Shopify Functions: The Developer's Guide to Checkout Extensibility

Shopify Functions represent the most significant expansion of Shopify's extensibility model since the platform launched. For years, checkout customization required Plus-plan workarounds, Script Editor (deprecated), or abandoning Shopify checkout entirely for a custom headless build. Functions change that calculation.

Here's what you actually need to know as a developer.

What Shopify Functions Are

Shopify Functions are small programs that run in Shopify's infrastructure at various points in the checkout, cart, and order management flows. They're written in Rust (compiled to WebAssembly) or JavaScript, and they execute with sub-millisecond latency inside Shopify's own infrastructure — not on your server.

The key distinction: unlike traditional webhooks (which are asynchronous and fire after the fact), Functions run synchronously inline with the checkout process. They can affect what happens — not just observe it.

What Functions Can Do

Discount Functions

Custom discount logic is the most common Function use case. Functions can:

• Apply volume-based discounts based on quantity thresholds
• Create buy-X-get-Y promotions with custom eligibility rules
• Apply customer-segment-specific pricing (B2B, loyalty tiers, wholesale)
• Stack or prevent discount combinations based on your rules

Example: A Function that applies a 20% discount if the customer's order contains at least 3 products from a specific collection AND they're a Shopify B2B customer — logic that's impossible to express in native Shopify discount rules.

Payment Customization Functions

Control which payment methods appear at checkout:

• Hide payment methods based on cart contents, customer tags, or geography
• Reorder payment methods to surface preferred options
• Show or hide payment methods based on order value thresholds

Shipping Customization Functions

Custom shipping logic:

• Hide shipping options for specific products (hazmat, oversized items)
• Rename shipping options with custom labels
• Sort shipping options by your preferred priority

Cart Transform Functions

Modify the cart before checkout:

• Automatically add free gifts based on cart conditions
• Bundle products into kits with custom pricing
• Replace products (subscription vs. one-time variants)

The WebAssembly Constraint

The most important constraint to understand: Functions run in a sandbox with strict limits. No network calls, no external API access, no persistence. Functions receive a payload (cart data, customer data) and return a transformation — that's it.

This is by design. Network calls would introduce latency and failure modes unacceptable at checkout. But it means any external data your Function needs must be baked into the payload via metafields at page load time.

Pattern for AI-driven Functions: Your server-side AI system makes decisions (e.g., which customers are eligible for a specific discount) and writes those decisions to customer metafields. The Function reads those metafields — no network calls — and applies them. The intelligence happens before the Function; the Function just enforces it.

Checkout UI Extensions

Alongside Functions, Checkout UI Extensions allow injecting custom UI components at specific checkout steps:

• Cart page: Upsells, cross-sells, custom product information
• Information step: Custom fields, loyalty points display
• Shipping step: Delivery date selection, custom shipping messaging
• Payment step: Order summary customization, loyalty/rewards display
• Thank you / Order status page: Post-purchase upsell, custom content

UI Extensions call your APIs directly — they can fetch real-time data (inventory, recommendations, loyalty balances) unlike Functions.

Getting Started

The Shopify CLI handles Function scaffolding:

shopify app generate extension --template discount

This creates a project structure with the Function code (JavaScript or Rust), the GraphQL query definition (what data your Function receives), and the configuration.

For most commerce use cases, JavaScript is sufficient and faster to iterate on. Rust is worth the complexity for Functions that need maximum performance or complex computation.

The deployment model is straightforward: Functions deploy with your Shopify app. Version control, CI/CD, and testing are handled through your standard app development workflow.