reference error event is not defined node.js

Question

I'm working with Notifications but the Notification is not working and getting this error, I am new to this so i can't fully understand this and getting tilted by the error. Any help would be appreciated, I don't find any related solutions about this

ReferenceError: event is not defined
at exports.sendNotification.functions.database.ref.onWrite (/user_code/index.js:9:31)
at cloudFunctionNewSignature (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:105:23)
at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:135:20)
at /var/tmp/worker/worker.js:744:24
at process._tickDomainCallback (internal/process/next_tick.js:135:7)

and Here is my index.js

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

exports.sendNotification = 

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

    const user_id = event.params.user_id;
    const notification = event.params.notification;

    console.log('We have a notification to send to: ', user_id);

        if(!event.data.val())
        {
            return console.log('A Notification has been deleted from database: ', notification_id);
        }

        const fromUser = admin.database().ref(`/notifications/${user_id}/${notification_id}`);
        return fromUser.then(fromUserResult => {

            const from_user_id = fromUserResult.val().from;

            console.log('You have new Notification from: ', from_user_id);

            const userQuery = admin.database().ref(`Users/${from_user_id}/name`).once('value');
            const deviceToken = admin.database().ref(`/Users/${user_id}/device_token`).once('value');

            return Promise.all([userQuery, deviceToken]).then(result => {


                const userName = result[0].val();
                const token_id = result[1].val();

                    const payload = {
                      notification: {
                        title: "Message Request",
                        body: `${userName} has sent you a Message Request`,
                        icon: "logo.png",
                        click_action: "com.example.gab.quadrantms_TARGET_NOTIFICATION"
                    },
                    data : {
                        from_user_id : from_user_id
                    }
                };

                return admin.messaging().sendtoDevice(token_id, payload).then(response =>{

                    console.log('This was the Notification Feature');
                    return true;
                });
            });
        });
    });

Answer 1

Just as the error says, event isn't defined anywhere. It looks like you should be using context instead.

const user_id = context.params.user_id;
const notification = context.params.notification;

Here are the docs for EventContext: https://firebase.google.com/docs/reference/functions/functions.EventContext

And RefBuilder.onWrite: https://firebase.google.com/docs/reference/functions/functions.database.RefBuilder#onWrite