cloud-functions.js

var functions = require('firebase-functions');
var admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.sendNotificationOnEventComment 
    = functions.database.ref('/eventComments/{eventUid}/{eventCommentUid}').onWrite(event => {

        if (event.data.exists()) {

            var eventUid = event.params.eventUid;
            var eventCommentUid = event.params.eventCommentUid;
            console.log("event id is", eventUid);
            console.log("event comment id is", eventCommentUid);
            console.log("event comment is", event.data.val());            
            
            var comments = event.data.val();
            var commenter = comments.senderName;
            console.log("commenter is", commenter);

            var payload = {
                notification : {
                    title : ""
                }
            };            

            var eventPromise = admin.database().ref('/events/' + eventUid).once("value");
            
            eventPromise
                .then(function (resultSnap) {
                    if (resultSnap.exists()) {
                        var eventData = resultSnap.val();
                        console.log("event is", eventData);            
                        return eventData;
                    }
                })
                .then(function (eventData){
                    if (eventData) {
                        payload.notification.title = "New comment on event " 
                            + eventData.name + " by " + commenter;
                        console.log("event payload", payload.notification.title);                                        
                        var invitees = eventData.invitees;

                        console.log("invitees are ", invitees);                                                                
                        var fcmTokenPromises = invitees.map(retrieveFcmToken);

                        return Promise.all(fcmTokenPromises).then(function(results){
                                
                            console.log("token promise result ", results);                                                                                    
                            tokens = [];
                            results.forEach(function(item){
                                if (item.val()) {
                                    tokens.push(item.val());
                                    console.log('token', item.val());
                                }
                            });
                            

                        admin.messaging().sendToDevice(tokens, payload)
                            .then(function(response) {
                                console.log("Successfully sent message:", response);
                            })
                            .catch(function(error) {
                                console.log("Error sending message:", error);
                            });

                        })
                        .catch(function (err){
                                console.log("error is", err);
                        });
                    }
                });
        }

    });

    var retrieveFcmToken = function(Uid) {
        return admin.database().ref('/users/' + Uid + '/fcmToken').once('value');
                
    }