Integrating Polar with Express.js

In this guide, we'll show you how to integrate Polar with Express.js.

tip

Consider following this guide while using the Polar Sandbox Environment. This will allow you to test your integration without affecting your production data. You can find the Sandbox environment here.

Install the Polar JavaScript SDK

To get started, you need to install the Polar JavaScript SDK. You can do this by running the following command:

pnpm install @polar-sh/sdk

Setting up environment variables

Polar Access Token

To authenticate with Polar, you need create an access token, and supply it to Node.js using a POLAR_ACCESS_TOKEN environment variable. You can create a personal access token on the Polar account settings page.

Polar Organization ID

Products are tied to organizations, not your personal account. You need to supply the organization ID to Node.js using a POLAR_ORGANIZATION_ID environment variable. Organization IDs for a given organization can be found on the organization's settings page.

# .env
POLAR_ACCESS_TOKEN="polar_pat..."
POLAR_ORGANIZATION_ID="********-****-****-****-************"

Configuring a Polar API Client

To interact with the Polar API, you need to create a new instance of the Polar class. This class uses the provided access token to authenticate with the Polar API.

// src/polar.ts
import { Polar } from "@polar-sh/sdk"

export const api = new Polar({
    accessToken: process.env.POLAR_ACCESS_TOKEN!,
    server: 'sandbox' // Use this option if you're using the sandbox environment - else use 'production' or omit the parameter
})

Remember to replace sandbox with production when you're ready to switch to the production environment.

Checkout Sessions

The first step is to create a Checkout session. For this you'll need at least your Product Price ID.

tip

You can retrieve your Product Price ID from Products in your dashboard, click on the ellipsis … button in front of your product and click on Copy Price ID.

The API will return you an object containing all the information about the session, including an URL where you should redirect your customer so they can complete their order.

import { Polar } from '@polar-sh/sdk'
import express, { Request, Response } from 'express'

const app = express()
const port = 3000
const polar = new Polar({
  accessToken: process.env['POLAR_ACCESS_TOKEN'] ?? '',
})

app.get('/checkout', async (req: Request, res: Response) => {
  const checkout = await polar.checkouts.custom.create({
    productPriceId: '00000000-0000-0000-0000-000000000000',
  })
  res.redirect(checkout.url)
})

app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`)
})

Set a success URL

By default, the customer will be redirected to a Polar success page. You can customize this so they are redirected to an URL in your application.

app.get('/checkout', async (req: Request, res: Response) => {
  const checkout = await polar.checkouts.custom.create({
    productPriceId: '00000000-0000-0000-0000-000000000000',
    successUrl: `${req.protocol}://${req.get('host')}/success?checkout_id={CHECKOUT_ID}`,
  })
  res.redirect(checkout.url)
})

app.get('/success', async (req: Request, res: Response) => {
  const checkoutId = req.query.checkout_id
  if (!checkoutId) {
    return res.status(400).send('Missing checkout_id')
  }
  const checkout = await polar.checkouts.custom.get(checkoutId)

  if (checkout.status === 'succeeded') {
    return res.send('<html><body><h1>Thank you!</h1></body></html>')
  }

  if (checkout.status === 'confirmed') {
    return res.send(
      '<html><body><h1>Please wait while we process the payment...</h1></body></html>',
    )
  }

  if (checkout.status === 'failed') {
    return res.send('<html><body><h1>An error occured!</h1></body></html>')
  }
})

Notice how you can pass the {CHECKOUT_ID} token to your URL: Polar will automatically replace it with the actual Checkout session ID. This allows you to retrieve the Checkout object easily on the success page.

Reconciliation with your app

In your app, it's probable you already know the customer or are in a specific state you don't want to lose during the checkout process. For this, you can pass arbitrary metadata to the Checkout object. This way, when you retrieve the Checkout on your success page (or through a webhook), you can read the metadata and link with your own application state.

app.get('/checkout', async (req: Request, res: Response) => {
  const checkout = await polar.checkouts.custom.create({
    productPriceId: '00000000-0000-0000-0000-000000000000',
    successUrl: `${req.protocol}://${req.get('host')}/success?checkout_id={CHECKOUT_ID}`,
    metadata: {
      userId: 'MY_USER_ID',
    },
  })
  res.redirect(checkout.url)
})

app.get('/success', async (req: Request, res: Response) => {
  const checkoutId = req.query.checkout_id
  if (!checkoutId) {
    return res.status(400).send('Missing checkout_id')
  }
  const checkout = await polar.checkouts.custom.get(checkoutId)
  const userId = checkout.metadata['userId']

  if (checkout.status === 'succeeded') {
    return res.send(`<html><body><h1>Thank you ${userId}!</h1></body></html>`)
  }

  if (checkout.status === 'confirmed') {
    return res.send(
      '<html><body><h1>Please wait while we process the payment...</h1></body></html>',
    )
  }

  if (checkout.status === 'failed') {
    return res.send('<html><body><h1>An error occured!</h1></body></html>')
  }
})

tip

The metadata you set on Checkout are automatically copied to the Order and/or Subscription that are created from that session.

It means that if you listen to new orders and subscriptions from webhooks, you'll get that metadata directly. Besides, they also have a checkout_id reference to the origin Checkout, so you can retrieve it if needed.

Prefill customer information

If you already know your customer, you can prefill some of the fields to speed-up the process. You can also provide their IP address, so we can automatically guess their billing country and have VAT computed directly on first load.

app.get('/checkout', async (req: Request, res: Response) => {
  const checkout = await polar.checkouts.custom.create({
    productPriceId: '00000000-0000-0000-0000-000000000000',
    successUrl: `${req.protocol}://${req.get('host')}/success?checkout_id={CHECKOUT_ID}`,
    customerEmail: 'user@example.com',
    customerName: 'John Doe',
    customerIpAddress: req.ip,
    metadata: {
      userId: 'MY_USER_ID',
    },
  })
  res.redirect(checkout.url)
})

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