How to fix messaging/UNREGISTERED: App instance unregistered from FCM in Firebase

// Node.js Admin SDK
const admin = require('firebase-admin');

async function sendNotification(token, payload) {
  try {
    const response = await admin.messaging().send({
      token: token,
      notification: payload,
    });
    console.log('Successfully sent message:', response);
    return { success: true };
  } catch (error) {
    if (error.code === 'messaging/registration-token-not-registered' ||
        error.code === 'messaging/invalid-argument') {
      console.log('Token is unregistered, removing from database');
      await removeTokenFromDatabase(token);
      return { success: false, removed: true };
    }
    throw error;
  }
}

async function removeTokenFromDatabase(token) {
  // Example using a generic database
  await db.collection('fcm_tokens').deleteOne({ token: token });
  console.log(`Removed invalid token: ${token}`);
}

// Or with PostgreSQL
async function removeTokenFromDatabase(token) {
  await pool.query('DELETE FROM fcm_tokens WHERE token = $1', [token]);
}

// When saving a new token
async function saveToken(userId, token) {
  await db.collection('fcm_tokens').updateOne(
    { userId: userId },
    {
      $set: {
        token: token,
        lastSeen: new Date(),
        createdAt: new Date(),
      }
    },
    { upsert: true }
  );
}

// Update lastSeen when app opens
async function updateTokenActivity(token) {
  await db.collection('fcm_tokens').updateOne(
    { token: token },
    { $set: { lastSeen: new Date() } }
  );
}

// Cloud Function running daily
exports.cleanupStaleTokens = functions.pubsub
  .schedule('every 24 hours')
  .onRun(async (context) => {
    const staleDate = new Date();
    staleDate.setDate(staleDate.getDate() - 250); // 250 days ago

    const staleTokens = await db.collection('fcm_tokens')
      .where('lastSeen', '<', staleDate)
      .get();

    const batch = db.batch();
    staleTokens.forEach((doc) => {
      batch.delete(doc.ref);
    });

    await batch.commit();
    console.log(`Removed ${staleTokens.size} stale tokens`);
  });

// React Native / JavaScript
import messaging from '@react-native-firebase/messaging';

async function checkAndRefreshToken() {
  const authStatus = await messaging().requestPermission();

  if (authStatus === messaging.AuthorizationStatus.AUTHORIZED) {
    const token = await messaging().getToken();

    // Send to your server
    await fetch('https://yourserver.com/api/tokens', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({
        userId: currentUserId,
        token: token,
        platform: Platform.OS
      }),
    });
  }
}

// Call on app startup
useEffect(() => {
  checkAndRefreshToken();
}, []);

// React Native
messaging().onTokenRefresh(async (newToken) => {
  console.log('Token refreshed:', newToken);

  await fetch('https://yourserver.com/api/tokens/refresh', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      userId: currentUserId,
      token: newToken
    }),
  });
});

// Android
FirebaseMessaging.getInstance().getToken()
    .addOnCompleteListener(task -> {
        if (task.isSuccessful()) {
            String token = task.getResult();
            sendTokenToServer(token);
        }
    });