Firebase Cloud Function to handle online/offline status

cloudfunction.js

const functions = require('firebase-functions');
const Firestore = require('@google-cloud/firestore');

const firestore = new Firestore();

exports.onUserStatusChanged = functions.database
  .ref('/status/{userId}') // Reference to the Firebase RealTime database key
  .onUpdate(event => {
    const usersRef = firestore.collection('/users'); // Create a reference to the Firestore Collection
  
    return event.data.ref.once('value')
      .then(statusSnapshot => snapShot.val()) // Get the latest value from the Firebase Realtime database
      .then(status => {
        // check if the value is 'offline'
        if (status === 'offline') {
          // Set the Firestore's document's online value to false
          usersRef
            .doc(event.params.userId)
            .set({
              online: false
            }, { merge: true });
        }
      })
  });

I have used above code.
but I am getting an Error
TypeError: Cannot read property 'userId' of undefined
at exports.onUserStatusChanged.functions.database.ref.onUpdate.event

please help me.