Add Stripe Customer Id to Auth0 user's metadata, and Auth0 id to Stripe Customer's metadata via custom rule

rule.js

function (user, context, callback) {
    user.app_metadata = user.app_metadata || {};
    
    // Checking if the Auth0 User already has a stripe customer id with it's metadata
    if ('stripe_customer_id' in user.app_metadata) {
        console.log(user);
        return callback(null, user, context);
    }

    var stripe = require('stripe')('sk_test_tyd8...');
    
    // Dict for creating Stripe Customer, with Auth0 User id
    var customer = {
        name: user.name,
        email: user.email,
        metadata: {
            'auth0_user_id': user.user_id,
        },
    };

    stripe.customers.create(customer, function (err, customer) {
        if (err) {
            return callback(err);
        }

        user.app_metadata.stripe_customer_id = customer.id;

        auth0.users.updateAppMetadata(user.user_id, user.app_metadata)
            .then(function () {
                console.log(user);
                callback(null, user, context);
            })
            .catch(function (err) {
                callback(err);
            });
    });
}