Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.

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 Runtime : Drop in Node compatible runtime (built on JavaScriptCore, implemented in Zig). Package manager : bun install is significantly faster than npm/yarn. Lockfile is bun.lock (text) by default in current Bun; older versions used bun.lockb (binary). Bundler : Built in bundler and transpiler for apps and libraries. Test runner : Built in bun test with 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. Vercel : Set runtime to Bun in project settings. Build: bun run build or bun build ./src/index.ts outdir=dist . Install: bun install frozen lockfile for reproducible deploys. Examples Run and install Scripts and env Testing Runtime API Best Practices Commit the lockfile ( bun.lock or bun.lockb ) for reproducible installs. Prefer bun run for scripts. For TypeScript, Bun runs .ts natively. Keep dependencies up to date; Bun and the ecosystem evolve quickly.

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