title: How to set up environment variables in react native
body: <p>I'm making a react native app that makes a request to my server hosted on heroku.</p>
  <p>Should I be hiding the URL of my server and if so how can I add an environment variable to a react native project?</p>
  <p>I have made a <code>.env</code> file and then have done this:</p>
  <pre><code>console.log(process.env.URL)
  </code></pre>
  <p>Which is returning undefined - I am also using expo if that makes a difference.</p>