JavaScript !! operators
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/784929">What is the !! (not not) operator in JavaScript?</a>  </p>
</blockquote>

<p>I've seen operator <code>!</code> used like <code>!!</code>. For example</p>

<pre>
<code>
var filter = !!(document.body.filters);
</code>
</pre>

<p>If I'm not wrong it's equivalent <code>var filters = typeof document.body.filters != 'undefined'</code>?</p>

<p>Is it a good practice to use <code>!!</code>? </p>
