Bun Runtime

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 "bunVersion": "1.x" in vercel.json — this runs Vercel Functions and Routing Middleware on the Bun runtime instead of Node.js (accepted values today: "1.x", "1.4.x"; Vercel manages minor versions). Build: bun run build or bun build ./src/index.ts --outdir=dist. Install: bun install --frozen-lockfile — or bun ci, which behaves the same — for reproducible deploys (both fail when bun.lock doesn't match package.json).

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

Testing

bun test
bun test --watch

// test/example.test.ts
import { expect, test } from "bun:test";

test("add", () => {
  expect(1 + 2).toBe(3);
});

Runtime API

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.