Cart Transform

Merge cart lines into bundles, expand bundle SKUs into components, and update line presentation with JavaScript.

A cart transform reshapes the lines of the cart itself — merge several lines into a single bundle line, expand one line into its components, or update how a line is presented. Your code inspects the cart and returns a list of operations.

var ops = [];

var bundleLines = input.cart.lines.filter(function (line) {
  var product = line.merchandise && line.merchandise.product;
  return product && product.hasTags && product.hasTags.some(function (t) { return t.hasTag; });
});

if (bundleLines.length >= 2) {
  ops.push({
    linesMerge: {
      cartLines: bundleLines.map(function (l) { return { cartLineId: l.id, quantity: l.quantity }; }),
      parentVariantId: 'gid://shopify/ProductVariant/…',
      title: 'Bundle',
      price: { percentageDecrease: { value: 10 } },
    },
  });
}

return { operations: ops };

A store can have only one cart transform function — the Functions Console lets you create a single one, in the General or Tags family. Its title is internal: buyers never see it.

Operations

Return an array of operations, or { operations: [...] }. Each operation is an object with exactly one of these keys:

Operation	Shape	Effect
Merge	{ linesMerge: { cartLines: [{ cartLineId, quantity }], parentVariantId, title?, image?, price? } }	Combines lines into one bundle line, presented as the parent variant
Expand	{ lineExpand: { cartLineId, title?, image?, expandedCartItems: [{ merchandiseId, quantity, price? }] } }	Splits one line into its component items
Update	{ lineUpdate: { cartLineId, title?, image?, price? } }	Overrides a line's title, image or price

Price shapes: merge uses price: { percentageDecrease: { value: 10 } }; expanded items use price: { adjustment: { fixedPricePerUnit: { amount: '10.0' } } }.

Returning an empty array leaves the cart unchanged. Operations of any other shape are ignored.

lineUpdate (price/title/image overrides) works on Shopify Plus and development stores only — prefer merge and expand for rules that must work everywhere.

The input

• Cart lines are input.cart.lines, each with an id, quantity, cost and merchandise (variant id, sku, product data — the Tags family adds product and customer hasTags).

• There is no cart-level cost — money lives per line in line.cost, and input.presentmentCurrencyRate at the root converts shop-currency amounts to what the buyer pays.

• The variant IDs you reference (parentVariantId, merchandiseId) must be existing, published variants — supply them via variables so merchants can set them without code changes.

How DataJet manages the transform

• The function runs on every cart change, so keep the code fast and return { operations: [] } early when there's nothing to do.

• Turning the function off doesn't delete anything — the off switch is stored in the function's configuration and the code simply stops producing operations.

• Deleting the function removes it from Shopify entirely, including its configuration — the code remains available in the version history.

• Cart transforms have no page in the Shopify admin; they're managed only from the Functions Console.

Example: expand a bundle SKU

Family: General — set BUNDLE_SKU and the component variant IDs as code variables.

var ops = [];
var componentIds = (vars.BUNDLE_COMPONENT_IDS || '').split(',');

input.cart.lines.forEach(function (line) {
  var variant = line.merchandise;
  if (variant && variant.sku === vars.BUNDLE_SKU && componentIds.length > 1) {
    ops.push({
      lineExpand: {
        cartLineId: line.id,
        expandedCartItems: componentIds.map(function (id) {
          return { merchandiseId: 'gid://shopify/ProductVariant/' + id.trim(), quantity: 1 };
        }),
      },
    });
  }
});

return { operations: ops };

Notes

• Cart transforms don't use the Run on steps — they apply whenever the cart changes.

• If your code throws, the cart is left unchanged (fail open).

• See also: Cart and Checkout Validation, Delivery Customization, Payment Customization, Discounts.