Push Notifications with Firebase

Real-world analogy

What is it?

Push notifications let your server send messages directly to a user's device (phone or browser) without the user needing to be on your app. Firebase Cloud Messaging (FCM) is the most popular service for this.

Real-world relevance

Every app you use sends push notifications — WhatsApp messages, Instagram likes, Uber ride updates, news alerts. FCM handles billions of notifications daily for free.

Key points

• Firebase Cloud Messaging (FCM) — FCM is Google's free push notification service for Android, iOS, and web. It handles delivery infrastructure — just call an API with the message and target. Delivers billions of messages daily at no cost.
• Device Tokens — When a user installs your app, FCM assigns a unique device token. Store tokens in your database linked to each user. Provide the token when sending and FCM routes it to the exact device. Refresh regularly.
• Topic Subscriptions — Users subscribe to topics like news, deals, or sports. One API call to a topic delivers to all subscribers automatically. Perfect for broadcasting announcements, breaking news, or promotions to large groups.
• Background Processing — Send notifications through a Bull queue instead of directly in your API handler. Queue the job, return instantly, let a background worker call FCM. Keeps your API responsive even for thousands of messages.
• Data Payloads — Notifications carry custom data beyond title and body. Include fields like screen: orderDetails and orderId: 123 so tapping navigates to the right screen. Data payloads create rich, interactive experiences.
• Rich Notifications — Go beyond plain text with images, action buttons, and expandable content. Show product images, add Reply and Mark as Read buttons, or display delivery progress. Rich notifications boost user engagement.
• Notification Channels — Android requires notification channels so users control which types they receive. Create channels for messages, updates, and promotions with different importance levels. High-priority wakes the screen silently.
• Token Management — FCM tokens become invalid when users uninstall, clear data, or switch devices. Handle refresh events in your client app and update the server. Remove stale tokens when FCM returns errors to keep data clean.
• Notification Analytics — Track delivery rates, open rates, and click-through rates to measure engagement. FCM provides delivery receipts. Use this data to optimize send times, personalize content, and stop sending what users ignore.
• Silent Notifications — Send data-only messages without visible alerts to trigger background work. Use silent notifications to sync data, update cache, or refresh badge counts. Users see nothing, but your app stays current when opened.

Code example

// 1. Initialize Firebase Admin SDK 🔧
import * as admin from 'firebase-admin';

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

// 2. Send to a specific device 📱
async function sendToDevice(token: string, title: string, body: string) {
  await admin.messaging().send({
    token: token, // device's unique FCM token
    notification: {
      title,
      body,
    },
    data: {
      // Custom data your app can read
      screen: 'notifications',
      actionId: '12345',
    },
  });
}

// 3. Send to a topic (all subscribers) 📢
async function sendToTopic(topic: string, title: string, body: string) {
  await admin.messaging().send({
    topic: topic, // e.g., 'new-properties', 'deals'
    notification: { title, body },
  });
}

// 4. In NestJS — queue it! 🎯
@Injectable()
export class NotificationService {
  constructor(
    @InjectQueue('notifications') private notifQueue: Queue
  ) {}

  async notifyUser(userId: string, message: string) {
    const user = await this.prisma.user.findUnique({
      where: { id: userId },
      select: { fcmTokens: true },
    });

    // Queue notification for each device
    for (const token of user.fcmTokens) {
      await this.notifQueue.add('push', {
        token,
        title: 'New Update!',
        body: message,
      });
    }
  }
}

Line-by-line walkthrough

1. 1. 1. Initialize Firebase Admin SDK 🔧
2. 2. Importing required dependencies
3. 3.
4. 4.
5. 5.
6. 6.
7. 7.
8. 8. 2. Send to a specific device 📱
9. 9. Declaring a function
10. 10. Waiting for an async operation to complete
11. 11.
12. 12.
13. 13.
14. 14.
15. 15. Closing block
16. 16.
17. 17. Custom data your app can read
18. 18.
19. 19.
20. 20. Closing block
21. 21.
22. 22. Closing block
23. 23.
24. 24. 3. Send to a topic (all subscribers) 📢
25. 25. Declaring a function
26. 26. Waiting for an async operation to complete
27. 27.
28. 28.
29. 29.
30. 30. Closing block
31. 31.
32. 32. 4. In NestJS — queue it! 🎯
33. 33. Decorator that adds metadata or behavior
34. 34. Exporting for use in other files
35. 35.
36. 36. Decorator that adds metadata or behavior
37. 37.
38. 38.
39. 39.
40. 40. Declaring a variable
41. 41.
42. 42.
43. 43.
44. 44.
45. 45. Queue notification for each device
46. 46. Loop iteration
47. 47. Waiting for an async operation to complete
48. 48.
49. 49.
50. 50.
51. 51.
52. 52. Closing block
53. 53. Closing block
54. 54. Closing block

Spot the bug

async sendNotification(userId: string, msg: string) {
  const user = await this.prisma.user.findUnique({ where: { id: userId } });
  await admin.messaging().send({
    token: user.fcmToken,
    notification: { title: "Update", body: msg },
  });
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Firebase Cloud Messaging (Firebase Official)
• FCM Architecture Overview (Firebase Official)
• Firebase Push Notifications (Fireship)