Shopify Functions are in checkout extensibility developer preview. The documentation will be updated as Shopify introduces new features and refines existing functionality.
Shopify Functions allow developers to customize the backend logic that powers parts of Shopify. The first place where we will allow you to extend logic is the discounting logic that lives inside of Checkout. Over time you will be able to manipulate the backend of many parts of Shopify.
Like a typical function you define an input, write the logic of your function, and then write out the output. Let’s dissect how each of these work:
Apps that contain functions can be distributed to any merchant through the App Store. If you want to build an app with functions for a specific merchant, then the merchant must be on a Shopify Plus plan.