Add Stripe Customer Id to Auth0 via custom rule

rule.js

function (user, context, callback) {
  user.app_metadata = user.app_metadata || {};

  if ('stripe_customer_id' in user.app_metadata) {
    context.idToken['https://example.com/stripe_customer_id'] = user.app_metadata.stripe_customer_id;
    return callback(null, user, context);
  }

  var stripe = require('stripe')('sk_....');
  var customer = {
    email: user.email
  };
  
  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() {
        context.idToken['https://example.com/stripe_customer_id'] = user.app_metadata.stripe_customer_id;
        callback(null, user, context);
      })
      .catch(function(err) {
        callback(err);
      });
  });
}