title: what does this "a: ?number = null;" mean?
body: <p>In Facebook's <a href="https://facebook.github.io/react-native/docs/geolocation.html" rel="nofollow noreferrer">React Native's Geolocation</a> documentation I see this line of code:</p>
  
  <pre><code> watchID: ?number = null;
  </code></pre>
  
  <p>I am clueless about it's nature. Is this even javascript?</p>