Sometimes firebase.auth().onAuthStateChanged returns user of null when I refresh the page

Precondition

$npm install --save [email protected]

issue

I'm using firebase authentication on my web application. In my app, I implemented onAuthStateChanged for client side js like below.

firebase.auth().onAuthStateChanged((user) => {
  if(user) {
    //logged in
  } else {
    //do sth
  }
});

After login, I confirmed this method will return actual user obj, but if I refresh the page, then user might be null. Curiously, sometimes user won't be null. I'm afraid there are some limitation of calling onAuthStateChanged, but currently I have no idea.

How should I deal with this issue?

update

Let me share my minimal example.

My app is working with express.js. There are two URLs like below. /login /main

In the login page, I implemented authentication method. If the login is successfully finished, then user will be redirected to '/main'.

//login.js
import firebase from 'firebase';
var config = {...};
firebase.initializeApp(config);

var provider = new firebase.auth.GoogleAuthProvider();
firebase.auth().signInWithPopup(provider)
.then((result) => {
  return result.user.getIdToken(true);
}).then((idToken) => {
  if(idToken) {
    location.href = '/main';
  }
});

In the main page, there is no login method. main.js is only checking whether user is logged in.

//main.js
import firebase from 'firebase';
var config = {...};
firebase.initializeApp(config);

firebase.auth().onAuthStateChanged((user) => {
  if (user) {
    //initialize main page.
  } else {
    location.href = '/login';
  }
}

I think login status is stored on LocalStorage of web browser. This means that, after finishing loading of main.js, onAuthStateChanged will be automatically fired with user information, but not working as I expected. I'm sure that persistence of login information is correct because official document says the default setting is LOCAL for web client.

https://firebase.google.com/docs/auth/web/auth-state-persistence

my question

Should I implement onAuthStateChanged with another way? How can I ensure user is logged in after reload?

e.g.

import $ from 'jquery';
$(document).on('ready', () => {
  onAuthStateChanged((user) => {...});
});

Or could you show me the correct way?

Workaround

I decided to remove session and set redirection to login page if null is returned. This is not a solution, but a workaround currently...