On May 14, the open-source JavaScript runtime Bun merged pull request #30412, rewriting its entire codebase from Zig to Rust. The change, led by Bun’s core team, targets performance gains, memory safety, and broader cross-platform support. Bun, created as a faster alternative to Node.js and Deno, now runs on a Rust foundation while maintaining compatibility with existing JavaScript and TypeScript applications.
The rewrite was announced in a GitHub pull request titled 'Rewrite Bun in Rust,' which was merged on May 14. Bun’s creator, Jarred Sumner, described the shift as a strategic move to leverage Rust’s performance and safety features. 'Rust’s zero-cost abstractions and memory safety guarantees make it ideal for a runtime like Bun,' Sumner wrote in the pull request description. The change follows Bun’s rapid growth, with over 100,000 weekly downloads on npm and adoption by companies like Vercel and Cloudflare.
Bun was originally written in Zig, a low-level programming language designed for performance and safety. However, the team cited Rust’s larger ecosystem and tooling as key factors in the switch. 'Rust’s compiler, package manager, and community support are unmatched,' the pull request noted. The rewrite retains Bun’s core features, including its built-in bundler, test runner, and package manager, while aiming to reduce memory usage and improve startup times for JavaScript applications.
Performance benchmarks included in the pull request show Bun’s Rust rewrite achieving a 20-30% reduction in memory usage compared to its Zig-based predecessor. Startup times for Node.js-compatible applications also improved by 15-25%, according to internal tests. The team emphasized that the rewrite does not break existing APIs, ensuring backward compatibility for developers. 'We’ve worked hard to make the transition seamless,' Sumner stated in a comment on the pull request.
The decision to rewrite Bun in Rust aligns with a broader trend in the JavaScript ecosystem, where Rust is increasingly used for performance-critical components. Deno, another JavaScript runtime, also incorporates Rust for its core modules, while projects like SWC (a Rust-based JavaScript compiler) have gained traction for their speed. Bun’s team highlighted Rust’s ability to handle concurrency and parallelism as a key advantage for modern web applications.
Bun’s Rust rewrite includes support for WebAssembly (Wasm), enabling developers to run Wasm modules alongside JavaScript. This feature is particularly useful for high-performance computing tasks, such as image processing or data analysis. The team also noted that Rust’s cross-platform capabilities will simplify Bun’s deployment on Windows, macOS, and Linux, reducing the need for platform-specific optimizations.
The pull request generated significant discussion in the developer community, with over 500 comments and 2,000 reactions on GitHub. Many developers praised the move, citing Rust’s growing popularity and its potential to improve Bun’s long-term maintainability. 'This is a huge win for the JavaScript ecosystem,' wrote one commenter. Others raised concerns about the complexity of maintaining a Rust codebase, particularly for contributors unfamiliar with the language.
Bun’s team addressed these concerns by committing to extensive documentation and community support for the Rust rewrite. 'We’ll provide detailed guides and tutorials to help contributors get up to speed,' Sumner wrote in response to a question about onboarding. The team also plans to release a series of blog posts and benchmarks comparing Bun’s performance before and after the rewrite, with the first post expected in June.