Authentication trigger functions in Firebase - Mini Project: Build & Apply

Firebase Authentication Trigger Functions

📖 Scenario: You are building a simple app that needs to track when users sign up and sign in. You will use Firebase Authentication trigger functions to react to these events and store user data in the database.

🎯 Goal: Create Firebase Authentication trigger functions that run when a user signs up and when a user signs in. These functions will write user information to the Firestore database.

📋 What You'll Learn

Create a Firestore collection called users to store user data.

Write a Firebase function called onUserCreate that triggers when a user signs up.

Write a Firebase function called onUserSignIn that triggers when a user signs in.

In onUserCreate, add a document to users with the user's uid, email, and createdAt timestamp.

In onUserSignIn, update the user's document with the lastSignIn timestamp.

💡 Why This Matters

🌍 Real World

Authentication triggers are used in real apps to keep user data updated and perform actions automatically when users sign up or sign in.

💼 Career

Understanding Firebase Authentication triggers is important for backend and full-stack developers working with serverless architectures and real-time databases.

1

Set up Firestore and import Firebase functions

Write the code to import functions from firebase-functions and admin from firebase-admin. Initialize the Firebase admin app and get a Firestore database reference called db.

Firebase

// Your code here to import and initialize Firebase admin and Firestore

Hint

Use require to import modules. Initialize admin with admin.initializeApp(). Get Firestore with admin.firestore().

2

Create the user creation trigger function

Write a Firebase Authentication trigger function called onUserCreate that triggers on user creation using functions.auth.user().onCreate. Inside the function, create a document in the users collection with the user's uid as the document ID. Store the user's email and the current timestamp as createdAt.

Firebase

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const db = admin.firestore();

// Your code here to create onUserCreate function

Hint

Use functions.auth.user().onCreate to trigger on user signup. Use db.collection('users').doc(user.uid).set() to save data.

3

Create the user sign-in trigger function

Write a Firebase Authentication trigger function called onUserSignIn that triggers on user sign-in using functions.auth.user().onLogin. Inside the function, update the user's document in the users collection with the current timestamp as lastSignIn. (Note: Firebase does not have a built-in onLogin trigger, so simulate this with a callable function named onUserSignIn that accepts a uid parameter.)

Firebase

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const db = admin.firestore();

exports.onUserCreate = functions.auth.user().onCreate(async (user) => {
  return db.collection('users').doc(user.uid).set({
    email: user.email,
    createdAt: admin.firestore.FieldValue.serverTimestamp()
  });
});

// Your code here to create onUserSignIn callable function

Hint

Firebase does not have an onLogin trigger. Use a callable function with functions.https.onCall that accepts uid and updates the user's lastSignIn.

4

Export both functions for deployment

Ensure both onUserCreate and onUserSignIn functions are exported properly in the module exports so they can be deployed to Firebase.

Firebase

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const db = admin.firestore();

exports.onUserCreate = functions.auth.user().onCreate(async (user) => {
  return db.collection('users').doc(user.uid).set({
    email: user.email,
    createdAt: admin.firestore.FieldValue.serverTimestamp()
  });
});

exports.onUserSignIn = functions.https.onCall(async (data) => {
  const uid = data.uid;
  return db.collection('users').doc(uid).update({
    lastSignIn: admin.firestore.FieldValue.serverTimestamp()
  });
});

// Your code here to export functions if needed

Hint

Both functions should be exported using exports.functionName = ... syntax.

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