Answer to https://twitter.com/gabe_ragland/status/1261006295806562304

create-customer.js

const stripe = require("stripe")(process.env.STRIPE_SECRET_KEY);
const requireAuth = require("./_require-auth.js");
const { updateUser } = require("./_db.js");

export default requireAuth((req, res) => {
  const user = req.user;

  // If user already has a stripeCustomerId then return it in success response
  if (user.stripeCustomerId) {
    return res.send({
      status: "success",
      data: stripeCustomerId,
    });
  }

  // Check if there's already a customer in Stripe with this email
  // We use the email from the JWT token (decoded by requireAuth middleware)
  return getExistingCustomer(user.email)
    .then((existingCustomer) => {
      // If there's an existing customer then return it in success response
      if (existingCustomer) {
        return res.send({
          status: "success",
          data: existingCustomer.id,
        });
      }
      // Create a new Stripe customer
      return stripe.customers.create({
        email: user.email,
      })
        .then((customer) => {
          // Update user in database with Stripe customer.id and pass customer object to next step
          return updateUser(user.uid, {
            stripeCustomerId: customer.id,
          }).then(() => customer);
        })
        .then((customer) => {
          // Return success response
          res.send({
            status: "success",
            data: customer.id,
          });
        })
    })
    .catch((error) => {
      // Handle errors and return error response
      res.send({
        status: "error",
        code: error.code,
        message: error.message,
      });
    });
});

// Fetch a customer from Stripe by email
function getExistingCustomer(email) {
  // Stripe allows multiple customers with the same email so we
  // limit to 1 and then return first customer in response.
  return stripe.customers
    .list({
      email: email,
      limit: 1,
    })
    .then((response) => {
      return response.data.length && response.data[0];
    });
}