how to tell on the client side that the user is logged in when using node js and react

I used the following code (https://github.com/firebase/quickstart-nodejs/blob/master/auth-sessions/app.js) to log a user in using React and Nodejs and Firebase. I can tell if someone is logged in using NodeJs, but not on the ReactJs side. I used the code and that does not work after a few minutes:

`app.auth().onAuthStateChanged((user) => {`