Bun Runtime

Bun is a fast all-in-one JavaScript runtime and toolkit: runtime, package manager, bundler, and test runner.

When to Use

• Prefer Bun for: new JS/TS projects, scripts where install/run speed matters, Vercel deployments with Bun runtime, and when you want a single toolchain (run + install + test + build).
• Prefer Node for: maximum ecosystem compatibility, legacy tooling that assumes Node, or when a dependency has known Bun issues.

Use when: adopting Bun, migrating from Node, writing or debugging Bun scripts/tests, or configuring Bun on Vercel or other platforms.

How It Works

• Version baseline: guidance below targets Bun 1.4.x. Check the actual version with bun --version before relying on flag names, since CLI flags can change between releases.
• Runtime: Drop-in Node-compatible runtime, built on JavaScriptCore. As of Bun 1.4 the runtime itself is implemented in Rust (migrated from Zig). Not 100% Node-compatible, some native addons, less-common node: internal APIs, and packages that depend on Node-specific internals can still fail; verify before depending on it in production.
• Package manager: Bun includes a package manager; installation performance depends on the project, network conditions, and cache state. Lockfile is bun.lock (text) by default in current Bun; older versions used bun.lockb (binary), Bun still reads that format for migration, but new projects should use bun.lock.
• Bundler: Built-in bundler and transpiler for apps and libraries.
• Test runner: Built-in bun test with a Jest-like API.

Migration from Node: Replace node script.js with bun run script.js or bun script.js. Run bun install in place of npm install; most packages work. Use bun run for npm scripts; bun x for npx-style one-off runs. Node built-ins are supported; prefer Bun APIs where they exist for better performance.