Custom authentication tokens in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a custom token with Firebase Admin SDK.

Firebase

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

admin.initializeApp();

const uid = 'some-uid';
admin.auth().[1](uid)
  .then((customToken) => {
    console.log(customToken);
  })
  .catch((error) => {
    console.log('Error creating custom token:', error);
  });

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

AcreateCustomToken

BgenerateToken

CsignInWithCustomToken

DgetCustomToken

Attempts:

3 left

💡 Hint

Common Mistakes

Using generateToken instead of createCustomToken

Trying to use client SDK methods like signInWithCustomToken on the server

The Firebase Admin SDK uses createCustomToken(uid) to generate a custom authentication token for the given user ID.

2fill in blank

medium

Complete the code to add custom claims to the token.

Firebase

const uid = 'user123';
const additionalClaims = { admin: true };

admin.auth().createCustomToken(uid, [1])
  .then((token) => {
    console.log(token);
  });

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

AextraClaims

BadditionalClaims

CcustomClaims

Dclaims

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the claims object with a wrong variable name

Not passing any claims object when needed

The second argument to createCustomToken is an object with additional claims, commonly named additionalClaims.

3fill in blank

hard

Fix the error in the code to verify a custom token on the client side.

Firebase

firebase.auth().[1](customToken)
  .then((userCredential) => {
    console.log('User signed in');
  })
  .catch((error) => {
    console.error(error);
  });

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

AauthenticateWithToken

BverifyIdToken

CcreateCustomToken

DsignInWithCustomToken

Attempts:

3 left

💡 Hint

Common Mistakes

Using verifyIdToken which is a server-side method

Using createCustomToken on the client side

On the client side, use signInWithCustomToken(token) to sign in with a custom token.

4fill in blank

hard

Fill both blanks to decode and verify a custom token on the server.

Firebase

admin.auth().[1](idToken)
  .then((decodedToken) => {
    const uid = decodedToken.[2];
    console.log('User ID:', uid);
  })
  .catch((error) => {
    console.error('Error verifying token:', error);
  });

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

AverifyIdToken

BdecodeToken

Cuid

DuserId

Attempts:

3 left

💡 Hint

Common Mistakes

Using decodeToken which is not a Firebase Admin method

Accessing userId instead of uid

Use verifyIdToken to verify the token and access the user's ID with decodedToken.uid.

5fill in blank

hard

Fill all three blanks to create a custom token with claims and handle errors properly.

Firebase

const uid = 'user456';
const additionalClaims = { premiumAccount: true };

admin.auth().[1](uid, [2])
  .then((token) => {
    console.log('Token:', token);
  })
  .catch(([3]) => {
    console.error('Failed to create token:', error);
  });

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

AcreateCustomToken

Bclaims

Cerror

DadditionalClaims

Attempts:

3 left

💡 Hint

Common Mistakes

Using claims instead of additionalClaims as variable name

Not naming the catch parameter error

Use createCustomToken(uid, additionalClaims) and catch errors with a parameter named error.

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