Tiered volume discount by quantity

api_version = "2026-10"

[[extensions]]
name = "Tiered volume discount"
handle = "tiered-volume-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
      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. */
const DEFAULT_CONFIG = {
  // Quantity of one product (all its variants together) → percentage off that product.
  tiers: [
    { minQuantity: 3, percentage: 5 },
    { minQuantity: 5, percentage: 10 },
    { minQuantity: 10, percentage: 15 },
  ],
};

/** @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 };
  const tiers = [...config.tiers].sort((a, b) => b.minQuantity - a.minQuantity);

  /** @type {Map<string, {quantity: number, lineIds: string[]}>} */
  const products = new Map();
  for (const line of input.cart.lines) {
    if (line.merchandise.__typename !== "ProductVariant") continue;
    const entry = products.get(line.merchandise.product.id) ?? { quantity: 0, lineIds: [] };
    entry.quantity += line.quantity;
    entry.lineIds.push(line.id);
    products.set(line.merchandise.product.id, entry);
  }

  const candidates = [];
  for (const { quantity, lineIds } of products.values()) {
    const tier = tiers.find((t) => quantity >= t.minQuantity);
    if (!tier) continue;
    candidates.push({
      message: `${tier.percentage}% off ${tier.minQuantity}+`,
      targets: lineIds.map((id) => ({ cartLine: { id } })),
      value: { percentage: { value: String(tier.percentage) } },
    });
  }
  if (candidates.length === 0) return NO_DISCOUNT;

  // ALL applies every product's candidate; FIRST would discount only one product.
  return { operations: [{ productDiscountsAdd: { candidates, selectionStrategy: "ALL" } }] };
}