Firebase javascript function to send push notification when a new item added in firestore

pushNotifiations.js

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

const admin = require("firebase-admin");
admin.initializeApp();

exports.notifyNewMessage = functions.firestore
    .document('365BetaCannels2/{chanl}/thrad/{messe}')
    .onCreate((docSnapshot, context) => {
      const message = docSnapshot.data();
      const recipientId = message['recipientID'];
      const senderFirstName = message['senderFirstName'];
      const senderLastName = message['senderLastName'];


      return admin.firestore().doc('PhowUsers/' + recipientId)
        .get().then(userDoc => {
            const registrationTokens = userDoc.get('firebaseCloudMessagingTokens');
            const notificationBody = (message['content']) ?  message['content'] :  "You received a new image message" ;
            const payload = {
              notification: {
                title: senderFirstName + " " + senderLastName + " sent you a message.",
                body: notificationBody,
                clickAction: "ChatActivity"
              },
              data: {
                USER_NAME: senderFirstName,
                USER_ID: message['senderID']
              }
            };

            return admin.messaging().sendToDevice(registrationTokens, payload).then(
              response => {
                const stillRegisteredTokens = registrationTokens;

                response.results.forEach((result, index) => {
                  const error = result.error;
                    if (error) {
                      const failedToken =  registrationTokens[index];
                      console.error('invalid token will be removed automatically', failedToken, error);
                      if (error.code === 'messaging/invalid-registration-token' || error.code ==='messaging/registration-token-not-registered'){
                            const failedIndex = stillRegisteredTokens.indexOf(failedToken);
                            if (failedIndex > -1) {
                                stillRegisteredTokens.splice(failedIndex, 1);
                            }
                      }
                    }
                });
                return admin.firestore().doc("PholowUsers/" + recipientId).update({
                    firebaseCloudMessagingTokens: stillRegisteredTokens
                });
            });
        });
    });