title: does process.env.NODE_ENV sets to 'production' automatically?
body: <p>I was reading someone's code and came across this line.</p>
  
  <pre><code>if (process.env.NODE_ENV === 'production') { ...
  </code></pre>
  
  <p>This is a node.js application with express server and reactjs front end. So lets say we deploy it to heroku will it get this NODE_ENV variable automatically(or set by heroku itself) or do I have to set it manually as we set other env variables in development server?  </p>