Shopify Functions Setup for Custom Discount/Shipping Logic

Shopify Functions for Custom Discount and Shipping Logic

Shopify Functions — mechanism for running user code directly in Shopify infrastructure. Unlike webhook apps, Functions execute synchronously during checkout: Shopify calls the function, gets a response, and applies the result.

Where Functions Work

Functions written in Rust (official language, compiles to WebAssembly) or JavaScript/TypeScript (Javy runtime). Rust preferred for complex logic—faster and more economical.

Installation and Initialization

shopify app generate extension --template discount --name custom-volume-discount
cd extensions/custom-volume-discount

Structure: src/run.ts (or run.rs), input.graphql, shopify.extension.toml, package.json.

Example: Volume Discount Function (JavaScript)

input.graphql describes what data Shopify passes:

query RunInput {
  cart {
    lines {
      id
      quantity
      merchandise {
        ... on ProductVariant {
          id
          product {
            id
            tags
          }
        }
      }
    }
  }
}

src/run.ts function logic:

export function run(input: RunInput): FunctionRunResult {
  const discounts = [];

  for (const line of input.cart.lines) {
    const variant = line.merchandise;
    if (variant.__typename !== "ProductVariant") continue;

    if (!variant.product.tags.includes("volume-eligible")) continue;

    discounts.push({
      targets: [{ cartLine: { id: line.id } }],
      value: { percentage: { value: String(line.quantity > 5 ? 10 : 5) } },
      message: `${line.quantity > 5 ? 10 : 5}% bulk discount`,
    });
  }

  return { discounts, discountApplicationStrategy: "FIRST" };
}

Activation via Shopify Admin

Functions don't work automatically—create a Discount in Admin and select the function:

const CREATE_DISCOUNT = `
  mutation discountAutomaticAppCreate($automaticAppDiscount: DiscountAutomaticAppInput!) {
    discountAutomaticAppCreate(automaticAppDiscount: $automaticAppDiscount) {
      automaticAppDiscount { discountId title startsAt }
      userErrors { field message }
    }
  }
`;

Function Limitations

• Execution time: 5ms for Rust/WASM, 10ms for JS
• Memory: 10 MB
• No network requests: all data via input.graphql
• No database access: complex config via Metafields
• No Checkout UI Extensions: separate mechanism

Testing

npm run test
shopify app deploy
shopify app logs

Timeline

Simple discount function (percentage by tag/collection): 2–4 days. Complex multi-level function with metafield config and Admin UI: 1–2 weeks. Shipping function with geologic and external tariff integration: 1–2 weeks.