Shopify Functions
Shopify Functions
Shopify Functions are your own code executed inside Shopify’s back end at specific points of the cart and checkout: discounts, validations, line transformation, shipping and payment methods. They are written in JavaScript or Rust, compiled to WebAssembly, and a custom Function can only be installed on stores with Shopify Plus.
Also called: Functions, function extension
Functions are Shopify’s answer to “I need a business rule the platform does not ship”. They replaced Shopify Scripts and differ in something important: they do not run in the browser or on a server of yours, they run inside Shopify, before the checkout is drawn.
The budget, which is the part that surprises people
A Function is not an ordinary function. It has an execution budget — on the order of 11 million instructions — and a 128 kB cap on its input. That changes how you write one:
- The input query is half the work: a Function only sees what it asks for, and an order pad with 200 lines cannot afford stray fields.
- There are no network calls. You cannot query your ERP from inside a Function.
- Data the Function needs that is not native goes in metafields, which is what does arrive in the input.
The plan limit
This is the point that breaks the most quotes: only stores on Shopify Plus can install custom apps containing Functions. On Basic, Grow and Advanced you can use functions that ship inside a public App Store app, but not one written for your store. The practical difference: with a public app you adapt to its data model, with your own Function you set the model.
A complete example
A validation Function that blocks checkout when a line is not a multiple of the warehouse case pack, with its input query and its tests, is published in shopify-checkout-lab and can be poked at live in the Checkout Lab on the home page.
Official documentation: Shopify Functions.