Firebase Cloud Function to send a push notification everytime you have a new entry in your Cloud Storage

index.js

// The Cloud Functions for Firebase SDK to create Cloud Functions and setup triggers.
const functions = require('firebase-functions');

// The Cloud Functions for Firebase SDK to access database
const admin = require('firebase-admin');

// Init
admin.initializeApp(functions.config().firebase);

/**
 * Create a 'sendPushNotification' function triggered
 * everytime a new item is create in storage.
 */
exports.sendPushNotification = functions.storage.object()
.onFinalize(event => {
    const payload = {
        notification: {
            body : 'Hello world !',
            badge : '1',
            sound : 'default'
        }
    }

    return admin.database().ref('REF_TO_FCM_TOKENS').once('value').then(tokens => {
        if(!tokens.val()) {
            return;
        }
        const token = Object.keys(tokens.val());
        return admin.messaging().sendToDevice(token, payload)
        .then(function(response) {
            return 'Notification sent: ', response;
        })
        .then(function(error) {
            throw('Notification sent: ', error);
        })
    })
})