Firebase stop listening onAuthStateChanged

As of version ^3.0.0, I'm having a difficult time removing the auth state change listener.

To start the listener per the documentation:

firebase.auth().onAuthStateChanged(function (user) {
    // handle it
});

However, I cannot find anywhere in the documentation that refers to a remove auth state change listener. There is peculiar function on the Firebase.Auth class called removeAuthTokenListener. Unfortunately it's not documented (firebase docs reference).

Via your browser's web console.

var auth = firebase.auth();
auth.removeAuthTokenListener;

prints a function definition that takes one parameter. I tried to do the following:

this.authListener = firebase.auth().onAuthStateChanged(function (user) {...});
firebase.auth().removeAuthTokenListener(this.authListener);

but that didn't do anything.

According to the documentation, the onAuthStateChanged() function returns

> The unsubscribe function for the observer.

So you can just:

var unsubscribe = firebase.auth().onAuthStateChanged(function (user) {
    // handle it
});

And then:

unsubscribe();

This has already been answered really well by Frank van Puffelen, but here is my use case for React components that are getting user data. These components need to unsubscribe when the component is unmounted or there will be a memory leak for each of these components.

React.useEffect(() => {
  let unsubscribe;
  const getUser = async () => {
    unsubscribe = await firebase.checkUserAuth(user => setUser(user));
  };
  getUser();
  return unsubscribe;
}, []);