How to connect the Firebase authentication API to a web application

In order to connect your application to the Firebase authentication API, you should start by creating a Firebase application instance:

let config = {
  apiKey: 'YourAPIKey',
  databaseURL: 'YourDBURL',
  authDomain: 'YourAuthDomain'
}
let app = firebase.initializeApp(config)

You can find the necessary keys and URLs in the popup that opens if you click on the Web Setup button:

The setup config to use Firebase in a web application

Now you can use the app instance to access the auth() object and its methods. Check out the official Firebase documentation regarding the authentication API: https://firebase.google.com/docs/auth/users.

The most important part of the API for us is the methods to create and sign in a user, and the method that listens to the changes in the authentication state:

app.auth().createUserWithEmailAndPassword(email, password)

Or:

app.auth().signInWithEmailAndPassword(email, password)

The method that listens to the...