Shopify Functions

When to use this skill

Use this skill when:

• Creating custom discount logic
• Customizing delivery options
• Implementing payment method rules
• Validating cart or checkout
• Building order routing logic
• Extending Shopify's backend behavior

What are Shopify Functions?

Functions are serverless WebAssembly modules that extend Shopify's backend logic. They:

• Run on Shopify's infrastructure
• Execute in milliseconds
• Scale automatically
• Are upgrade-safe

Function Types

Getting Started

1. Create a Function

# In an existing app
shopify app generate extension

# Select from function types:
# - Delivery customization
# - Product discount
# - Order discount
# - Cart & Checkout Validation
# - etc.

2. Choose a Language

Rust (Recommended) - Best performance, handles large carts

JavaScript - Easier to learn, good for simpler logic

3. Function Structure

extensions/
└── my-discount/
    ├── src/
    │   └── run.rs (or run.js)
    ├── input.graphql
    ├── shopify.extension.toml
    └── Cargo.toml (for Rust)

Function Anatomy

Configuration (shopify.extension.toml)

api_version = "2025-01"

[[extensions]]
name = "Volume Discount"
handle = "volume-discount"
type = "function"

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

[extensions.build]
command = "cargo wasi build --release"
path = "target/wasm32-wasi/release/volume-discount.wasm"

Input Query (input.graphql)

query RunInput {
  cart {
    lines {
      id
      quantity
      merchandise {
        ... on ProductVariant {
          id
          product {
            id
            title
            hasAnyTag(tags: ["discount-eligible"])
          }
        }
      }
      cost {
        amountPerQuantity {
          amount
          currencyCode
        }
      }
    }
  }
  discountNode {
    metafield(namespace: "volume-discount", key: "config") {
      value
    }
  }
}

Product Discount Function (Rust)

// src/run.rs
use shopify_function::prelude::*;
use shopify_function::Result;

#[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
    let mut discounts = vec![];

    // Parse configuration from metafield
    let config: Config = input.discount_node.metafield
        .as_ref()
        .map(|m| serde_json::from_str(&m.value).unwrap())
        .unwrap_or_default();

    for line in input.cart.lines {
        let quantity = line.quantity;

        // Check if eligible for volume discount
        if quantity >= config.minimum_quantity {
            let merchandise = match &line.merchandise {
                input::InputCartLinesMerchandise::ProductVariant(variant) => variant,
                _ => continue,
            };

            // Check for eligible tag
            if merchandise.product.has_any_tag {
                discounts.push(output::Discount {
                    targets: vec![output::Target::ProductVariant(
                        output::ProductVariantTarget {
                            id: merchandise.id.clone(),
                            quantity: None,
                        },
                    )],
                    value: output::Value::Percentage(output::Percentage {
                        value: Decimal::from_str(&config.discount_percentage).unwrap(),
                    }),
                    message: Some(format!("{}% volume discount", config.discount_percentage)),
                });
            }
        }
    }

    Ok(output::FunctionRunResult {
        discounts,
        discount_application_strategy: output::DiscountApplicationStrategy::FIRST,
    })
}

#[derive(Default, serde::Deserialize)]
struct Config {
    minimum_quantity: i64,
    discount_percentage: String,
}

Product Discount Function (JavaScript)

// src/run.js
// @ts-check
import { DiscountApplicationStrategy } from "../generated/api";

/**
 * @param {RunInput} input
 * @returns {FunctionRunResult}
 */
export function run(input) {
  const config = JSON.parse(
    input.discountNode.metafield?.value ??
      '{"minimumQuantity": 5, "percentage": "10"}',
  );

  const discounts = [];

  for (const line of input.cart.lines) {
    const variant = line.merchandise;

    // Check quantity threshold
    if (line.quantity >= config.minimumQuantity) {
      // Check for eligible products
      if (
        variant.__typename === "ProductVariant" &&
        variant.product.hasAnyTag
      ) {
        discounts.push({
          targets: [
            {
              productVariant: {
                id: variant.id,
              },
            },
          ],
          value: {
            percentage: {
              value: config.percentage,
            },
          },
          message: `${config.percentage}% volume discount`,
        });
      }
    }
  }

  return {
    discounts,
    discountApplicationStrategy: DiscountApplicationStrategy.First,
  };
}

Delivery Customization

// Rename, hide, or reorder delivery options
use shopify_function::prelude::*;
use shopify_function::Result;

#[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
    let mut operations = vec![];

    for method in input.cart.delivery_groups[0].delivery_options.iter() {
        // Hide express shipping for heavy orders
        if method.title.contains("Express") && cart_weight_exceeds_limit(&input) {
            operations.push(output::Operation::Hide(output::HideOperation {
                delivery_option_handle: method.handle.clone(),
            }));
        }

        // Rename delivery option
        if method.title.contains("Standard") {
            operations.push(output::Operation::Rename(output::RenameOperation {
                delivery_option_handle: method.handle.clone(),
                title: Some("Economy Shipping (5-7 days)".to_string()),
            }));
        }
    }

    Ok(output::FunctionRunResult { operations })
}

Payment Customization

// src/run.js
export function run(input) {
  const cart = input.cart;
  const operations = [];

  // Calculate cart total
  const total = cart.cost.totalAmount.amount;

  // Hide COD for orders over $500
  if (parseFloat(total) > 500) {
    const codMethod = input.paymentMethods.find((method) =>
      method.name.includes("Cash on Delivery"),
    );

    if (codMethod) {
      operations.push({
        hide: {
          paymentMethodId: codMethod.id,
        },
      });
    }
  }

  // Reorder payment methods
  operations.push({
    move: {
      paymentMethodId: input.paymentMethods[0].id,
      index: 2,
    },
  });

  return { operations };
}

Cart & Checkout Validation

use shopify_function::prelude::*;
use shopify_function::Result;

#[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
    let mut errors = vec![];

    // Check minimum order value
    let total: f64 = input.cart.cost.total_amount.amount.parse().unwrap();
    if total < 25.0 {
        errors.push(output::FunctionError {
            localized_message: "Minimum order value is $25.00".to_string(),
            target: output::Target::Cart,
        });
    }

    // Check product availability by region
    for line in &input.cart.lines {
        if let input::InputCartLinesMerchandise::ProductVariant(variant) = &line.merchandise {
            if is_restricted_product(&variant, &input.cart.buyer_identity) {
                errors.push(output::FunctionError {
                    localized_message: format!(
                        "{} is not available in your region",
                        variant.product.title
                    ),
                    target: output::Target::CartLine(output::CartLineTarget {
                        id: line.id.clone(),
                    }),
                });
            }
        }
    }

    Ok(output::FunctionRunResult { errors })
}

Cart Transform

Modify cart contents dynamically:

// src/run.js
export function run(input) {
  const operations = [];

  for (const line of input.cart.lines) {
    const variant = line.merchandise;

    // Add free gift for orders with specific products
    if (variant.product.hasAnyTag && line.quantity >= 3) {
      operations.push({
        expand: {
          cartLineId: line.id,
          expandedCartItems: [
            {
              merchandiseId: variant.id,
              quantity: line.quantity,
            },
            {
              merchandiseId: "gid://shopify/ProductVariant/FREE_GIFT_ID",
              quantity: 1,
            },
          ],
        },
      });
    }
  }

  return { operations };
}

Testing Functions

Local Testing

# Test with sample input
shopify app function run --path extensions/my-function

# Provide input via stdin
cat input.json | shopify app function run --path extensions/my-function

Sample Input JSON

{
  "cart": {
    "lines": [
      {
        "id": "gid://shopify/CartLine/1",
        "quantity": 5,
        "merchandise": {
          "__typename": "ProductVariant",
          "id": "gid://shopify/ProductVariant/123",
          "product": {
            "id": "gid://shopify/Product/456",
            "title": "Test Product",
            "hasAnyTag": true
          }
        },
        "cost": {
          "amountPerQuantity": {
            "amount": "10.00",
            "currencyCode": "USD"
          }
        }
      }
    ]
  },
  "discountNode": {
    "metafield": {
      "value": "{\"minimumQuantity\": 3, \"discountPercentage\": \"15\"}"
    }
  }
}

Deployment

# Deploy function with app
shopify app deploy

# Function will be available to configure in admin

Configuration UI

Create an admin UI to configure function settings:

// app/routes/app.discount.jsx
import { authenticate } from "../shopify.server";
import { Form, TextField, Button, Card } from "@shopify/polaris";

export async function action({ request }) {
  const { admin } = await authenticate.admin(request);
  const formData = await request.formData();

  // Create discount with function
  await admin.graphql(
    `
    mutation CreateDiscount($discount: DiscountAutomaticAppInput!) {
      discountAutomaticAppCreate(automaticAppDiscount: $discount) {
        automaticAppDiscount {
          discountId
        }
        userErrors {
          field
          message
        }
      }
    }
  `,
    {
      variables: {
        discount: {
          title: formData.get("title"),
          functionId: "YOUR_FUNCTION_ID",
          startsAt: new Date().toISOString(),
          metafields: [
            {
              namespace: "volume-discount",
              key: "config",
              value: JSON.stringify({
                minimumQuantity: parseInt(formData.get("minQty")),
                discountPercentage: formData.get("percentage"),
              }),
              type: "json",
            },
          ],
        },
      },
    },
  );

  return redirect("/app/discounts");
}

Performance Best Practices

1. Use Rust for large carts - JavaScript can timeout
2. Minimize input query - Only request needed data
3. Avoid complex loops - Keep logic simple
4. Cache configuration - Parse metafields once
5. Test with real data - Test large cart scenarios

CLI Commands Reference

Resources

• Functions Overview
• Functions API Reference
• Discount Functions
• Delivery Customization
• Payment Customization
• JavaScript for Functions
• Rust for Functions

For checkout UI, see the checkout-customization skill.