The front-end of the application originally was just static pages, but has since been refactored into a fairly simple React application. The main focus here will be on using serverless functions for doing our processing though. The first step in getting our instant checkout links created requires setting up OAuth in order to get our user’s access token. For this, we will create our first two functions, named “authorize” and “code”. Google does a great job of explaining how to use Firebase CLI on their site so we’ll skip that.

If you’re not familiar with OAuth or want to know more about OAuth with Square, checkout (pun intended) my previous blog post “OAuth, wherefor art thou?”.

Now let’s dig into some sweet, sweet code examples.

We’re simply generating the link to redirect our users to give us permission to access their account.

Here we are just creating a string to send to the client-side for redirecting our user. The main purpose here is more about generating our state, creating the cookie, and passing it along for us to verify later when we get the callback from Square redirecting our user back to us.

We’re doing a bit more here, but still basically getting our token from Square and then creating a user.

We’re definitely doing a bit more here than just getting our access token from the callback. We are looking up the user’s info using their access token and then creating an account in Firebase for that user’s email. The intention here was to disallow a user to create an account using any email they wanted, and strictly having it tied to the email that is set for their Square account. We’ve omitted the createFirebaseAccount() function definition, since it’s just another detail peculiar to Firebase for creating accounts.

We’re in a pretty good place! We have authorized our user, received an access token, set the user up in Firebase, stored their token, and now we can start using Square’s API’s for generating our instant checkout links.

After our user gets their sign-in link in their email and is redirected back to our app to sign in, we need to generate our links to send to the user. This is where we’ll need the catalog function.

We have a LOT going on here.

This function is a little bit dense. We’re taking the token send to us, decoding the Firebase token, looking up our user, getting their Square access token, querying the catalog API, iterating over the catalog items, creating a list of item objects, and creating short url links for checking out with.

Take a breath, there was a lot in that list.

This is a not so pretty example of our checkout link.

Okay, break’s over. Back to work.

In the catalog links, we’re constructing a URL the follows the format /merchantId/catalogVariantId/ which allows us to look up our Square merchant and then what item is being purchased. This might be a little confusing, since we haven’t looked at the final function yet. We have one more function to go over: the checkout function. Our checkout function will handle the process of looking up a merchant, looking up the catalog item, creating a Square checkout URL, and then redirecting the user to that checkout url. Let’s take a look at what this function looks like.

We wrapped our function in an Express app to more easily handle routing.

We switched things up here by wrapping our function up as an Express application. This allows an easier time in parsing route variables so we could look up our merchant and catalog IDs. Since our prices are already set in Square, all we need to do is place the item as a line item in our order, generate our checkout link, and then redirect!