title: Getting the request origin in Express
body: <p>How do I actually get the origin? When I use the following under <code>router.get('/', ...)</code>, it just returns undefined.</p>
  
  <pre><code>var origin = req.get('origin');
  </code></pre>
  
  <p>(I am trying to use the origin domain as a lookup key, to then construct the right URL parameters to post to an API)</p>