Bun (JavaScript runtime)

--- name: bun-runtime description: Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support. origin: ECC --- # 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 ``bash # 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 `bash bun run --env-file=.env dev FOO=bar bun run script.ts ` ### Testing `bash bun test bun test --watch ` `typescript // test/example.test.ts import { expect, test } from "bun:test"; test("add", () => { expect(1 + 2).toBe(3); }); ` ### Runtime API `typescript const file = Bun.file("package.json"); const json = await file.json(); Bun.serve({ port: 3000, fetch(req) { return new Response("Hello"); }, }); ` ## 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.