Authentication trigger functions in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Firebase Authentication trigger function?

Consider this Firebase Cloud Function that triggers when a user is created. What will be logged when a new user with email '[e-mail]' is created?

Firebase

exports.userCreated = functions.auth.user().onCreate((user) => {
  console.log(`New user created: ${user.email}`);
  return null;
});

A New user created: [e-mail]

B New user created: undefined

C Error: user.email is not defined

D No output is logged

💡 Hint

The user object passed to the trigger contains the user's email property.

The onCreate trigger receives a user object with properties including email. Logging user.email prints the new user's email.

🧠 Conceptual

intermediate

Which Firebase Authentication trigger runs when a user deletes their account?

Firebase Authentication provides triggers for user lifecycle events. Which trigger function runs when a user deletes their account?

A functions.auth.user().onDestroy()

B functions.auth.user().onRemove()

C functions.auth.user().onDelete()

D functions.auth.user().onExpire()

💡 Hint

The trigger name includes 'Delete' for account deletion.

The onDelete() trigger runs when a user account is deleted in Firebase Authentication.

📝 Syntax

advanced

Identify the syntax error in this Firebase Authentication trigger function

Which option contains the correct syntax for a Firebase Authentication onCreate trigger function?

Firebase

exports.newUser = functions.auth.user().onCreate((user) => {
  console.log('User created:', user.uid);
  return null;
});

A

exports.newUser = functions.auth.user().onCreate((user) => {
  console.log('User created:', user.uid);
  return null;
}

B

exports.newUser = functions.auth.user().onCreate((user) => {
  console.log('User created:', user.uid)
  return null;
});

C

exports.newUser = functions.auth.user().onCreate(user => {
  console.log('User created:', user.uid);
  return null;
});

D

exports.newUser = functions.auth.user().onCreate((user) => {
  console.log('User created:', user.uid);
  return null;
});

💡 Hint

Check for missing semicolons and balanced brackets.

Option D has correct arrow function syntax, balanced braces, and semicolons. Others have missing semicolon or brace.

❓ optimization

advanced

How to optimize a Firebase Authentication onCreate trigger to minimize cold start time?

You want your Firebase Authentication onCreate trigger to run quickly when a user signs up. Which practice helps reduce cold start delays?

A Use global imports and initialize heavy libraries outside the trigger function

B Import only necessary modules inside the trigger function, not globally

C Write all code inside the trigger function without external imports

D Avoid using async/await in the trigger function

💡 Hint

Cold start is affected by how much code loads before the function runs.

Initializing heavy libraries globally allows Firebase to reuse the instance, reducing cold start time.

🔧 Debug

expert

Why does this Firebase Authentication onDelete trigger fail to run?

Given this code, why does the onDelete trigger not execute when a user deletes their account? exports.userDeleted = functions.auth.user().onDelete((user) => { console.log('User deleted:', user.uid); return null; });

A The user parameter is undefined in onDelete triggers

B The function is not deployed to Firebase after code changes

C The trigger should be onRemove, not onDelete

D Firebase Authentication does not support onDelete triggers

💡 Hint

Check if the function is active in your Firebase project.

If the function is not deployed, it won't run even if the code is correct.

Practice

1. What does the Firebase Authentication trigger function onCreate do?

easy

A. It sends a password reset email.

B. It deletes a user from the database.

C. It runs automatically when a new user signs up.

D. It updates user profile information.

Solution

1. Step 1: Understand the purpose of onCreate

   The onCreate trigger runs when a new user account is created in Firebase Authentication.

2. Step 2: Match the trigger to the correct action

   Since it runs on new user sign-up, it does not delete users or send emails directly.

3. Final Answer:

   It runs automatically when a new user signs up. -> Option C

4. Quick Check:

   onCreate = New user signup trigger [OK]

Hint: Remember: onCreate = new user signup trigger [OK]

Common Mistakes:

• Confusing onCreate with onDelete trigger
• Thinking onCreate sends emails automatically
• Assuming onCreate updates user data

2. Which of the following is the correct syntax to define a Firebase Authentication trigger function for user deletion in Node.js?

easy

A. functions.auth.user().onCreate((user) => { /* code */ });

B. exports.userDeleted = functions.auth.user().onDelete((user) => { /* code */ });

C. exports.userDeleted = functions.auth.user().onRemove((user) => { /* code */ });

D. functions.auth.user().delete((user) => { /* code */ });

Solution

1. Step 1: Identify the correct trigger for user deletion

   The correct trigger for user deletion is onDelete attached to functions.auth.user().

2. Step 2: Check the syntax for exporting the function

   The function should be exported using exports.functionName = ... syntax.

3. Final Answer:

   exports.userDeleted = functions.auth.user().onDelete((user) => { /* code */ }); -> Option B

4. Quick Check:

   onDelete trigger syntax = exports.userDeleted = functions.auth.user().onDelete((user) => { /* code */ }); [OK]

Hint: Use exports.functionName = functions.auth.user().onDelete(...) [OK]

Common Mistakes:

• Using onCreate instead of onDelete for deletion
• Using non-existent onRemove trigger
• Not exporting the function properly

3. Given the following Firebase Authentication trigger function, what will be logged when a user signs up?

exports.welcomeUser = functions.auth.user().onCreate((user) => {
  console.log(`Welcome, ${user.email}!`);
});

medium

A. Welcome, undefined!

B. Welcome, user.email!

C. No output, function does not run on sign-up.

D. Welcome, [e-mail]!

Solution

1. Step 1: Understand the onCreate trigger behavior

   The function runs automatically when a new user signs up, receiving the user object with an email property.

2. Step 2: Analyze the console.log statement

   The template string inserts the user's email, so it logs "Welcome, [e-mail]!" if the user's email is [e-mail].

3. Final Answer:

   Welcome, [e-mail]! -> Option D

4. Quick Check:

   onCreate logs user email correctly [OK]

Hint: onCreate passes user object with email property [OK]

Common Mistakes:

• Assuming user.email is undefined
• Thinking function does not run on sign-up
• Misreading template string syntax

4. Identify the error in this Firebase Authentication trigger function code:

exports.deleteUserData = functions.auth.user().onDelete(user => {
  const uid = user.uid;
  admin.database().ref(`/users/${uid}`).remove();
});

medium

A. admin is not initialized or imported.

B. Incorrect trigger function; should use onCreate instead of onDelete.

C. Missing return statement for the database operation.

D. The user parameter should be destructured as { uid }.

Solution

1. Step 1: Check for admin SDK initialization

   The code uses admin.database() but does not show admin being imported or initialized, which causes runtime errors.

2. Step 2: Verify other parts of the code

   The trigger and user parameter usage are correct; missing return is not mandatory but recommended.

3. Final Answer:

   admin is not initialized or imported. -> Option A

4. Quick Check:

   admin SDK must be initialized before use [OK]

Hint: Always import and initialize admin SDK before database calls [OK]

Common Mistakes:

• Confusing onDelete with onCreate trigger
• Ignoring admin SDK initialization
• Thinking return statement is mandatory

5. You want to send a welcome email only to users who sign up with an email ending in @example.com. Which code snippet correctly implements this using Firebase Authentication trigger functions?

hard

A. exports.sendWelcomeEmail = functions.auth.user().onCreate((user) => {
if (user.email.endsWith('@example.com')) {
sendEmail(user.email);
}
});

B. exports.sendWelcomeEmail = functions.auth.user().onDelete((user) => {
if (user.email.includes('@example.com')) {
sendEmail(user.email);
}
});

C. exports.sendWelcomeEmail = functions.auth.user().onCreate((user) => {
if (user.email === '@example.com') {
sendEmail(user.email);
}
});

D. exports.sendWelcomeEmail = functions.auth.user().onCreate((user) => {
if (user.email.indexOf('@example.com') === 0) {
sendEmail(user.email);
}
});

Solution

1. Step 1: Choose the correct trigger for new user sign-up

   Use onCreate to run code when a user signs up, not onDelete.

2. Step 2: Check the email condition for domain matching

   Use endsWith('@example.com') to check if the email ends with the domain. Using includes or equality check is incorrect here.

3. Final Answer:

   exports.sendWelcomeEmail = functions.auth.user().onCreate((user) => { if (user.email.endsWith('@example.com')) { sendEmail(user.email); } }); -> Option A

4. Quick Check:

   Use onCreate + endsWith() for domain check [OK]

Hint: Use onCreate and endsWith('@domain.com') to filter emails [OK]

Common Mistakes:

• Using onDelete instead of onCreate
• Checking email equality instead of endsWith
• Using indexOf === 0 instead of endsWith