Shopify Scripts Are Gone: Here’s How to Master the Migration to Functions

If you’re here, you already know Shopify Scripts are being replaced by Shopify Functions, and you’re likely wondering: How much work is this migration going to take?

Let’s get one thing straight—this isn’t just about swapping one tool for another. Migrating from Scripts to Functions is an opportunity to rethink how you approach customizations, workflows, and scaling. Functions don’t just check the boxes; they bring better performance, deeper integration, and unmatched flexibility to the table.

That’s where this guide comes in. We’ll break down exactly what’s changing, what you need to know, and how you can confidently navigate this transition—step by step.

Why did scripts have to go?

Shopify Scripts brought a level of customization that, for a time, was revolutionary—allowing Shopify Plus merchants to tailor discounts, shipping, and payment options to fit their unique needs. But as e-commerce evolved, Scripts struggled to keep pace with the growing demands of modern workflows and merchant expectations.

Here’s why Scripts no longer fit the bill:

i. Limited Scope: Scripts were built exclusively for checkout customizations. Modern businesses need solutions that span the entire buyer journey—covering dynamic cart rules, real-time delivery adjustments, and B2B workflows. Scripts simply couldn’t handle these broader requirements.

ii. Scalability Issues: Scripts come with strict resource limits on CPU, memory, and execution time, making them prone to failure under high-traffic conditions. Flash sales or viral campaigns can overwhelm Scripts, turning them into bottlenecks instead of enablers.

iii. Disconnected from Shopify's growing ecosystem: As Shopify’s ecosystem expanded with tools like Flow, Checkout Extensibility, and GraphQL APIs, Scripts were left behind. Operating in isolation, they couldn’t integrate seamlessly with modern workflows, limiting merchants’ ability to automate and innovate.

iv. Maintenance Complexity: Updating Scripts meant manual edits in the Script Editor for each store, relying on developers for even minor changes. This outdated process wasted time and increased the risk of inconsistencies.

How Functions outshine Scripts: Key differences

1. Supported Languages

Shopify Functions, powered by WebAssembly (Wasm), bring unmatched performance and flexibility. Here’s why this shift matters:

- Work with what you know: Functions let you code in modern, high-performance languages like Rust, Go, and AssemblyScript and already familiar to many developers. This eliminates the need for Shopify-specific tools and allows you to work faster using technologies you already trust.

- Performance without limits: Wasm executes custom logic in under 5 milliseconds, delivering consistent performance even during traffic spikes. For developers, this ensures reliable scaling and faster workflows.

- Part of a bigger ecosystem: Wasm isn’t confined to Shopify—it’s a widely adopted standard across industries. That means better tools, a larger developer community, and more resources at your fingertips. Investing in Wasm doesn’t just make sense today; it ensures your skills and workflows stay relevant for the future.

2. Areas of Applications

Shopify Functions unlock a new level of flexibility, enabling custom logic across the entire buyer journey—from cart-level adjustments to post-checkout workflows and even accelerated checkout flows like Shop Pay. Here’s how Functions expand your toolkit:

i. Cart-level customizations:

Functions enable real-time modifications to cart behavior, allowing you to create dynamic and personalized shopping experiences:

- Dynamic Bundle Discounts: Automatically adjust bundle pricing based on quantity or product combinations (e.g., "Buy 2, Get 1 Free").

- Age Validation: Block restricted items like alcohol unless the customer meets legal requirements.

- Upsell Recommendations: Present personalized product suggestions to boost average order value (AOV), such as offering complementary items based on cart contents.

ii. Draft order workflows:

For B2B and wholesale operations, traditional checkout flows don’t always fit. Functions integrate with draft orders to tailor these workflows:

- Custom pricing tiers: Automatically apply specific discounts or pricing rules for segmented customer groups.

- Approval workflows: Create automated rules to flag high-value or complex orders for manual review before processing.

iii. Real-time delivery customizations:

With Functions, merchants can dynamically fine-tune delivery options, offering smarter, more relevant choices:

- Geography-based rules: Restrict same-day delivery to urban areas while offering free shipping to rural zones.

- Customer segmentation: Prioritize faster shipping for VIP customers or urgent orders.

- Inventory awareness: Enable pre-orders or restrict delivery based on real-time stock availability.

iv. Support for accelerated checkouts:

Functions work seamlessly with fast-track checkout flows like Shop Pay, ensuring that custom logic—whether discounts, delivery customizations, or cart validations—operates flawlessly at speed.

3. Integration Depth

Scripts operated in isolation. They let you control checkout customizations, but integrating them with Shopify’s broader ecosystem was often a patchwork effort.

i. Centralized Control in Shopify Admin: Functions are managed alongside native tools like discounts, delivery rules, and payment settings. This eliminates the need for external apps or disconnected workflows. Everything is in one place, simplifying operations.

ii. Seamless API Integration: Functions integrate natively with Shopify’s evolving ecosystem, enabling merchants and developers to create dynamic, flexible solutions:

- Checkout Extensibility API: Build custom checkout workflows while staying compatible with Shopify’s rapid updates.

- Shopify Flow: Automate operational tasks by triggering actions based on Function outputs.

- GraphQL Admin API: Share data across Shopify apps effortlessly, keeping everything connected and in sync.

iii. Developer-First Design: Functions are built with modern development workflows in mind, making updates and collaboration easier than ever:

- Version Control: Functions integrate with Git, so you can manage, track, and roll back changes when needed.

- Centralized Updates: Update a Function once, and it’s deployed across all stores using it—no more manual, store-by-store edits.

iv. App-based deployment and testing: Functions are deployed as apps, eliminating manual Script Editor updates. Developers can test locally with real-world scenarios using Shopify CLI and JSON payloads. Once ready, updates roll out automatically across all stores, ensuring consistency and saving time.

4. Scalability

Shopify Functions were designed to meet high-traffic and complex workflows. Here’s how they scale where Scripts couldn’t:

i. Dynamic Resource Allocation: Functions intelligently adjust CPU, memory, and execution time based on the cart’s complexity and traffic intensity.

• Effortlessly handle carts with 200+ items and 25+ simultaneous discounts.
• Ensure smooth user experiences, even during flash sales or holiday rushes.

ii. Parallel Execution: Thanks to WebAssembly (Wasm), Functions can run multiple customizations simultaneously without compromising performance:

• Apply discounts across multiple cart items concurrently.
• Execute complex workflows, like multi-location fulfillment or advanced payment logic, in parallel.

iii. High-Load Resilience: Functions thrive under extreme traffic conditions, maintaining sub-5ms execution times for thousands of transactions per minute.

• Whether it’s a viral flash sale or a global campaign, Functions keep transactions seamless and error-free.

Step by Step Guide to Migrate from Scripts to Functions

Planning is everything. A well-thought-out strategy not only simplifies the transition but also ensures you’re fully equipped to leverage the power of Shopify Functions. Follow these steps to prepare:

1. Audit Existing Scripts: Streamline your migration process by cataloging and evaluating your active Scripts. Here’s how:

- Catalog Active Scripts: Create a list of all active Scripts, grouped by their functionality (e.g., Discounts, Shipping, Payment Logic).

- Identify Key Triggers: Document what conditions activate each Script.

- Evaluate Dependencies: Note any external tools, apps, or processes tied to the Script.

- Review Performance: Analyze CPU usage, memory consumption, and execution times to identify bottlenecks or inefficiencies.

This step helps you determine which Scripts should be migrated, optimized, or retired, creating a clear foundation for the transition to Functions.

2. Map Scripts to Functions: Match your Scripts to Shopify Functions APIs like Order Discount or Delivery Customization. For unsupported use cases, assess Shopify’s roadmap or alternative strategies. Begin with simpler use cases and roll out more complex logic incrementally.

3. Equip Your Team: Ensure your team is up to speed with Functions’ syntax, supported languages like Rust or JavaScript, and tools like GraphQL. Mock scenarios and debug locally to fine-tune your approach.

4. Plan for Edge Cases: Anticipate scenarios where Functions might face limitations and build fallback logic. Test extensively for scalability to ensure your setup holds up under pressure, such as high-SKU carts or peak-season traffic.

With the groundwork laid, here’s how to execute the migration, step by step.

Step 1: Inventory and Evaluate Existing Scripts

i. Access Script Inventory: Navigate to Apps > Script Editor in your Shopify Admin to list all active scripts.

ii. Categorize by Functionality: Group scripts into Discounts, Shipping Rates, and Payment Customizations to map their scope.

iii. Analyze Usage:

• Identify frequency: Are scripts triggered universally or for specific campaigns?
• Measure performance: Use metrics from the Script Editor to review CPU, memory usage, and execution time. Identify bottlenecks.

iv. Evaluate for Migration:

• Cross-check functionality with supported Shopify Functions APIs (e.g., Discounts, Delivery Customization).
• Highlight unsupported logic or dependencies, such as those tied to checkout.liquid, to plan alternative solutions.

Step 2: Prepare Your Development Environment

i. Install Required Tools:

- Shopify CLI: Download and set up for managing Functions locally OR install the Shopify CLI by following the official Shopify CLI installation guide.

curl -s https://shopify.dev/cli | bash 

- Rust: Shopify Functions default to Rust. Install via

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh 

- Node.js and NPM (Optional): For working with AssemblyScript if Rust isn’t preferred.

ii. Clone Function Examples: Use Shopify’s Function Examples Repository to kickstart your migration.

git clone https://github.com/Shopify/function-examples 
cd function-examples 

Review scripts for discounts, delivery, and payments to understand structure.

Step 3: Build a New Function

- Generate a Scaffold:

shopify app generate extension --type=function 

• Select the desired function type (e.g., Discount, Delivery Customization).
• Provide a meaningful name for the Function.

- Define GraphQL Input: Create an input schema defining data available for your Function. Example for a discount function:

query { 

  cart { 

    lines { 

      merchandise { 

        ... on ProductVariant { 

          price 

        } 

      } 

    } 

  } 

} 

Write Your Logic in Rust: Example: Apply a $10 discount to products over $100.

use shopify_function::prelude::*; 

  
#[shopify_function] 

fn apply_discounts(input: Input) -> Result<Output> { 

// Filter cart lines to identify items priced above $100 

    let discounts: Vec<Discount> = input 

        .cart 

        .lines 

        .iter() 

        .filter_map(|line| { 

            if line.merchandise.price > 100.0 { 

                Some(Discount { 

                    value: 10.0,  // Apply $10 discount 

                    target: line.id.clone(),  // Target the specific line item 

                }) 

            } else { 

                None 

            } 

        }) 

        .collect(); 

// Return the computed discounts 

    Ok(Output { discounts }) 

} 

Step 4: Test Your Function Locally

1. Run Local Tests Using Shopify CLI: Testing your Shopify Functions locally is a critical step in ensuring your migration is smooth and free from errors. Here's how to test effectively:

• Use the command below to test your Function locally with a sample JSON payload:

shopify app dev --extension=function 

When prompted, provide an example JSON input that matches the GraphQL schema you defined for your Function. Example Input -

{ 

  "cart": { 

    "lines": [ 

      { "merchandise": { "price": 150.0 }, "id": "line-item-1" }, 

      { "merchandise": { "price": 50.0 }, "id": "line-item-2" } 

    ] 

  } 

} 

Observe the CLI output to validate that the discounts or logic applied to the input data meet your expectations.

2. Enable Debug Mode: If the results are unexpected or errors occur, replay the test in debug mode:

shopify app dev --extension=function –debug 

- Debug Mode Benefits:

• Displays detailed logs of how your Function processes input data.
• Helps identify issues like invalid inputs or unexpected behavior in your logic.

3. Validate Input/Output: Compare the output JSON with the expected structure. Ensure that all necessary fields are present and correctly formatted. Example Output :

Json 

{ 

  "discounts": [ 

    { "value": 10.0, "target": "line-item-1" } 

  ] 

} 

4. Test Edge Cases:

• Empty carts.
• Carts with items that don't meet discount criteria.
• Carts with a high number of SKUs to check for scalability.

5. Integration Testing: After local testing, deploy the Function to a development store and test it end-to-end in the Shopify checkout flow.

Step 5: Package and Deploy

Once your Function is tested, it’s ready for deployment.

1. Build the Function: Convert the Rust code to WebAssembly:

cargo build --target wasm32-unknown-unknown --release 

2. Deploy to Shopify: Use the CLI to upload your Function to Shopify.

Step 6: Configure in Shopify Admin

i. Navigate to the relevant settings in Shopify Admin:

• For discounts: Settings > Discounts.
• For delivery: Settings > Shipping and Delivery.

ii. Add and configure your deployed Function.

Step 7: Monitor Performance

i. Check Function Metrics: Use Shopify CLI or Partner Dashboard to track:

• Execution frequency.
• Time taken (ensure <5ms).
• Input/output validation.

ii. Use Shopify Analytics: Monitor key metrics like cart abandonment, redemption rates, or shipping performance.

Resources for Migration to Shopify Functions

Shopify has a wealth of resources to guide you through every step. Here are the must-haves to keep handy:

• Shopify Functions Documentation: Detailed guides on how Functions work, including APIs, workflows, and use cases.

• Shopify Migration Guide: A detailed roadmap to evaluate your current Scripts and plan your migration.

• GraphQL Admin API Reference: Everything you need to create and manage input and output structures for Functions.

• GitHub Examples: Pre-built Shopify Functions examples to help you get started quickly.

• Rust Learning Resources: Get up to speed with Rust, Shopify’s recommended language for Functions.