Zoho Deluge - Connect to Xero API

What

This is an article to document a standalone function which generates an access token from within Zoho CRM so you can access your Xero instance. There are 3rd-party API libraries and SDKs out there but as usual with financial technologies (fintech), I don't trust them. Here's how to do it in pure Deluge with a Zoho CRM...

Why

It is quite a common request so I'm using this article to refine a generic method of connecting to any client's Xero. The aim is to have a function that initializes a valid connection and then append code to do what we want to do in Xero such as synchronizing invoices or contacts.

How

So the key thing to remember here is that a Refresh Token in Xero expires immediately after use. You use a Refresh Token to generate a new Access Token which can be reused over the next 30 minutes. In the response, you will also get a new Refresh Token which you should store away for later use when you want to generate...