Firebase Admin SDK (Node.js) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What is the output of this Firebase Admin SDK code snippet?

Consider this Node.js code using Firebase Admin SDK to fetch a user by UID:

import admin from 'firebase-admin';

admin.initializeApp();

async function getUser() {
  try {
    const userRecord = await admin.auth().getUser('nonexistentUID');
    console.log(userRecord.uid);
  } catch (error) {
    console.log(error.code);
  }
}

getUser();

What will be printed to the console?

Firebase

import admin from 'firebase-admin';

admin.initializeApp();

async function getUser() {
  try {
    const userRecord = await admin.auth().getUser('nonexistentUID');
    console.log(userRecord.uid);
  } catch (error) {
    console.log(error.code);
  }
}

getUser();

A SyntaxError

B auth/user-not-found

C undefined

D null

💡 Hint

Think about what happens when you request a user that does not exist in Firebase Authentication.

The Firebase Admin SDK throws an error with code auth/user-not-found when trying to get a user that does not exist. The catch block logs this error code.

❓ Configuration

intermediate

Which option correctly initializes Firebase Admin SDK with a service account JSON file?

You want to initialize Firebase Admin SDK in Node.js using a service account JSON file located at ./serviceAccountKey.json. Which code snippet correctly does this?

A

const admin = require('firebase-admin');
const serviceAccount = require('./serviceAccountKey.json');

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount)
});

B

import admin from 'firebase-admin';

admin.initializeApp({
  credential: admin.credential.applicationDefault()
});

C

import admin from 'firebase-admin';
import serviceAccount from './serviceAccountKey.json' assert { type: 'json' };

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount)
});

D

import admin from 'firebase-admin';

admin.initializeApp({
  credential: admin.credential.cert('./serviceAccountKey.json')
});

💡 Hint

Use ES modules import syntax and ensure the JSON file is imported correctly.

In ES modules, JSON files must be imported with assert { type: 'json' }. Then pass the imported JSON object to admin.credential.cert(). Option C does this correctly.

❓ Architecture

advanced

Which Firebase Admin SDK feature best supports sending notifications to multiple devices efficiently?

You want to send a push notification to thousands of devices using Firebase Admin SDK in Node.js. Which approach is best to optimize performance and cost?

A Call <code>admin.messaging().send()</code> individually for each device token in a loop.

B Send notifications via Firebase Realtime Database triggers only.

C Use <code>admin.messaging().sendToDevice()</code> with a single token repeatedly.

D Use <code>admin.messaging().sendMulticast()</code> to send to up to 500 tokens at once.

💡 Hint

Look for a method that supports batch sending to multiple tokens.

sendMulticast() allows sending a message to up to 500 device tokens in one call, which is efficient for large scale notifications. Sending individually (A) is inefficient. Option D is unrelated to sending messages directly. Option D sends to one token repeatedly, which is inefficient.

❓ security

advanced

What is the safest way to manage Firebase Admin SDK credentials in a production Node.js app?

You deploy a Node.js app using Firebase Admin SDK. Which practice ensures your service account credentials remain secure?

A Store the service account JSON file in environment variables or a secure secrets manager and load it at runtime.

B Commit the service account JSON file to the public GitHub repository for easy access.

C Hardcode the service account credentials directly in the source code.

D Share the service account JSON file via email to all developers.

💡 Hint

Think about how to keep sensitive data out of source code and public repos.

Storing credentials in environment variables or a secure secrets manager keeps them out of source code and version control, reducing risk of leaks. Committing to public repos or hardcoding exposes credentials. Sharing via email is insecure.

🧠 Conceptual

expert

What error will this Firebase Admin SDK code produce and why?

Analyze this Node.js code snippet:

import admin from 'firebase-admin';

admin.initializeApp();

async function createUser() {
  const user = await admin.auth().createUser({
    email: '[e-mail]',
    emailVerified: true,
    password: 'short',
  });
  console.log(user.uid);
}

createUser();

What error will occur when running this code and why?

Firebase

import admin from 'firebase-admin';

admin.initializeApp();

async function createUser() {
  const user = await admin.auth().createUser({
    email: '[e-mail]',
    emailVerified: true,
    password: 'short',
  });
  console.log(user.uid);
}

createUser();

A auth/invalid-password - Password must be at least 6 characters long.

B auth/email-already-exists - Email is already registered.

C auth/invalid-email - Email format is incorrect.

D No error, user is created successfully.

💡 Hint

Check Firebase password requirements for creating users.

Firebase requires passwords to be at least 6 characters. The password 'short' has 5 characters, so auth/invalid-password error is thrown.

Practice

1. What is the primary purpose of the Firebase Admin SDK in Node.js?

easy

A. To build user interfaces for Firebase apps

B. To securely manage Firebase services from a server environment

C. To run Firebase apps directly in the browser

D. To replace Firebase client SDK on mobile devices

Solution

1. Step 1: Understand Firebase Admin SDK role

   The Admin SDK is designed for server-side use to manage Firebase services securely.

2. Step 2: Differentiate from client SDKs

   Client SDKs run on user devices for app interaction, while Admin SDK runs on servers for management tasks.

3. Final Answer:

   To securely manage Firebase services from a server environment -> Option B

4. Quick Check:

   Admin SDK = server management [OK]

Hint: Admin SDK is for backend server tasks, not frontend apps [OK]

Common Mistakes:

• Confusing Admin SDK with client SDK
• Thinking Admin SDK runs in browsers
• Assuming Admin SDK builds UI

2. Which of the following is the correct way to initialize the Firebase Admin SDK in Node.js using a service account JSON file named serviceAccountKey.json?

easy

A. const admin = require('firebase-admin');
admin.initializeApp({credential: admin.credential.cert(require('./serviceAccountKey.json'))});

B. const admin = require('firebase-admin');
admin.initializeApp({serviceAccount: 'serviceAccountKey.json'});

C. const admin = require('firebase-admin');
admin.initializeApp({credential: admin.credential.applicationDefault()});

D. const admin = require('firebase-admin');
admin.initializeApp({credential: admin.credential.cert(serviceAccountKey.json)});

Solution

1. Step 1: Load service account JSON correctly

   The service account JSON must be imported using require to pass the object, not a string path.

2. Step 2: Use admin.credential.cert with the JSON object

   Pass the imported JSON object to admin.credential.cert for proper initialization.

3. Final Answer:

   const admin = require('firebase-admin'); admin.initializeApp({credential: admin.credential.cert(require('./serviceAccountKey.json'))}); -> Option A

4. Quick Check:

   Use require() to load JSON for cert [OK]

Hint: Use require() to load JSON, not a string path [OK]

Common Mistakes:

• Passing file path string instead of JSON object
• Using applicationDefault() when service account is needed
• Incorrect property name like 'serviceAccount'

3. Given the following Node.js code using Firebase Admin SDK, what will be the output if the user with UID 'user123' exists?

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

admin.auth().getUser('user123')
  .then(userRecord => console.log(userRecord.uid))
  .catch(error => console.log('Error:', error.message));

medium

A. user123

B. Error: User not found

C. undefined

D. null

Solution

1. Step 1: Understand getUser() behavior

   getUser() returns a Promise resolving to a user record if the user exists.

2. Step 2: Check the then() callback output

   The code logs userRecord.uid, which will be 'user123' if the user exists.

3. Final Answer:

   user123 -> Option A

4. Quick Check:

   getUser() returns user data, logs UID [OK]

Hint: getUser() returns user info; log uid to see user ID [OK]

Common Mistakes:

• Assuming getUser() returns error if user exists
• Expecting undefined or null instead of UID
• Not handling Promise correctly

4. Identify the error in the following Firebase Admin SDK Node.js code snippet:

const admin = require('firebase-admin');
admin.initializeApp({
  credential: admin.credential.cert(serviceAccountKey.json)
});

admin.auth().createUser({
  email: '[e-mail]',
  password: 'secret123'
}).then(userRecord => console.log(userRecord.uid));

medium

A. initializeApp() cannot accept credential property

B. Password must be at least 10 characters

C. createUser() requires a displayName field

D. The service account JSON file path should be required, not passed as a string

Solution

1. Step 1: Check initialization of credential

   admin.credential.cert() expects a JSON object, not a string path. The file must be loaded with require().

2. Step 2: Validate other parameters

   Password length and displayName are optional or valid; initializeApp() accepts credential property.

3. Final Answer:

   The service account JSON file path should be required, not passed as a string -> Option D

4. Quick Check:

   Use require() to load JSON for cert [OK]

Hint: Load JSON with require(), not string path [OK]

Common Mistakes:

• Passing string path instead of JSON object
• Assuming password length error
• Thinking displayName is mandatory

5. You want to send a push notification to a device using Firebase Admin SDK in Node.js. Which sequence of steps correctly applies to achieve this?

hard

A. Use Firebase client SDK to send notification directly from server without Admin SDK

B. Initialize Admin SDK with credentials, call admin.auth().sendNotification(token, message), then handle response

C. Initialize Admin SDK with credentials, build a message object with token and notification, then call admin.messaging().send(message)

D. Initialize Admin SDK without credentials, build message, then call admin.database().send(message)

Solution

1. Step 1: Initialize Admin SDK properly

   Admin SDK must be initialized with credentials to access Firebase services securely.

2. Step 2: Create message object and send

   Build a message with device token and notification details, then use admin.messaging().send() to send it.

3. Final Answer:

   Initialize Admin SDK with credentials, build a message object with token and notification, then call admin.messaging().send(message) -> Option C

4. Quick Check:

   Use admin.messaging().send() with message object [OK]

Hint: Use admin.messaging().send() with message object and token [OK]

Common Mistakes:

• Using auth() instead of messaging() for notifications
• Trying to send without initializing SDK
• Using database() to send notifications