title: using process.env in TypeScript
body: <p>How do I read node environment variables in TypeScript? </p>
  
  <p>If i use <code>process.env.NODE_ENV</code> I have this error :</p>
  
  <pre><code>Property 'NODE_ENV' does not exist on type 'ProcessEnv'
  </code></pre>
  
  <p>I have installed <code>@types/node</code> but it didn't help.</p>