title: !! c operator, is a two NOT?
body: <p>I reading this <a href="https://gist.github.com/909424#L115" rel="noreferrer">code</a>, and have this line</p>
  
  <pre><code> switch (!!up + !!left) {
  </code></pre>
  
  <p>what is <code>!!</code> operator ? two logical NOT ?  </p>