Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.
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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
# Install dependencies (creates/updates bun.lock or bun.lockb)
bun install
# Run a script or file
bun run dev
bun run src/index.ts
bun src/index.ts
Scripts and env
bun run --env-file=.env dev
FOO=bar bun run script.ts