Bun runtime overview

Bun is an all-in-one JavaScript runtime and toolkit that provides a runtime, package manager, bundler, and test runner with a focus on speed.

When to choose Bun

• Prefer Bun for new JavaScript/TypeScript projects where install and startup speed matter, and a small tooling surface is desirable.
• Prefer Node when you require the widest ecosystem compatibility, depend on native tooling that assumes Node, or have dependencies with known Bun incompatibilities.

Use cases: adopting Bun for new packages, migrating from Node, writing or debugging Bun scripts and tests.

What Bun provides

• Runtime: a fast drop-in runtime implemented on JavaScriptCore (written in Zig) with many Node-compatible APIs.
• Package manager: bun install is significantly faster than many alternatives; lockfile is bun.lock (text). Older versions used bun.lockb (binary).
• Bundler: built-in bundler and transpiler for applications and libraries.
• Test runner: bun test with a Jest-like API.

Migrating from Node