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Sumeet Shroff Freelancer
Sumeet Shroff Freelancer

Posted on • Originally published at prateeksha.com

Shopify Scripts Functions: A Practical Migration Playbook

#backend #devops #tutorial

A concise, developer-focused playbook to move Script Editor logic into Shopify Functions: audit, prototype, test, and stage rollouts with QA, monitoring, and rollback steps.

Shopify Scripts → Functions: A Practical Migration Playbook

Quick summary

  • Audit active scripts, map business intent, and prioritize by impact.
  • Prototype a single high-value Function, validate parity, then iterate.
  • Use staged rollouts, robust QA, and observability to reduce launch risk.

Introduction

Shopify is standardizing customization around Shopify Functions. If your store still relies on the Script Editor, planning a migration reduces the chance of customer-facing surprises. This guide presents a practical, developer-oriented workflow: audit, map use-cases, prototype, test, and roll out safely.

Why migrate now

Shopify Functions run as sandboxed WebAssembly modules and plug into Shopify’s native evaluation points (discounts, shipping, payments). The migration brings:

  • Better performance and scalability
  • Clearer extension boundaries and versioning
  • Reduced maintenance debt for long-term compatibility

Conceptual changes: Scripts vs Functions

Key differences to keep in mind:

  • Execution: Scripts ran in the Script Editor runtime; Functions are invoked by Shopify’s server-side evaluation and must be deterministic.
  • Language & tooling: Scripts were Ruby-based; Functions use languages that compile to WASM (Rust is the most common path).
  • Scope: Functions are tied to extension points (discounts, shipping, payments), not arbitrary checkout mutations.

Design with determinism and idempotence in mind: Functions should produce the same output for the same cart input every time.

Quick comparison

  • Deployment: Script Editor (per-shop) vs app extension + Shopify CLI (versionable).
  • Surface: General cart scripting vs specific extension points (discounts/shipping/payments).
  • Maintainability: Monolithic shop scripts vs modular, testable functions.
  • Performance: Legacy runtime vs sandboxed WASM with lower latency potential.

Migration overview — step-by-step

Follow this task-oriented flow.

  1. Inventory & Audit

    • Export every active script and note where it runs and which data it uses (line items, customer tags, shipping address).
    • Capture business intent: discounts, bundling, conditional shipping, or payment routing.
    • Flag scripts that rely on private or deprecated APIs.

  2. Prioritize & Scope

    • Tag scripts as High (revenue-critical), Medium (UX/ops), Low (infrequent).
    • Choose one high-impact script for your pilot.

  3. Map to Functions

    • Match business intent to the right extension point (Discounts, Shipping Rates, Payment).
    • Re-express logic as pure calculations that return updated pricing or rate objects.

  4. Prototype

    • Build a minimal Function that covers core behavior.
    • Run locally with Shopify CLI and a dev store to confirm baseline parity.

  5. Implement & Integrate

    • Expand logic, introduce shared utilities, and add unit and integration tests.
    • Add structured logging and feature flags to control rollout.

  6. Test & QA

    • Unit test every rule branch and edge case.
    • Integration tests with product fixtures and end-to-end checkout flows.
    • Validate rounding, tax interactions, and unusual cart compositions.

  7. Staged Rollout

    • Canary by percentage, storefront, or market.
    • Monitor key metrics and hold a rollback window.

  8. Post-launch

    • Validate telemetry, monitor errors, and iterate.

Mapping common use-cases

How common Script patterns translate to Functions:

  • SKU or collection discounts → Discount Function with deterministic matching rules.
  • Buy X get Y / bundle promotions → Discount Function that groups line items and calculates line-level adjustments.
  • Conditional shipping (by tag, weight, destination) → Shipping Function that returns authorized shipping rates.
  • Payment routing or gating → Payments extension with server-side checks if needed.

Always replace side-effects with returned, repeatable data structures — Functions should not rely on mutable external state.

Real-world scenarios (short)

  • Holiday bundle: Prototype caught subtle rounding differences; staged rollout enabled quick fixes.
  • Tag-based free shipping: Added tests for international destinations that previous Scripts overlooked.
  • Legacy tiered pricing: Split logic between a Discount Function and app-side membership checks to simplify rules.

QA and testing checklist

Pre-migration

  • List active scripts and business intents
  • Capture analytics and revenue attribution per script
  • Document any API limitations

Development

  • Create Function prototype with unit tests
  • Add structured logs and feature flags
  • Build integration tests against a dev store

Staging & Rollout

  • Smoke test on limited storefront
  • Monitor AOV, conversion rate, discount application rate
  • Prepare clear rollback steps

Post-launch

  • Monitor errors, latency, and revenue math
  • Run periodic parity checks between legacy and Function outputs before retiring scripts

Tip: Keep a canonical test suite that runs both the old Script outputs (when possible) and the new Function outputs against the same cart fixtures to spot drift early.

Rollout strategy

Options to reduce blast radius:

  • Canary by session percentage or feature flag
  • Deploy to one storefront or market first
  • Schedule deployments during lower-traffic windows
  • Keep stakeholders (marketing, support, ops) informed and ready to respond

Monitoring & observability

Key signals to track:

  • Discount application rate vs historical baseline
  • Checkout conversion and abandonment
  • Average order value (AOV)
  • Function latency and error rate
  • Structured logs showing unmatched cases or unexpected inputs

Integrate with your centralized logging and alerting stack and define thresholds for automated alerts.

Common pitfalls and how to avoid them

  • Expecting a straight 1:1 code translation — instead, map business intent first.
  • Incomplete test coverage — build deterministic fixtures and automated checks.
  • Preserving clever one-off rules — refactor reusable parts into shared helpers.

Post-migration optimization

After parity is confirmed:

  • Consolidate duplicated rules into shared libraries
  • Tune telemetry to find performance bottlenecks
  • Consider warm-up strategies if cold starts are visible in latency

Conclusion

Migrating from Shopify Scripts to Shopify Functions reduces technical risk and aligns stores with Shopify’s supported extensibility. Start with an audit, prototype a high-value use-case, validate with rigorous tests, and roll out in stages with clear monitoring and rollback plans. Want help planning or executing a migration? Reach out and we can map a pilot together.

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