Auth state observer in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start observing authentication state changes.

Firebase

firebase.auth().onAuthStateChanged([1]);

Drag options to blanks, or click blank then click option'

Auser => console.log(user)

Bcallback

CsignIn

DauthState

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string instead of a function.

Using a variable name that is not defined.

Forgetting to pass any argument.

The onAuthStateChanged method takes a callback function that runs when the auth state changes. Here, user => console.log(user) is a valid callback.

2fill in blank

medium

Complete the code to check if a user is signed in inside the observer.

Firebase

firebase.auth().onAuthStateChanged(user => {
  if ([1]) {
    console.log('User signed in');
  } else {
    console.log('No user signed in');
  }
});

Drag options to blanks, or click blank then click option'

Auser.exists

Buser

Cuser.isSignedIn

Duser === null

Attempts:

3 left

💡 Hint

Common Mistakes

Checking user.isSignedIn which does not exist.

Using user === null to check signed in (this checks opposite).

The user object is null if no user is signed in. Checking if (user) means the user is signed in.

3fill in blank

hard

Fix the error in the observer callback to correctly access the user's email.

Firebase

firebase.auth().onAuthStateChanged(user => {
  if (user) {
    console.log('Email:', user[1]);
  }
});

Drag options to blanks, or click blank then click option'

A.email

B.email()

C.getEmail()

D.userEmail

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses like user.email() which causes an error.

Trying to call a method getEmail() which does not exist.

The user's email is accessed as a property user.email, not as a method.

4fill in blank

hard

Fill both blanks to unsubscribe the auth state observer correctly.

Firebase

const unsubscribe = firebase.auth().onAuthStateChanged([1]);

// Later when no longer needed
[2]();

Drag options to blanks, or click blank then click option'

Auser => console.log(user)

Bunsubscribe

Cunsubscribe()

Dfirebase.auth().signOut

Attempts:

3 left

💡 Hint

Common Mistakes

Calling unsubscribe() immediately instead of storing it.

Using firebase.auth().signOut which signs out user, not unsubscribes.

The onAuthStateChanged returns an unsubscribe function. Store it in a variable and call it later with parentheses to unsubscribe.

5fill in blank

hard

Fill all three blanks to log a message only when a user signs out.

Firebase

firebase.auth().onAuthStateChanged(user => {
  if (user [1] null) {
    console.log('User signed in');
  } else if (user [2] null) {
    console.log([3]);
  }
});

Drag options to blanks, or click blank then click option'

A!==

B===

C'User signed out'

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using != instead of !== which is less strict.

Logging wrong message on sign out.

Use !== null to check user signed in, === null to check signed out, and log the message string.

Practice

1. What is the main purpose of an auth state observer in Firebase?

easy

A. To store user data permanently in the database

B. To watch for user sign-in or sign-out events and respond accordingly

C. To send emails to users automatically

D. To create new user accounts without user input

Solution

1. Step 1: Understand the role of auth state observer

   An auth state observer listens for changes in user authentication status, such as sign-in or sign-out.

2. Step 2: Identify the correct purpose

   It helps the app respond instantly to these changes by updating the UI or user info.

3. Final Answer:

   To watch for user sign-in or sign-out events and respond accordingly -> Option B

4. Quick Check:

   Auth state observer = watch sign-in/out [OK]

Hint: Auth observer tracks sign-in/out changes instantly [OK]

Common Mistakes:

• Thinking it stores user data permanently
• Confusing it with email sending service
• Assuming it creates accounts automatically

2. Which of the following is the correct syntax to add an auth state observer in Firebase JavaScript SDK?

easy

A. firebase.auth().onAuthStateChanged(user => { /* code */ });

B. firebase.auth().addUserObserver(user => { /* code */ });

C. firebase.auth().watchUserState(user => { /* code */ });

D. firebase.auth().observeAuth(user => { /* code */ });

Solution

1. Step 1: Recall Firebase auth observer method

   The correct method to listen for auth state changes is onAuthStateChanged.

2. Step 2: Match the syntax

   firebase.auth().onAuthStateChanged(user => { /* code */ }); uses the exact method name and callback syntax as per Firebase docs.

3. Final Answer:

   firebase.auth().onAuthStateChanged(user => { /* code */ }); -> Option A

4. Quick Check:

   Correct method = onAuthStateChanged [OK]

Hint: Look for onAuthStateChanged method name [OK]

Common Mistakes:

• Using non-existent methods like addUserObserver
• Mixing method names with watchUserState
• Using observeAuth which is invalid

3. Given this code snippet, what will be logged if a user is signed in?

firebase.auth().onAuthStateChanged(user => {
  if (user) {
    console.log('User ID:', user.uid);
  } else {
    console.log('No user signed in');
  }
});

medium

A. Nothing will be logged

B. No user signed in

C. An error message about missing user

D. User ID: followed by the user's unique ID

Solution

1. Step 1: Understand the callback behavior

   The callback runs whenever auth state changes. If a user is signed in, the user object is not null.

2. Step 2: Check the if condition

   Since user exists, it logs 'User ID:' plus the user's unique ID (user.uid).

3. Final Answer:

   User ID: followed by the user's unique ID -> Option D

4. Quick Check:

   Signed-in user logs ID [OK]

Hint: If user exists, logs user.uid [OK]

Common Mistakes:

• Assuming it logs 'No user signed in' when user is present
• Expecting an error message incorrectly
• Thinking nothing logs without explicit call

4. Identify the error in this Firebase auth state observer code:

firebase.auth().onAuthStateChanged(user => {
  if (user) {
    console.log('Welcome ' + user.email);
  } else {
    console.log('User signed out');
  }
});
firebase.auth().onAuthStateChanged();

medium

A. Missing semicolon after console.log statements

B. Using user.email is invalid; should use user.id

C. Calling onAuthStateChanged without a callback causes an error

D. No error; code works fine

Solution

1. Step 1: Review the first observer usage

   The first call to onAuthStateChanged correctly passes a callback function.

2. Step 2: Check the second call

   The second call to onAuthStateChanged has no callback, which is required and causes an error.

3. Final Answer:

   Calling onAuthStateChanged without a callback causes an error -> Option C

4. Quick Check:

   Callback required for onAuthStateChanged [OK]

Hint: Always provide a callback to onAuthStateChanged [OK]

Common Mistakes:

• Thinking user.email is invalid (it is valid)
• Ignoring missing callback error
• Assuming missing semicolons cause runtime errors

5. You want your app to update the UI immediately when the user signs in or out, and also fetch user profile data only when signed in. Which approach using Firebase auth state observer is best?

hard

A. Use onAuthStateChanged to detect user, then fetch profile data inside the callback only if user exists

B. Fetch profile data once at app start, ignoring auth state changes

C. Use onAuthStateChanged but fetch profile data outside the callback unconditionally

D. Do not use onAuthStateChanged; instead poll user status every 5 seconds

Solution

1. Step 1: Understand the need for immediate UI update

   Using onAuthStateChanged lets the app react instantly to sign-in or sign-out events.

2. Step 2: Fetch profile data conditionally

   Fetching profile data inside the callback only when user exists avoids unnecessary calls and ensures data is current.

3. Final Answer:

   Use onAuthStateChanged to detect user, then fetch profile data inside the callback only if user exists -> Option A

4. Quick Check:

   Fetch profile inside auth callback if user present [OK]

Hint: Fetch profile only inside auth callback when user exists [OK]

Common Mistakes:

• Fetching profile data regardless of auth state
• Polling user status wastes resources
• Ignoring auth state changes for UI updates