Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.
Bun is a fast all-in-one JavaScript runtime and toolkit: runtime, package manager, bundler, and test runner.
Use when: adopting Bun, migrating from Node, writing or debugging Bun scripts/tests, or configuring Bun on Vercel or other platforms.
bun install is significantly faster than npm/yarn. Lockfile is bun.lock (text) by default in current Bun; older versions used bun.lockb (binary).bun testMigration 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.
# 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
bun run --env-file=.env dev
FOO=bar bun run script.ts
bun test
bun test --watch
// test/example.test.ts
import { expect, test } from "bun:test";
test("add", () => {
expect(1 + 2).toBe(3);
});
const file = Bun.file("package.json");
const json = await file.json();
Bun.serve({
port: 3000,
fetch(req) {
return new Response("Hello");
},
});
bun.lock or bun.lockb) for reproducible installs.bun run for scripts. For TypeScript, Bun runs .ts natively.