I have this firebase.js file:

// Import the functions you need from the SDKs you need
import { initializeApp } from "firebase/app";

const firebaseConfig = {...};

// Initialize Firebase
export const app = initializeApp(firebaseConfig);
const analytics = getAnalytics(app);


import { getAuth, onAuthStateChanged } from "firebase/auth";

export const auth = getAuth(app);
export let uid = null;

onAuthStateChanged(auth, (user) => {
  if (user) {
    uid = user.uid;
  } else {
    uid = null;
  }
});

In another function I have been trying to do:

import {uid, auth} from "./firebase.js";
if (auth.currentUser)

or

if (uid)

However, neither works consistently. After sign the user will exist but after a page reload they disappear again. Is this the right way to check the user is signed in?