Shopify Functions represent a new frontier of backend customization on the platform, allowing developers to write their own business logic for critical areas like discounts, shipping, and payments. These functions are compiled to WebAssembly (Wasm), a high-performance, sandboxed runtime that ensures they run quickly and securely on Shopify’s infrastructure.
When you generate a new function using the Shopify CLI, you are presented with a choice of languages, with the two most prominent being Rust and JavaScript.
While JavaScript is the familiar choice for the vast majority of Shopify developers, Rust is often touted as the “native” language of WebAssembly, promising unparalleled performance and safety. This leaves many developers wondering: which one should I choose?
This guide will provide a professional, technical comparison to help you make an informed decision based on the needs of your project and the skills of your team.
The Case for JavaScript: Familiarity and Speed of Development
For most developers in the Shopify ecosystem, JavaScript (or TypeScript) is the path of least resistance. It’s the language of the web, the language of theme development, and a first-class citizen in the Shopify Functions environment.
Pros of Using JavaScript:
- Familiar Syntax and Ecosystem: You don’t need to learn a new language. You can leverage your existing knowledge of JavaScript, along with the vast ecosystem of npm packages, to get up and running quickly.
- Faster Iteration and Development: The development cycle for JavaScript is generally faster. There’s no complex compilation step during local development, and the language itself is more forgiving than Rust.
- Excellent Tooling and AI Support: The Shopify CLI provides great boilerplate and type generation for JavaScript Functions. Furthermore, AI coding assistants are extensively trained on JavaScript, making them highly effective partners for writing and debugging your code.
Cons of Using JavaScript:
- Potential for Runtime Errors: As a dynamically typed language, JavaScript is more prone to runtime errors (e.g., “undefined is not a function”) that can only be caught through rigorous testing. While TypeScript helps, it’s not a complete guarantee of safety.
- Slight Performance Overhead: While Shopify’s Wasm runtime is incredibly fast, JavaScript will always have a slight performance overhead compared to a language like Rust that compiles directly to highly optimized machine code.
Choose JavaScript when: Your top priorities are speed of development and leveraging your team’s existing skills. For the vast majority of custom discount or validation logic, the performance of JavaScript is more than sufficient.
The Case for Rust: Performance and Safety
Rust is a modern, systems-level programming language that is renowned for its performance and, most importantly, its memory safety. It’s a language that is “fast, reliable, and productive,” and it’s a favorite in the WebAssembly community.
Pros of Using Rust:
- Blazing-Fast Performance: Rust compiles to a highly optimized Wasm binary with minimal overhead. For functions that need to perform complex calculations or run millions of times a day, this can be a significant advantage.
- Unmatched Safety and Reliability: Rust’s strict compiler and ownership model prevent entire classes of common bugs, such as null pointer exceptions and data races. If your Rust code compiles, you can have a very high degree of confidence that it is free of memory-related errors.
- First-Class WebAssembly Support: Rust has the most mature and well-supported toolchain for compiling to WebAssembly in the entire industry.
Cons of Using Rust:
- Steep Learning Curve: Rust is a fundamentally different and more complex language than JavaScript. Its concepts of ownership, borrowing, and lifetimes can be challenging for developers coming from a scripting background.
- Slower Development Cycle: The strictness of the Rust compiler means you’ll spend more time upfront satisfying the compiler’s checks. While this leads to more reliable code, it can slow down the initial development process.
Choose Rust when: Your function is performance-critical and will run at a massive scale, or when the logic is so complex that the compile-time safety guarantees of Rust become a major asset in preventing bugs.
Head-to-Head Comparison
| Factor | JavaScript | Rust |
|---|---|---|
| Performance | Very Good | Exceptional |
| Safety & Reliability | Good (especially with TypeScript) | Exceptional (guaranteed by the compiler) |
| Ease of Learning | Easy (for existing web developers) | Difficult (steep learning curve) |
| Development Speed | Faster | Slower (more upfront effort) |
| Ecosystem & Libraries | Massive (npm) | Growing (Crates.io) |
| Best For | Most business logic, rapid development | Performance-critical, high-scale tasks |
Final Thoughts: The Pragmatic Choice
For the professional Shopify developer, the choice between Rust and JavaScript should be a pragmatic one.
Start with JavaScript. For 95% of Shopify Functions, its performance is more than adequate, and the speed of development and familiar ecosystem make it the most efficient choice.
Reach for Rust when you have a clear, justifiable reason. If you are building a function for a Shopify Plus merchant that will handle a massive volume of traffic, or if the logic is so complex that Rust’s safety guarantees will save you from costly bugs down the line, then investing in Rust is a wise decision.
Ultimately, the best language is the one that allows you to ship a reliable, performant, and maintainable solution for your client. For most Shopify developers, most of the time, that language is JavaScript.
❓ Have you built a Shopify Function in Rust? What was your experience, and was the performance benefit worth the learning curve?