Bun is a JavaScript and TypeScript runtime that packs a runtime, an npm-compatible package manager, a bundler and a test runner into a single binary.
It is written in Zig and built on JavaScriptCore, Safari's JavaScript engine, whereas Node.js and Deno use V8. Created by Jarred Sumner, it reached version 1.0 in September 2023.
Its main selling point is speed: fast startup, very short dependency installs, and direct execution of TypeScript files with no compile step. It aims for compatibility with Node.js APIs, so a large share of existing code runs without being rewritten.
You mostly find it in development tooling (scripts, tests, package installs) and in newer back-end services, less often as the main production runtime of an older application.
Why it matters when hiring
Bun alone does not justify a hire: a good Node.js or TypeScript developer picks it up in a few days. The real signal in an interview is the ability to explain what Bun changes and what it does not (incomplete compatibility with some native modules, production maturity). Asking for “3 years of Bun” in a job ad is a trap: few candidates can show it, and it is not the right criterion.