Shopify Scripts Functions: A Practical Migration Playbook

Introduction ## Why migrate now ## Conceptual changes: Scripts vs Functions ## Quick comparison ## Migration overview — step-by-step ## Mapping common use-cases ## Real-world scenarios (short) ## QA and testing checklist ## Rollout strategy ## Monitoring & observability ## Common pitfalls and how to avoid them ## Post-migration optimization ## Conclusion 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 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. Shopify Functions run as sandboxed WebAssembly modules and plug into Shopify’s native evaluation points (discounts, shipping, payments). The migration brings: Key differences to keep in mind: Design with determinism and idempotence in mind: Functions should produce the same output for the same cart input every time. Follow this task-oriented flow. Implement & Integrate How common Script patterns translate to Functions: Always replace side-effects with returned, repeatable data structures — Functions should not rely on mutable external state. 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. Options to reduce blast radius: Key signals to track: Integrate with your centralized logging and alerting stack and define thresholds for automated alerts. After parity is confirmed: 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. Home: https://prateeksha.com Blog: https://prateeksha.com/blog Canonical: https://prateeksha.com/blog/migrate-shopify-scripts-to-shopify-functions-playbook Templates let you quickly answer FAQs or store snippets for re-use. Are you sure you want to hide this comment? It will become hidden in your post, but will still be visible via the comment's permalink. Hide child comments as well For further actions, you may consider blocking this person and/or reporting abuse - 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. - Better performance and scalability - Clearer extension boundaries and versioning - Reduced maintenance debt for long-term compatibility - 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. - 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. - 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. - 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. - Prioritize & Scope Tag scripts as High (revenue-critical), Medium (UX/ops), Low (infrequent). Choose one high-impact script for your pilot. - Tag scripts as High (revenue-critical), Medium (UX/ops), Low (infrequent). - Choose one high-impact script for your pilot. - 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. - 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. - Prototype Build a minimal Function that covers core behavior. Run locally with Shopify CLI and a dev store to confirm baseline parity. - Build a minimal Function that covers core behavior. - Run locally with Shopify CLI and a dev store to confirm baseline parity. - Implement & Integrate Expand logic, introduce shared utilities, and add unit and integration tests. Add structured logging and feature flags to control rollout. - Expand logic, introduce shared utilities, and add unit and integration tests. - Add structured logging and feature flags to control rollout. - 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. - 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. - Staged Rollout Canary by percentage, storefront, or market. Monitor key metrics and hold a rollback window. - Canary by percentage, storefront, or market. - Monitor key metrics and hold a rollback window. - Post-launch Validate telemetry, monitor errors, and iterate. - Validate telemetry, monitor errors, and iterate. - 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. - Tag scripts as High (revenue-critical), Medium (UX/ops), Low (infrequent). - Choose one high-impact script for your pilot. - 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. - Build a minimal Function that covers core behavior. - Run locally with Shopify CLI and a dev store to confirm baseline parity. - Expand logic, introduce shared utilities, and add unit and integration tests. - Add structured logging and feature flags to control rollout. - 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. - Canary by percentage, storefront, or market. - Monitor key metrics and hold a rollback window. - Validate telemetry, monitor errors, and iterate. - 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. - 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. - List active scripts and business intents - Capture analytics and revenue attribution per script - Document any API limitations - Create Function prototype with unit tests - Add structured logs and feature flags - Build integration tests against a dev store - Smoke test on limited storefront - Monitor AOV, conversion rate, discount application rate - Prepare clear rollback steps - Monitor errors, latency, and revenue math - Run periodic parity checks between legacy and Function outputs before retiring scripts - 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 - 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 - 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. - Consolidate duplicated rules into shared libraries - Tune telemetry to find performance bottlenecks - Consider warm-up strategies if cold starts are visible in latency