title: What is process.env.PORT in Node.js?
body: <p>what is <code>process.env.PORT || 3000</code> used for in Node.js? I saw this somewhere:</p>
  
  <pre><code> app.set('port', process.env.PORT || 3000);
  </code></pre>
  
  <p>If it is used to set <code>3000</code> as the listening port, can I use this instead?</p>
  
  <pre><code>app.listen(3000);
  </code></pre>
  
  <p>If not why?</p>