Jarred Sumner on Bun: hyperfast JavaScript toolkit

Sumner describes Bun as a JavaScript runtime, package manager, task runner, and test runner built to simplify common workflows. He says the ecosystem has become complicated and pieced together, and argues that faster tools help developers spend less time waiting and switching contexts. Bun aims to work with existing tools: developers can keep using Yarn, npm, and familiar build tools.

Sumner credits Bun’s speed to many small optimizations informed by profiling. On macOS, `bun install` uses `clonefile` for copy-on-write directory cloning; Bun also reduces path copying, optimizes lockfile serialization, and uses SIMD to process text in larger chunks. He says the transpiler uses SIMD when parsing large inputs such as source maps.

Bun began after Sumner found that refreshing his browser voxel game took 30 seconds to a minute. He built a transpiler, then added a runtime after aiming to support Next.js, embedding JavaScriptCore after studying its code. He chose Zig because he found it productive and predictable, with explicit memory management and development checks.

Sumner says `bun install` is likely Bun’s easiest feature to adopt, while Bun test still lacked `extends`, inline snapshots, and mock modules at the time. Bun’s binary lockfile and file operations contribute to install speed; dependency postinstall scripts were disabled by default, with an allowlist planned for packages that need them. For React Native, he discusses possible future bundler or on-device runtime uses, but stresses these ideas were speculative and unimplemented.

Sumner says Bun aims to replace Node.js while implementing most of its core modules, though some were still missing. Windows support was limited, and he cited the small team and hiring needs as factors. He also described plans for hosting through the Bun CLI, but gave no release date and said bug fixing remained the priority.