bun-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. ---

# 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.