OAuth Providers and Anonymous Auth

Providers, guests and linking

Federated providers such as Google, Apple, GitHub and Microsoft use provider classes (GoogleAuthProvider, OAuthProvider...) with signInWithPopup or signInWithRedirect. Anonymous auth (signInAnonymously) creates a real user with a uid but no credentials, useful for letting people try the app before signing up. When the guest later chooses a provider, account linking (linkWithPopup, linkWithCredential) attaches that credential to the same uid, so their data stays with them. If the credential already belongs to another account, linking fails with auth/credential-already-in-use and you must decide how to merge.

Google sign-in and upgrading a guest

TypeScript.

import {
  getAuth, GoogleAuthProvider, signInWithPopup,
  signInAnonymously, linkWithPopup,
} from "firebase/auth";

const auth = getAuth(app);
const google = new GoogleAuthProvider();

export const signInWithGoogle = () => signInWithPopup(auth, google);

export const startAsGuest = () => signInAnonymously(auth);

export async function upgradeGuestToGoogle() {
  const user = auth.currentUser;
  if (!user?.isAnonymous) return;
  try {
    await linkWithPopup(user, google); // same uid, now with a Google credential
  } catch (e: any) {
    if (e.code === "auth/credential-already-in-use") {
      // the Google account already has a Firebase user: merge data deliberately
    }
    throw e;
  }
}

A visitor badge that becomes an employee badge

Anonymous auth gives a visitor a badge number; linking later prints their name on that same badge instead of issuing a new one.

Quick check: Why link an anonymous account instead of signing in fresh with Google?

• Anonymous accounts cannot be deleted
• The user keeps the same uid, so data written as a guest stays theirs
• Linking is required to use Firestore
• Google sign-in does not create a uid