bun-runtime

What this skill does

When applied, it prepends a system prompt before your request is sent — no extra calls and no change to how you are billed beyond the added tokens.

---
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

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  "messages": [{ "role": "user", "content": "…" }]
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