title: what is returned on firebase.auth().onAuthStateChanged?
body: <p>When I hover over <code>onAuthStateChanged</code> it shows this:</p>
  
  <pre><code>(method) firebase.auth.Auth.onAuthStateChanged(nextOrObserver: firebase.Observer&lt;any, any&gt; | ((a: firebase.User) =&gt; any), error?: (a: firebase.auth.Error) =&gt; any, completed?: firebase.Unsubscribe): firebase.Unsubscribe
  </code></pre>
  
  <p>It also seems to work if I do <code>user: firebase.User</code></p>
  
  <pre><code>  firebase.auth().onAuthStateChanged((user) =&gt; {
      console.log("data from stateChanged: ", user);
    })
  </code></pre>
  
  <p>But when I try to access the properties of user, i.e. user.uid then I get error basically saying inaccessible. So what exactly is the return type here and how do I access that?</p>