Firebase functions: cannot read property 'user_id' of undefined

You need to install the latest firebase-functions and firebase-admin:

npm install firebase-functions@latest firebase-admin@latest --save
npm install -g firebase-tools

to be able to use the new API, check here for more info:

https://firebase.google.com/docs/functions/get-started#set_up_and_initialize

Change this:

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

exports.sendNotification = functions.database.ref('/notifications/{user_id}').onWrite((event) => {

console.log('Testing stuff', event.params.user_id);

into this:

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

exports.sendNotification = functions.database.ref('/notifications/{user_id}').onWrite((change, context) => {

console.log('Testing stuff', context.params.user_id);

For onWrite and onUpdate events, the data parameter has before and after fields. Each of these is a DataSnapshot with the same methods available in admin.database.DataSnapshot

params

An object containing the values of the wildcards in the path parameter provided to the ref() method for a Realtime Database trigger.

more info here:

Cloud functions v1.0 Changes

EventContext#params

Change