# Bun

> Bun — fast all-in-one JavaScript/TypeScript runtime, package manager, bundler, and test runner. Use when building with Bun, running TypeScript, managing packages with bun install, writing tests with bun test, or asking about Bun APIs, configuration, or Node.js migration. Fetch live documentation for up-to-date API details.

- Skill: `mikkelkrogsholm/bun` (Agent Skill)
- Install (CLI): `npx skillmds@latest add mikkelkrogsholm/bun`
- Raw SKILL.md: https://api.skillmd.com/api/skills/mikkelkrogsholm/bun/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Docs & Writing
- Author: mikkelkrogsholm (https://skillmd.com/u/mikkelkrogsholm)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/mikkelkrogsholm/bun

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# Bun

> **CRITICAL: Your training data for Bun is unreliable.** APIs change between versions and memorized patterns may be wrong or deprecated. Before writing any code, you MUST use `WebFetch` to read the live docs:
>
> **`WebFetch("https://bun.sh/llms.txt")`**
>
> Do not proceed without fetching this URL first. Never assume an API exists — verify against current docs.

Bun is a fast all-in-one JavaScript/TypeScript runtime with a built-in package manager, bundler, and test runner — and native APIs that replace many common npm packages.
## Key Capabilities

Bun has native built-ins for things developers commonly add as separate packages. **Always prefer these over npm equivalents:**

- **SQL (SQLite & PostgreSQL)**: use `Bun.sql()` / `new Bun.sqlite()` — not `better-sqlite3`, `pg`, or `mysql2`
- **Redis**: use `Bun.redis()` — not `redis` or `ioredis`
- **S3**: use `Bun.s3()` — not `@aws-sdk/client-s3`
- **Password hashing**: use `Bun.password.hash()` / `Bun.password.verify()` — not `bcrypt`
- **Shell scripting**: use `Bun.$\`command\`` — not `shelljs` or `execa`
- **Testing**: use `bun test` (Jest-compatible, built-in) — not Jest
- **File globbing**: use `new Bun.Glob()` — not the `glob` package
- **UUID**: use `Bun.randomUUID()` — not the `uuid` package
- **TypeScript execution**: run `.ts` files directly — no `ts-node` or `tsx` needed

## Best Practices

- **`.env` auto-loads** — Bun reads `.env` automatically without `dotenv`. Be aware when migrating from Node.js or running in environments with existing `.env` files.
- **Lockfile is `.lockb`** — not compatible with npm/yarn by default. Use `--yarn` flag if a yarn-compatible lockfile is required.
- **Test file naming is strict** — files must match `*.test.ts`, `*.spec.ts`, etc. and live in `__tests__/` or `test/`. Wrong naming means tests are silently skipped.
- **`import.meta.main` replaces entrypoint checks** — use `if (import.meta.main)` instead of the Node.js `if (require.main === module)` pattern.