A cloud function which sends data notification to all users stored in Firestore.

index.js

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

// // Create and Deploy Your First Cloud Functions
// // https://firebase.google.com/docs/functions/write-firebase-functions
//
// exports.helloWorld = functions.https.onRequest((request, response) => {
//  response.send("Hello from Firebase!");
// });

// Import the Firebase SDK for Google Cloud Functions.
const functions = require('firebase-functions');
// Import and initialize the Firebase Admin SDK.
const admin = require('firebase-admin');
admin.initializeApp();

exports.sendNotificationApi = functions.https.onRequest((req, res) => {
    sendMessage().then(val => {
        res.status(200).send(val);
    }).catch(val => {
        res.status(500).send(val);
    });
});

async function sendMessage() {
    // Notification details.
    const payload = {
        data: {
            title: "Cloud Messaging",
            message: "Open the app right now, please.",
        }
    };

    // Get the list of device tokens.
    const allTokens = await admin.firestore().collection('fcmTokens').get();
    const tokens = [];
    allTokens.forEach((tokenDoc) => {
        tokens.push(tokenDoc.id);
    });

    if (tokens.length > 0) {
        // Send notifications to all tokens.
        const response = await admin.messaging().sendToDevice(tokens, payload);
        await cleanupTokens(response, tokens);
        console.log('Notifications have been sent and tokens cleaned up.');
    }

    return "OK";
}

// Cleans up the tokens that are no longer valid.
function cleanupTokens(response, tokens) {
    // For each notification we check if there was an error.
    const tokensDelete = [];
    response.results.forEach((result, index) => {
        const error = result.error;
        if (error) {
            console.error('Failure sending notification to', tokens[index], error);
            // Cleanup the tokens who are not registered anymore.
            if (error.code === 'messaging/invalid-registration-token' ||
                error.code === 'messaging/registration-token-not-registered') {
                const deleteTask = admin.firestore().collection('messages').doc(tokens[index]).delete();
                tokensDelete.push(deleteTask);
            }
        }
    });
    return Promise.all(tokensDelete);
}