Move custom business rules to Shopify Functions.

How I untangle legacy scripts into dedicated Function APIs

When Shopify retired Ruby Scripts, the biggest architectural shift I had to explain to merchants was that one monolithic script no longer mutates everything in the cart at once. Instead, Shopify Functions separate business logic by domain: Discount Functions, Delivery Customization Functions, Payment Customization Functions, and Cart Transform Functions. Before I touch the keyboard, I sit down with the operations team to document the exact inputs, customer eligibility rules, and intended outputs.

Because Functions compile to WebAssembly and run inside Shopify’s infrastructure under strict instruction and payload budgets, I keep input GraphQL queries minimal and store dynamic promo thresholds inside configuration metafields so merchants can adjust campaigns without a code deploy.

Why I isolate the pure decision logic for unit testing

Debugging a compiled WebAssembly binary inside a live checkout is painful if your core math is mixed up with GraphQL boilerplate. My habit is to write the core business rule as a pure, zero-dependency function that operates on integer minor currency units (while respecting zero-decimal currencies like JPY or IDR in the adapter layer).

In the example below, I validate every input defensively and sort tiers deterministically so that unit tests can verify edge cases in milliseconds before the Function adapter formats the final Shopify operation.

export function chooseDiscount({eligible, subtotalMinor, tiers}) {
  if (!Number.isSafeInteger(subtotalMinor) || subtotalMinor < 0) {
    throw new TypeError("Invalid subtotal");
  }
  for (const tier of tiers) {
    if (!Number.isSafeInteger(tier.minimumMinor) || tier.minimumMinor < 0 ||
        !Number.isInteger(tier.percent) || tier.percent < 0 || tier.percent > 100) {
      throw new TypeError("Invalid tier");
    }
  }
  if (!eligible) return 0;
  const matching = [...tiers]
    .sort((a, b) => b.minimumMinor - a.minimumMinor)
    .find(tier => subtotalMinor >= tier.minimumMinor);
  return matching?.percent ?? 0;
}

How I validate a Function before production release

1. I run automated unit tests covering eligible buyers, guest shoppers, missing metafields, and empty carts.
2. I test boundary values at `minimumMinor - 1`, exact `minimumMinor`, and `minimumMinor + 1` for every tier.
3. I verify discount combination rules and multi-currency conversion behavior in a development store.
4. I validate generated GraphQL input/output types against the exact pinned Shopify Function API version.
5. I run `shopify app function replay` against real test cart traces and document recovery procedures before enabling the rule in production.