Shopify Functions: A Practical Guide for Merchants

Who it is for

Merchants replacing Scripts, and junior devs

Time needed

1 to 2 hours to plan, longer to build

What you need

Your checkout rules and your Shopify plan

Shopify Functions sit underneath some of the most valuable logic on a store: the tiered discount, the "hide express shipping for heavy items" rule, the "no cash on delivery over $500" rule. Merchants rarely see them directly. They install an app, set a few options and the logic runs at checkout. When an app doesn't exist for the rule they need, that's where a developer comes in.

I build functions for client stores, and most of the questions I get are the same: which plan do I need, can an app do this, and what replaced my old Scripts. This guide answers those first, then shows the developer side for anyone building one. Everything below was checked on shopify.dev and the Shopify Help Center on 7 November 2026.

What Shopify Functions are

A function is a piece of code that Shopify runs at a specific point in its own logic, called a target. It works in three steps:

1. Input. The developer writes a GraphQL input query that asks for the data the function needs, such as cart lines, the customer's tags or a product metafield. Shopify sends that data as JSON.
2. Logic. The code decides what to do. It's compiled to WebAssembly, and Shopify provides templates for Rust and JavaScript. Shopify strongly recommends Rust for performance on large carts.
3. Output. The function returns a JSON list of operations, such as "apply 10% to these lines" or "hide this delivery option". Shopify applies them.

Functions run on Shopify's infrastructure as shoppers move through the cart and checkout. You never call one by URL, and they don't slow your theme down, because they aren't loaded in the browser. They are packaged as extensions inside an app, so every function belongs to an app, whether it's one from the App Store or one built for your store.

The Shopify Functions APIs and what each one does

Each Function API controls one part of Shopify's logic. These are the ones merchants run into most:

There are also Pickup Point and Local Pickup Delivery Option Generator APIs for stores that offer pickup. Shopify's Function API reference also sets the order they run in: cart transform first, then line item discounts, fulfillment and routing, then delivery options and delivery discounts, then payment methods, with validation last.

Some limits are per store. You can activate up to 25 delivery customization functions and up to 25 validation functions on each store, which is far more than most stores need.

Which plans can use Shopify Functions

This is the rule that decides most projects:

• Public apps from the App Store that contain functions work on any plan, unless a specific API says otherwise.
• Custom apps that contain functions only work on Shopify Plus. A Basic, Grow or Advanced store can't install a custom-built function, however simple it is.
• Payment customizations in the US and Canada are limited to non-credit-card payment methods on plans below Plus. Customizing the credit card option needs Plus.

Replacing Shopify Scripts with functions

Shopify's Help Center is direct about it: Scripts and the Script Editor app were retired on 30 June 2026, and scripts no longer run on any store. If your store relied on them, the logic stopped on that date. Here's how the old script types map to functions:

When I migrate script logic, I don't port it line by line. I list what each script actually did for shoppers, check which rules Shopify's built-in discounts now handle without code, and only build functions for what's left. Built-in automatic discounts and discount combinations cover more than they did when most scripts were written.

App, custom function or built-in feature

Plenty of App Store apps are essentially configurable functions: tiered discounts, delivery rules, payment rules. If one fits, it's cheaper than a build. A custom function makes sense when the rule depends on your own data, such as a customer metafield from your ERP, or when you'd otherwise stack two or three apps to get one behavior. My guide to custom apps vs subscriptions walks through the cost comparison.

How to switch on a function in your admin

Installing an app with a function doesn't change anything until you activate it. The path depends on the type:

1. Discounts. Go to Discounts, click Create discount and pick the discount type the app provides. Set its options, combinations and dates, then save.
2. Delivery rules. Go to Settings > Shipping and delivery. In Delivery customizations, click Add delivery customization, choose the app's option, configure it and activate it.
3. Payment rules. Go to Settings > Payments. In Payment customizations, click Manage, then add, configure and activate the customization.
4. Validation. Go to Settings > Checkout. Under Checkout rules, click Add rule, select the validation, then click Activate and save.
5. Test. Place test orders that should and shouldn't trigger the rule, on desktop and phone, including express checkouts such as Shop Pay.

Every installed function is also listed on the app's page under Settings > Apps, in its extensions and functions section, which is useful when you inherit a store and need to know what's running.

How developers build a Shopify function

For developers, functions are built with Shopify CLI inside an app. The short version:

1. Create or open an app. Run shopify app init for a new app, or work in an existing one.
2. Generate the function. Run shopify app generate extension --template discount --name volume-discount and choose Rust or JavaScript. Other templates cover delivery, payment, validation and cart transform.
3. Write the input query. Edit the .graphql file so it requests only the fields you need. The query must stay under 3,000 bytes and a calculated cost of 30.
4. Write the logic. Return operations in the shape the API expects.
5. Run it locally. shopify app dev previews the app on a development store and prints each function run. shopify app function replay replays a logged run on your machine so you can debug without placing another order.
6. Deploy. shopify app deploy releases a new app version, and the function becomes available to stores with the app installed.

The extension's shopify.extension.toml ties each target to its input query and entry point. For a discount function it looks like this:

[[extensions.targeting]]
target = "cart.lines.discounts.generate.run"
input_query = "src/cart_lines_discounts_generate_run.graphql"
export = "cart_lines_discounts_generate_run"

Settings that merchants change, such as a percentage or a list of collections, are usually stored in a metafield on the discount or customization and read through the input query. That keeps the function itself generic.

Shopify Functions API limits

The instruction limit is the one that bites. A JavaScript function that loops over every line and every discount can pass with a 5-item test cart and fail with a 150-item wholesale order. That's why Shopify pushes Rust, and why I test with the biggest realistic cart a store gets.

Common mistakes with Shopify Functions

1. Commissioning a custom function on a non-Plus store. It won't install. Check the plan before any build starts.
2. Testing only small carts. Test the largest cart you see in your order history.
3. Forgetting express checkouts. Validation functions apply to Shop Pay, PayPal, Apple Pay and Google Pay too. Test each one you offer.
4. Hiding delivery options for PO boxes with built-in rates. Shopify warns the address data needed may not be available unless rates come from a carrier service or shipping app.
5. Two apps changing the same thing. Two delivery customization apps can hide each other's options. Keep one owner per rule.
6. Porting every old script. Many old script rules are now built-in discount settings.

Shopify Functions checklist

Next steps

List every rule your checkout should enforce, then go through the flow above for each one. Most stores end up with one or two apps and a handful of built-in discounts, and no custom code at all. If you're on Plus and a rule depends on your own data, a custom function is usually a small, contained build.

If you need one built, my Shopify app development service starts from $800, and every project gets a fixed quote. Send me the rule and your plan, and I'll tell you whether an existing app already does it.

Frequently asked questions

Sources

1. Function APIs, Shopify.dev
2. About Shopify Functions, Shopify.dev
3. Customizing payment methods and delivery options, Shopify Help Center
4. Shopify Scripts and the Script Editor, Shopify Help Center
5. Delivery Customization Function API, Shopify.dev
6. app function replay, Shopify.dev

Related services and work

Written by Ashraf Siddik

I'm a Shopify developer and CRO specialist. I have built and improved Shopify stores since 2018, completed more than 1,000 projects, and hold a 4.96 average rating across 719 client reviews.

About me · Client reviews