title: Environment variable is not assigning with Typescript
body: <p>My code is: </p>
  
  <pre><code>const port: Number = process.env.PORT || 3000;
  
  [ts]
  Type 'string | 3000' is not assignable to type 'Number'.
    Type 'string' is not assignable to type 'Number'.
  </code></pre>
  
  <p>I tried</p>
  
  <pre><code>const port: Number = parseInt(process.env.PORT, 10) || 3000;
  </code></pre>
  
  <p>But it gives me another error:</p>
  
  <pre><code>Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
    Type 'undefined' is not assignable to type 'string'.
  (property) NodeJS.Process.env: NodeJS.ProcessEnv
  </code></pre>
  
  <p>What am I doing wrong?</p>