Buy X get Y discount

api_version = "2026-10"

[[extensions]]
name = "Buy X get Y discount"
handle = "buy-x-get-y-discount"
type = "function"

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

  [extensions.build]
  command = ""
  path = "dist/function.wasm"

query CartLinesDiscountsGenerateRunInput {
  cart {
    lines {
      id
      quantity
      cost {
        amountPerQuantity {
          amount
        }
      }
      merchandise {
        __typename
        ... on ProductVariant {
          product {
            id
          }
        }
      }
    }
  }
  discount {
    discountClasses
    metafield(namespace: "$app", key: "function-configuration") {
      jsonValue
    }
  }
}

// @ts-check

/**
 * @typedef {import("../generated/api").CartLinesDiscountsGenerateRunInput} CartLinesDiscountsGenerateRunInput
 * @typedef {import("../generated/api").CartLinesDiscountsGenerateRunResult} CartLinesDiscountsGenerateRunResult
 */

/**
 * Settings stored as JSON in the discount's metafield. The product lists are
 * empty by default, so the discount does nothing until it's configured.
 * Keep the two lists separate: a product in both counts only as "buy".
 */
const DEFAULT_CONFIG = {
  buyProductIds: /** @type {string[]} */ ([]),
  buyQuantity: 2,
  getProductIds: /** @type {string[]} */ ([]),
  getQuantity: 1,
  // 100 makes the Y items free.
  percentage: 100,
  // Shown at checkout; null builds it from the numbers above ("Buy 2, get 1 free").
  message: /** @type {string | null} */ (null),
};

/**
 * @param {{buyQuantity: number, getQuantity: number, percentage: number}} config
 */
const defaultMessage = ({ buyQuantity, getQuantity, percentage }) =>
  `Buy ${buyQuantity}, get ${getQuantity} ${percentage >= 100 ? "free" : `${percentage}% off`}`;

/** @type {CartLinesDiscountsGenerateRunResult} */
const NO_DISCOUNT = { operations: [] };

/**
 * @param {CartLinesDiscountsGenerateRunInput} input
 * @returns {CartLinesDiscountsGenerateRunResult}
 */
export function cartLinesDiscountsGenerateRun(input) {
  // A Function may only return operations for the classes the discount was created with.
  if (!input.discount.discountClasses.includes("PRODUCT")) return NO_DISCOUNT;
  const config = { ...DEFAULT_CONFIG, ...input.discount.metafield?.jsonValue };

  /** @param {(typeof input.cart.lines)[number]} line */
  const productId = (line) => (line.merchandise.__typename === "ProductVariant" ? line.merchandise.product.id : null);
  const buyLines = input.cart.lines.filter((line) => config.buyProductIds.includes(productId(line)));
  const getLines = input.cart.lines.filter(
    (line) => !buyLines.includes(line) && config.getProductIds.includes(productId(line)),
  );

  const boughtUnits = buyLines.reduce((sum, line) => sum + line.quantity, 0);
  let freeUnits = Math.floor(boughtUnits / config.buyQuantity) * config.getQuantity;
  if (freeUnits === 0 || getLines.length === 0) return NO_DISCOUNT;

  // Discount the cheapest Y units first, as customers expect from "get one free".
  const targets = [];
  const cheapestFirst = [...getLines].sort(
    (a, b) => Number(a.cost.amountPerQuantity.amount) - Number(b.cost.amountPerQuantity.amount),
  );
  for (const line of cheapestFirst) {
    if (freeUnits === 0) break;
    const quantity = Math.min(line.quantity, freeUnits);
    targets.push({ cartLine: { id: line.id, quantity } });
    freeUnits -= quantity;
  }

  return {
    operations: [
      {
        productDiscountsAdd: {
          candidates: [{ message: config.message ?? defaultMessage(config), targets, value: { percentage: { value: String(config.percentage) } } }],
          selectionStrategy: "FIRST",
        },
      },
    ],
  };
}