Firebase login: the easiest way, with promises

Authentication.js

/**
 * Handles logging in to Firebase.
 */
class Authentication {
  authPromise;

  constructor() {
    this.authPromise = new Promise((resolve, reject) => {
      firebase.auth().onAuthStateChanged(function(user) {
        if (user) {
          // User is signed in.
          resolve(user);
        } else {
          // User is signed out.
          reject(new Error('Not logged in'));
        }
      });
    });
  }

  /**
   * Returns a Promise with the current logged in user, or logs in and then returns the user.
   *
   * It is safe to call this every time you need to do an operation requiring an authenticated user;
   * it will not re-login every time.
   */
  signIn() {
    const auth = firebase.auth();
  
    if (auth.currentUser) {
      return Promise.resolve(auth.currentUser);
    }
  
    return this.authPromise
      .catch((err) => {
        // You can replace the provider here with whichever one you're using.
        const provider = new firebase.auth.GoogleAuthProvider();
      
        return auth.signInWithPopup(provider)
          .then(function(result) {
            // This gives you a Google Access Token. You can use it to access the Google API.
            var token = result.credential.accessToken;
            // The signed-in user info.
            var user = result.user;
            
            return user;
          })
      });
  }
}