Shopify functions

Shopify Functions

Overview

Shopify Functions execute custom business logic in a WebAssembly sandbox at checkout: discounts, payment filtering, delivery customization, and cart transforms. They run server-side with strict constraints (11ms execution, 1MB memory, no network access) and replace Shopify Scripts.

Prerequisites

• Shopify CLI 3.x+ installed (npm install -g @shopify/cli)
• Shopify Partners account with a development store
• App configured for the target function API type

Instructions

Step 1: Configure the Function Extension

# extensions/product-discount/shopify.extension.toml
api_version = "2025-01"
type = "product_discounts"

[[targeting]]
target = "purchase.product-discount.run"
input_query = "extensions/product-discount/input.graphql"
export = "run"

Step 2: Define the Input Query

The input query declares what data your function receives at runtime:

# extensions/product-discount/input.graphql
query Input {
  cart {
    lines {
      quantity
      merchandise {
        ... on ProductVariant {
          id
          product { hasAnyTag(tags: ["VIP-DISCOUNT"]) }
        }
      }
    }
  }
  discountNode {
    metafield(namespace: "$app:discount-config", key: "percentage") { value }
  }
}

Step 3: Implement the Function

// extensions/product-discount/src/run.ts
import type { Input, FunctionRunResult } from "../generated/api";

export function run(input: Input): FunctionRunResult {
  const percentage = parseFloat(input.discountNode?.metafield?.value ?? "0");
  if (percentage === 0) return { discounts: [], discountApplicationStrategy: "FIRST" };

  const targets = input.cart.lines
    .filter((line) => line.merchandise.product.hasAnyTag)
    .map((line) => ({ productVariant: { id: line.merchandise.id } }));

  return {
    discounts: [{
      targets,
      value: { percentage: { value: percentage.toString() } },
      message: `${percentage}% VIP Discount`,
    }],
    discountApplicationStrategy: "FIRST",
  };
}

Step 4: Test and Deploy

shopify app function typegen          # Generate types from input query
shopify app function build            # Build to WASM
shopify app function run --input test-input.json  # Test locally
shopify app deploy                    # Deploy with app

See function-types.md for all function types, input-output-schemas.md for I/O shapes, and wasm-constraints.md for sandbox limitations.

Output

• WASM binary deployed as a Shopify Function extension
• Custom discount/payment/delivery logic running server-side at checkout
• Type-safe input/output via generated TypeScript types
• Configurable via metafields (merchant-editable without code changes)

Error Handling

Error	Cause	Solution
FunctionError	Runtime panic or unhandled exception in WASM	Add error handling; check optional fields for null
FUNCTION_TOO_LARGE	WASM binary exceeds 256KB	Tree-shake deps; use Rust for smaller binaries
Input query validation	Query references unavailable fields	Match api_version to available schema fields
Timeout (>11ms)	Function exceeds execution limit	Reduce iterations; pre-compute in metafields

Examples

Creating a VIP Discount Function

Build a product discount function that applies a configurable percentage off for items tagged "VIP-DISCOUNT" using metafield-driven configuration.

See Function Types for all available function types and their targeting.

Defining Input Queries and Output Schemas

Design the GraphQL input query that declares what cart data your function receives, and return the correctly shaped FunctionRunResult.

See Input/Output Schemas for I/O shapes per function type.

Working Within WASM Sandbox Limits

Your function hits the 11ms execution timeout or 256KB binary size limit. Apply optimization strategies for the constrained WASM environment.

See WASM Constraints for sandbox limitations and workarounds.

Resources

• Shopify Functions Overview
• Product Discount Tutorial
• Function APIs Reference
• WASM Limitations