index.js

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

admin.initializeApp(functions.config().firebase);

const firestore = admin.firestore();
const settings = {timestampInSnapshots: true};
firestore.settings(settings);

const stripe = require('stripe')(functions.config().stripe.token);
const currency = functions.config().stripe.currency || 'USD';

// Adds a customer's card information to Stripe for later billing.
// Returns the encrypted card information for processing on our end.
exports.addStripeSource = functions.firestore
    .document('cards/{userId}/tokens/{tokenId}')
    .onCreate(async (tokenSnap, context) => {
        var customer;
        const data = tokenSnap.data();
        if(data == null) {
            return null
        }
        const token = data.tokenId;
        const snapshot = await firestore.collection('cards')
                            .doc(context.params.userId).get();
        const customerId = snapshot.data().custId;
        const customerEmail = snapshot.data().email;

        if(customerId == 'new') {
            customer = await stripe.customers.create({
                email: customerEmail,
            });
            firestore.collection('cards').doc(context.params.userId).update({
                custId: customer.id
            });
        }

        else { 
            customer = await stripe.customers.retrieve(customerId);
        }

        const customerSource = await stripe.customers
                                .createSource(customer.id, {source: token});

        return firestore.collection('cards').doc(context.params.userId)
            .collection('sources').doc(customerSource.fingerprint)
            .set(customerSource, {merge: true});
    }
)

// When a user deletes their account, clean up after them
exports.cleanupUser = functions.auth.user().onDelete(async (user) => {
    const snapshot = await admin.firestore().collection('cards')
                        .doc(user.uid).get();
    const customer = snapshot.data();
    await stripe.customers.del(customer.custId);
    firestore.collection('users').doc(user.email).delete();
    return firestore.collection('cards').doc(user.uid).delete();
  });

does this work for you

becuase for me it says the below error caan u please fix this out for me
TypeError: Cannot read property 'custId' of undefined
at exports.addStripeSource.functions.firestore.document.onCreate (/srv/index.js:21:43)