Custom authentication tokens in Firebase - Mini Project: Build & Apply

Custom Authentication Tokens with Firebase

📖 Scenario: You are building a secure app that needs to verify users with custom authentication tokens. These tokens allow your app to trust users who sign in through your own system.

🎯 Goal: Create a Firebase Cloud Function that generates a custom authentication token for a user with a specific user ID and a role claim.

📋 What You'll Learn

Create a variable with the user ID

Create a configuration object with custom claims

Generate a custom token using Firebase Admin SDK

Return the generated token

💡 Why This Matters

🌍 Real World

Custom authentication tokens let your app securely identify users with roles or permissions you define. This is useful when you have your own user system outside Firebase.

💼 Career

Many cloud jobs require integrating custom authentication with Firebase or other identity providers to control user access securely.

1

Set up the user ID variable

Create a variable called uid and set it to the string 'user123'.

Firebase

// Your code here

Hint

Use const to declare the user ID variable.

2

Create custom claims configuration

Create a constant called customClaims and set it to an object with a key role and value 'admin'.

Firebase

const uid = 'user123';
// Your code here

Hint

Use an object literal to set the role claim.

3

Generate the custom authentication token

Use admin.auth().createCustomToken(uid, customClaims) to generate a custom token and assign it to a constant called customToken. Use await since this returns a promise.

Firebase

const uid = 'user123';
const customClaims = { role: 'admin' };
// Your code here

Hint

Remember to use await because createCustomToken returns a promise.

4

Return the generated token

Return the customToken from the function.

Firebase

const uid = 'user123';
const customClaims = { role: 'admin' };
const customToken = await admin.auth().createCustomToken(uid, customClaims);
// Your code here

Hint

Use the return keyword to send back the token.

Practice

1. What is the main purpose of Firebase custom authentication tokens?

easy

A. To store user data in Firebase Realtime Database

B. To sign in users from your own system to Firebase

C. To send notifications to users

D. To create Firebase projects automatically

Solution

1. Step 1: Understand the role of custom tokens

   Custom tokens are used to authenticate users from an external system with Firebase.

2. Step 2: Identify the correct purpose

   They allow users to sign in securely using tokens generated on your server.

3. Final Answer:

   To sign in users from your own system to Firebase -> Option B

4. Quick Check:

   Custom tokens = sign in users [OK]

Hint: Custom tokens = external user sign-in method [OK]

Common Mistakes:

• Confusing tokens with data storage
• Thinking tokens send notifications
• Assuming tokens create projects

2. Which Firebase Admin SDK method is used to create a custom authentication token?

easy

A. admin.auth().createCustomToken(uid)

B. admin.database().createToken(uid)

C. firebase.auth().signInWithCustomToken(uid)

D. admin.storage().generateToken(uid)

Solution

1. Step 1: Identify the correct Admin SDK service

   Custom tokens are created using the Authentication service, not database or storage.

2. Step 2: Match the correct method

   The method to create a custom token is createCustomToken(uid) under admin.auth().

3. Final Answer:

   admin.auth().createCustomToken(uid) -> Option A

4. Quick Check:

   Custom token creation = admin.auth().createCustomToken [OK]

Hint: Use admin.auth().createCustomToken to generate tokens [OK]

Common Mistakes:

• Using database or storage methods incorrectly
• Confusing signInWithCustomToken with token creation
• Calling methods on firebase instead of admin SDK

3. What will the following code output if uid is 'user123' and no errors occur?

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

admin.auth().createCustomToken('user123')
  .then(token => console.log(typeof token))
  .catch(error => console.error(error));

medium

A. 'object'

B. An error message

C. 'undefined'

D. 'string'

Solution

1. Step 1: Understand createCustomToken return type

   The method returns a Promise that resolves to a string token.

2. Step 2: Check console.log output

   Logging typeof token will print 'string' because the token is a string.

3. Final Answer:

   'string' -> Option D

4. Quick Check:

   createCustomToken returns string token [OK]

Hint: createCustomToken returns a string token inside a Promise [OK]

Common Mistakes:

• Expecting an object instead of string
• Confusing token with undefined
• Assuming an error without cause

4. Identify the error in this code snippet for generating a custom token:

const admin = require('firebase-admin');

admin.auth().createCustomToken('12345')
  .then(token => console.log(token))
  .catch(error => console.error(error));

medium

A. Missing admin.initializeApp() call

B. Incorrect method name, should be generateCustomToken

C. createCustomToken does not return a Promise

D. UID must be a string, not a number

Solution

1. Step 1: Check initialization of Firebase Admin SDK

   The admin SDK must be initialized with admin.initializeApp() before use.

2. Step 2: Verify other parts

   The UID '12345' is a valid string, createCustomToken returns a Promise, and method name is correct.

3. Final Answer:

   Missing admin.initializeApp() call -> Option A

4. Quick Check:

   Initialize admin SDK before calling methods [OK]

Hint: Always call admin.initializeApp() before using admin SDK [OK]

Common Mistakes:

• Passing number instead of string UID
• Assuming createCustomToken is synchronous
• Using wrong method name

5. You want to add extra user info like role: 'admin' in a custom token. Which code snippet correctly adds this claim?

hard

A. admin.auth().setCustomUserClaims(uid, { role: 'admin' })

B. admin.auth().createCustomToken(uid).setClaims({ role: 'admin' })

C. admin.auth().createCustomToken(uid, { role: 'admin' })

D. admin.auth().createCustomToken(uid, { claims: { role: 'admin' } })

Solution

1. Step 1: Understand how to add extra claims in custom tokens

   The createCustomToken method accepts a second argument with additional claims as a plain object.

2. Step 2: Evaluate each option

   admin.auth().createCustomToken(uid, { role: 'admin' }) correctly passes extra claims as second argument. admin.auth().createCustomToken(uid).setClaims({ role: 'admin' }) is invalid chaining. admin.auth().setCustomUserClaims(uid, { role: 'admin' }) sets claims on user but not in token creation. admin.auth().createCustomToken(uid, { claims: { role: 'admin' } }) nests claims incorrectly.

3. Final Answer:

   admin.auth().createCustomToken(uid, { role: 'admin' }) -> Option C

4. Quick Check:

   Extra claims go as second argument in createCustomToken [OK]

Hint: Pass extra claims as second argument to createCustomToken [OK]

Common Mistakes:

• Trying to chain setClaims on token creation
• Using setCustomUserClaims instead of token claims
• Nesting claims inside another object