title: accessing heroku config var in my React app
body: <p>I have defined a config var "BASE_URL" in my Heroku app and I'm trying to access it in my React app as </p>
  
  <pre><code>process.env.BASE_URL
  </code></pre>
  
  <p>but it gives me undefined when I console.log it as it seems to be not existing.
  How can I access Heroku config vars in my React app?</p>