What does '!!' do in Javascript?
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/784929/what-is-the-operator-in-javascript">What is the !! operator in JavaScript?</a>  </p>
</blockquote>



<p>Seen in jQuery's <a href="https://github.com/jquery/jquery/blob/master/src/ajax.js" rel="nofollow" title="ajax.js">ajax.js</a> file (look near the bottom).</p>

<pre><code>jQuery.support.ajax = !!jQuery.ajaxSettings.xhr();
</code></pre>

<p>What does <code>!!</code> do? Isn't a double negative useless?</p>
