title: What is the !! (not not) operator in JavaScript?
body: <p>I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: <code>!!</code>. Can someone please tell me what this operator does?</p>
  
  <p>The context in which I saw this was,</p>
  
  <pre><code>this.vertical = vertical !== undefined ? !!vertical : this.vertical;
  </code></pre>