underscorejs.org/underscore.js</a>, you can see such a code block:</p>
  
  <pre><code>// Is a given variable undefined?
  _.isUndefined = function(obj) {
    return obj === void 0;
  };
  </code></pre>
  
  <p>I don't understand the <code>void 0</code> part, it's very strange. How to understand it?</p>