Bun is a fast, all-in-one JavaScript runtime, bundler, transpiler, and package manager. Designed as a drop-in replacement for Node.js, it aims to improve speed and efficiency. Bun uses JavaScriptCore, a JavaScript engine developed by Apple, instead of V8. It supports TypeScript and JSX out of the box, and its built-in package manager boasts faster installation speeds compared to npm, yarn, and pnpm. Bun simplifies web development workflows with its comprehensive feature set.