Firebase Cloud Functions get updated document Details - node.js

Question

I'm trying to write a cloud function where if my app changed some string in user's firestore DB. A cloud function need to send a push notification. Database architecture is Messages => {UID} => UpdatedMessages . The problem is I cannot figure How to retrive which updateMessage under which UID has been updated.
const functions = require('firebase-functions');
const admin = require('firebase-admin')
admin.initializeApp()
const toUpperCase = (string)=> string.toUpperCase()
var registrationToken = 'dfJY6hYzJyE:APdfsdfsdddfdfGt9HMfTXmei4QFtO0u1ePVpNYaOqZ1rnDpB8xfSjx7-G6tFY-vWQY3vDPEwn_iZVK2PrsGUVB0q9L_QoRYpLJ3_6l1SVHd_0gQxJb_Kq-IBlavyJCkgkIZ';
exports.sendNotification = functions.firestore
.document('messages/{userId}/{updatedMessage}')
.onUpdate((change, context) => {
var message = {
data: {
title: 'Update',
body: 'New Update'
},
token: registrationToken
};
// Send a message to the device corresponding to the provided
// registration token.
admin.messaging().send(message)
.then((response) => {
// Response is a message ID string.
console.log('Successfully sent messagesssss:', response);
})
.catch((error) => {
console.log('Error sending message:', error);
});
});
Simply I need to rerive "var registrationToken" from UID .

Answer 1

You have to use the params property of the context object as follows
exports.sendNotification = functions.firestore
.document('messages/{userId}/{updatedMessage}')
.onUpdate((change, context) => {
const userId = context.params.userId;
const updatedMessage = context.params.updatedMessage;
var message = {
data: {
title: 'Update',
body: updatedMessage //For example, use the value of updatedMessage here
},
//...
};
//IMPORTANT: don't forget to return the promise returned by the asynchronous send() method
return admin.messaging().send(message)
.then((response) => {
// Response is a message ID string.
console.log('Successfully sent messagesssss:', response);
return null;
})
.catch((error) => {
console.log('Error sending message:', error);
return null;
});
});
See https://firebase.google.com/docs/functions/firestore-events#wildcards-parameters and https://firebase.google.com/docs/reference/functions/functions.EventContext#params for more info.
About the remark in the above code noted as "IMPORTANT", you may watch the official Firebase video series here: https://firebase.google.com/docs/functions/video-series/. In particular watch the three videos titled "Learn JavaScript Promises" (Parts 2 & 3 especially focus on background triggered Cloud Functions, but it really worth watching Part 1 before).