Rewriting Bun: From Zig to Rust — A Technical Retrospective

The article details the journey of rewriting Bun, a JavaScript runtime, from Zig to Rust. It covers the original creation of Bun as a line-for-line port of esbuild's transpiler from Go to Zig, the massive scope of the project, and the subsequent decision to rewrite it in Rust. The author discusses the technical challenges, performance considerations, and the evolution of the project, including the acquisition by Anthropic in December 2025. The piece provides deep technical insight into systems programming language choices, compiler design, and runtime development.