Bun

Bun is a JavaScript and TypeScript toolkit in one binary: a runtime built as a drop-in replacement for Node.js, plus a package manager, test runner and bundler, and now a formatter (bun fmt) and type checker (bun check). It's free and MIT-licensed. Since December 2025 it's owned by Anthropic, which ships Claude Code on it.

Install is one curl command, and you can adopt it piece by piece: run bun install or bun test inside an existing Node project without switching runtimes. Bun 1.4 (August 2026) rewrote the whole thing from Zig to Rust and added more than 1,500 tests from Node's own suite. New releases land every few weeks, and there's nothing to host.

Solo through large teams: free. There's no paid tier and no Bun cloud. The cost is migration time, not money.

The catch: compatibility is close, not complete. Bun's own numbers show node:http, node:fs and node:stream passing 97% of Node's tests, so test your app before moving production. The October 1.4.4 release patched three high-severity bugs, including a brotli out-of-bounds write. And one AI company now steers your runtime. Bun promises it stays MIT; Node and Deno are your hedge.