How to know if Firebase Auth is currently trying to retrieve user? : ##Background
I am using `GoogleAuthProvider`, with the default `LOCAL` persistence.

When I navigate to the page, I do:
```js
firebase.initializeApp(firebaseConfig)
firebase.auth().currentUser // this is always null
firebase.auth().onAuthStateChanged(user => {
  console.log("authStateChanged", user)
})
```
If the user is logged in, the callback is called once, with the user.

If the user is *not* logged in, the callback is also called once, *with* the user.

This suggests I *could* wait until the first callback after navigating to the page to get the real login state before deciding what view to display, for instance. (I originally thought that it would not get called with `null`, and so I could wait indefinitely)

## Question
Is this idiomatic usage? Does it seem like it will be robust against updates to firebase? Where can I find this discussed in the official documentation?