User profile data in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

How does Firebase Authentication handle user profile updates?

When a user updates their profile information using Firebase Authentication's updateProfile method, what happens to the stored user data?

A The profile data update triggers a full user account deletion and recreation.

B The profile data is updated on Firebase servers and immediately reflected in the user's authentication token.

C The profile data is updated on Firebase servers but requires the user to sign out and sign in again to see changes.

D The profile data is updated only locally on the client and not saved to Firebase servers.

💡 Hint

Think about how Firebase keeps user data consistent across sessions.

Firebase Authentication updates the user profile data on its servers and refreshes the user's token so changes are immediately available.

❓ Architecture

intermediate

Best practice for storing user profile data in Firebase

Which Firebase service is best suited for storing detailed user profile data that can be queried and updated frequently?

A Firebase Cloud Firestore

B Firebase Realtime Database

C Firebase Authentication user profile fields only

D Firebase Cloud Storage

💡 Hint

Consider which service supports flexible queries and structured data.

Cloud Firestore is designed for structured, queryable, and frequently updated data like user profiles.

❓ security

advanced

Securing user profile data in Firebase Firestore

Given a Firestore collection 'users' where each document ID matches the user's UID, which security rule correctly restricts read and write access so users can only access their own profile data?

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth.uid == userId;
    }
  }
}

A request.auth.token.email_verified == true

B request.time < timestamp.date(2025, 1, 1)

C request.auth.uid != userId

D request.auth.uid == userId

💡 Hint

Think about how to match the authenticated user with the document ID.

The rule allows access only if the authenticated user's UID matches the document ID, ensuring users access only their own data.

❓ Configuration

advanced

Firebase Cloud Functions to update user profile data

You want to automatically update a user's profile document in Firestore whenever their Firebase Authentication profile changes. Which Cloud Function trigger should you use?

A functions.auth.user().onUpdate()

B functions.firestore.document('users/{userId}').onUpdate()

C functions.auth.user().onCreate()

D functions.database.ref('/users/{userId}').onWrite()

💡 Hint

Consider which event fires when a Firebase Authentication user profile changes.

The onUpdate trigger on functions.auth.user() fires when a user's authentication profile is updated.

✅ Best Practice

expert

Handling user profile data consistency across Firebase Authentication and Firestore

What is the best approach to keep user profile data consistent between Firebase Authentication and Firestore when users update their profile?

A Update Firebase Authentication profile only and ignore Firestore data.

B Manually update both Firebase Authentication and Firestore profiles on the client side without synchronization.

C Use a Cloud Function triggered on Firebase Authentication user update to sync changes to Firestore.

D Update Firestore profile only and do not update Firebase Authentication profile.

💡 Hint

Think about automating synchronization to avoid data mismatch.

Using a Cloud Function triggered by Firebase Authentication updates ensures Firestore data stays consistent automatically.

Practice

1. What is the main purpose of storing user profile data in Firebase Firestore?

easy

A. To host static website files

B. To run server-side scripts automatically

C. To send emails to users

D. To save personal information for each user securely

Solution

1. Step 1: Understand user profile data purpose

   User profile data holds personal details like name and email for each user.

2. Step 2: Identify Firebase Firestore role

   Firestore stores this data securely and allows easy access and updates.

3. Final Answer:

   To save personal information for each user securely -> Option D

4. Quick Check:

   User profile data = personal info storage [OK]

Hint: User profiles store personal info, not run scripts or host files [OK]

Common Mistakes:

• Confusing Firestore with hosting or email services
• Thinking Firestore runs backend code automatically
• Assuming user profiles are for sending emails

2. Which Firebase Firestore syntax correctly creates a document for a user profile with ID 'user123'?

easy

A. db.collection('users').doc('user123').set({name: 'Alice'})

B. db.users('user123').add({name: 'Alice'})

C. db.collection('users').add('user123', {name: 'Alice'})

D. db.doc('users/user123').create({name: 'Alice'})

Solution

1. Step 1: Review Firestore document creation syntax

   Use collection('name').doc('id').set(data) to create or overwrite a document.

2. Step 2: Match syntax with options

   db.collection('users').doc('user123').set({name: 'Alice'}) matches correct syntax: collection, doc, then set with data object.

3. Final Answer:

   db.collection('users').doc('user123').set({name: 'Alice'}) -> Option A

4. Quick Check:

   Correct Firestore doc creation = db.collection('users').doc('user123').set({name: 'Alice'}) [OK]

Hint: Use collection().doc().set() to create user profile docs [OK]

Common Mistakes:

• Using add() with document ID specified
• Wrong method names like create() instead of set()
• Incorrect chaining of methods

3. Given this code snippet, what will be the output if the user document does not exist?

const docRef = db.collection('users').doc('user999');
docRef.get().then(doc => {
  if (doc.exists) {
    console.log(doc.data().name);
  } else {
    console.log('No user found');
  }
});

medium

A. No user found

B. undefined

C. Error: Document not found

D. null

Solution

1. Step 1: Understand doc.exists property

   doc.exists is false if document does not exist in Firestore.

2. Step 2: Follow code logic for missing document

   If doc.exists is false, code logs 'No user found'.

3. Final Answer:

   No user found -> Option A

4. Quick Check:

   Missing doc triggers else block = 'No user found' [OK]

Hint: Check doc.exists before accessing data to avoid errors [OK]

Common Mistakes:

• Assuming doc.data() returns null or undefined
• Expecting an error instead of else block output
• Ignoring the else condition

4. Identify the error in this Firebase Firestore code to update a user's profile name:

db.collection('users').doc('user123').update({
  username: 'Bob'
});

medium

A. update() cannot be used to change existing fields

B. Missing await or then to handle promise

C. Field name should be 'name' not 'username'

D. Document ID 'user123' must be a number

Solution

1. Step 1: Check Firestore update method usage

   update() returns a promise and should be handled with await or then.

2. Step 2: Identify missing promise handling

   Code does not handle promise, which can cause silent failures.

3. Final Answer:

   Missing await or then to handle promise -> Option B

4. Quick Check:

   Firestore update returns promise, must handle it [OK]

Hint: Always handle promises with await or then in Firestore calls [OK]

Common Mistakes:

• Thinking update() cannot change fields
• Assuming field names must be fixed
• Believing document IDs must be numeric

5. You want to store user profiles with optional phone numbers. Which Firestore data structure best handles missing phone numbers without errors?

hard

A. Set phone number field to empty string if missing

B. Always set phone number field to null

C. Include phone number field only if user provides it

D. Create a separate collection for phone numbers

Solution

1. Step 1: Understand optional fields in Firestore

   Firestore allows documents to omit fields that are not provided.

2. Step 2: Choose best practice for optional data

   Including the phone number field only when available avoids storing empty or null values and reduces confusion.

3. Final Answer:

   Include phone number field only if user provides it -> Option C

4. Quick Check:

   Omit optional fields if missing to keep data clean [OK]

Hint: Omit optional fields instead of storing empty or null values [OK]

Common Mistakes:

• Storing empty strings which can confuse queries
• Using null values that may cause unexpected behavior
• Overcomplicating with separate collections for simple optional data