Anonymous authentication in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens after anonymous sign-in in Firebase?

When a user signs in anonymously using Firebase Authentication, what is the state of the user's identity immediately after sign-in?

Firebase

firebase.auth().signInAnonymously().then(userCredential => {
  console.log(userCredential.user.uid);
});

A The user is signed out immediately after sign-in.

B The user has a unique user ID but no email or password associated.

C The user is signed in with a Google account by default.

D The user is signed in with a temporary email and password.

💡 Hint

Think about what anonymous means in authentication.

Anonymous sign-in creates a unique user ID without requiring email or password. This allows temporary access without user credentials.

❓ security

intermediate

What is a security risk of anonymous authentication?

Which of the following is a potential security risk when using Firebase anonymous authentication in an app?

A Anonymous authentication requires users to enter a password.

B Anonymous users can access admin-only data by default.

C Users can lose their data if they uninstall the app without linking their account.

D Anonymous users are automatically upgraded to full accounts.

💡 Hint

Consider what happens if the anonymous user identity is lost.

Anonymous users have a unique ID stored locally. If the app is uninstalled, the ID is lost and data linked to that ID is inaccessible.

❓ Configuration

advanced

Which Firebase rule allows anonymous users to read data but not write?

Given Firebase Realtime Database rules, which rule snippet correctly allows anonymous users to read data but prevents them from writing?

A

{
  "rules": {
    ".read": "auth != null",
    ".write": "auth != null && auth.isAnonymous == false"
  }
}

B

{
  "rules": {
    ".read": "auth != null && auth.isAnonymous == true",
    ".write": "auth != null"
  }
}

C

{
  "rules": {
    ".read": "auth == null",
    ".write": "auth == null"
  }
}

D

{
  "rules": {
    ".read": "true",
    ".write": "false"
  }
}

💡 Hint

Check the conditions for read and write permissions based on auth state.

Rule B allows any authenticated user to read, but only non-anonymous users to write. This matches the requirement.

❓ Architecture

advanced

How to preserve user data when upgrading anonymous users?

What is the recommended approach to keep user data when an anonymous Firebase user upgrades to a permanent account?

A Sign out the anonymous user and sign in with the new account without linking.

B Create a new user and manually copy data from the anonymous user ID to the new user ID.

C Delete the anonymous user and create a new permanent user without data migration.

D Link the anonymous user credential to the new permanent credential using linkWithCredential().

💡 Hint

Think about how Firebase supports account linking.

linkWithCredential() merges the anonymous user with the permanent account, preserving data and user ID.

🧠 Conceptual

expert

What is the behavior of Firebase anonymous authentication token expiration?

Which statement correctly describes the token expiration behavior for Firebase anonymous authentication users?

A Anonymous user tokens expire after one hour and are automatically refreshed by the SDK.

B Anonymous user tokens expire immediately after sign-in.

C Anonymous user tokens expire after 24 hours and require manual refresh by the developer.

D Anonymous user tokens never expire and do not require refresh.

💡 Hint

Consider how Firebase handles token lifecycle for all users.

Firebase issues ID tokens valid for one hour for all users, including anonymous ones, and the SDK refreshes them automatically.

Practice

1. What is the main purpose of Firebase anonymous authentication?

easy

A. Store user data permanently without authentication

B. Require users to verify their email before using the app

C. Allow users to use the app without creating an account

D. Force users to sign in with Google or Facebook

Solution

1. Step 1: Understand anonymous authentication concept

   Anonymous authentication lets users access the app without signing up or providing credentials.

2. Step 2: Identify the purpose

   This reduces signup friction and improves user experience by creating a temporary user ID.

3. Final Answer:

   Allow users to use the app without creating an account -> Option C

4. Quick Check:

   Anonymous authentication = Allow use without account [OK]

Hint: Anonymous means no signup needed to start [OK]

Common Mistakes:

• Thinking anonymous auth requires email verification
• Confusing anonymous auth with social login
• Assuming anonymous users have permanent accounts

2. Which Firebase method is used to sign in anonymously in JavaScript?

easy

A. firebase.auth().signInWithEmailAndPassword()

B. firebase.auth().signOut()

C. firebase.auth().createUserWithEmailAndPassword()

D. firebase.auth().signInAnonymously()

Solution

1. Step 1: Recall Firebase anonymous sign-in method

   The correct method to sign in anonymously is signInAnonymously().

2. Step 2: Eliminate other methods

   Other methods are for email/password sign-in or sign-out, not anonymous sign-in.

3. Final Answer:

   firebase.auth().signInAnonymously() -> Option D

4. Quick Check:

   Anonymous sign-in method = signInAnonymously() [OK]

Hint: Anonymous sign-in uses signInAnonymously() method [OK]

Common Mistakes:

• Using email/password sign-in methods for anonymous login
• Confusing createUserWithEmailAndPassword with anonymous sign-in
• Trying to sign out instead of sign in

3. What will the following JavaScript code output if anonymous sign-in succeeds?

firebase.auth().signInAnonymously()
  .then(() => console.log('User ID:', firebase.auth().currentUser.uid))
  .catch(error => console.log('Error:', error.message))

medium

A. User ID: undefined

B. User ID: a unique temporary ID string

C. Error: User not signed in

D. User ID: null

Solution

1. Step 1: Understand signInAnonymously success behavior

   On success, Firebase creates a temporary user with a unique ID accessible via currentUser.uid.

2. Step 2: Analyze console.log output

   The code logs 'User ID:' followed by the unique temporary ID string.

3. Final Answer:

   User ID: a unique temporary ID string -> Option B

4. Quick Check:

   Anonymous sign-in returns unique temp ID [OK]

Hint: Success logs unique temporary user ID [OK]

Common Mistakes:

• Expecting undefined or null user ID after sign-in
• Assuming error message on success
• Confusing currentUser.uid with email or password

4. Identify the error in this Firebase anonymous sign-in code snippet:

firebase.auth().signInAnonymously
  .then(() => console.log('Signed in anonymously'))
  .catch(error => console.log(error))

medium

A. Missing parentheses after signInAnonymously

B. Incorrect use of .then() method

C. No catch block for errors

D. Using console.log inside then block

Solution

1. Step 1: Check method call syntax

   The method signInAnonymously must be called with parentheses ().

2. Step 2: Identify missing parentheses

   The code uses signInAnonymously without parentheses, so it does not invoke the function.

3. Final Answer:

   Missing parentheses after signInAnonymously -> Option A

4. Quick Check:

   Method calls need () to execute [OK]

Hint: Always add () to call functions [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Misplacing .then() or .catch() chaining
• Assuming console.log causes errors

5. You want to let users start anonymously but later upgrade to email/password without losing data. Which Firebase feature helps achieve this?

hard

A. Linking anonymous user to email/password credential

B. Deleting anonymous user and creating new account

C. Signing out anonymous user before email sign-up

D. Using separate Firebase projects for anonymous and email users

Solution

1. Step 1: Understand user upgrade process

   Firebase allows linking an anonymous user to an email/password credential to keep the same user ID and data.

2. Step 2: Evaluate options

   Deleting or signing out loses data; separate projects complicate management.

3. Final Answer:

   Linking anonymous user to email/password credential -> Option A

4. Quick Check:

   Link anonymous to email to keep data [OK]

Hint: Link accounts to upgrade without losing data [OK]

Common Mistakes:

• Deleting anonymous user before upgrade
• Signing out instead of linking accounts
• Using multiple projects unnecessarily