Why Custom authentication tokens in Firebase? - Purpose & Use Cases

The Big Idea

What if you could give users just the right access without endless code and mistakes?

The Scenario

Imagine you have a website where users log in, but you want to give some users special access based on their roles or external system data. You try to manage all these permissions by hand, checking user info every time they do something.

The Problem

Doing this manually means writing lots of repeated code everywhere. It's easy to make mistakes, forget to check permissions, or expose sensitive data. Also, updating user roles or syncing with other systems becomes a slow, confusing mess.

The Solution

Custom authentication tokens let you create secure, trusted tokens that include exactly the user info and permissions you want. These tokens are generated once and then used safely by your app, making user access smooth and reliable.

Before vs After

✗ Before

if (user.role === 'admin') { allowAccess(); } else { denyAccess(); }

✓ After

const token = createCustomToken(userId, { role: 'admin' });

What It Enables

You can securely control user access and integrate external data into your app's login system without messy, repeated checks.

Real Life Example

A company uses custom tokens to let employees log in with their company ID and automatically get access to the right internal tools based on their department.

Key Takeaways

Manual permission checks are slow and error-prone.

Custom tokens package user info securely for easy access control.

This makes apps safer and simpler to manage.

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