---
name: bun-runtime-fc0ff230f098
description: Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.
metadata:
  origin: ECC
---

# 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

- **Version baseline**: guidance below targets Bun 1.4.x. Check the actual version with `bun --version` before relying on flag names, since CLI flags can change between releases.
- **Runtime**: Drop-in Node-compatible runtime, built on JavaScriptCore. As of Bun 1.4 the runtime itself is implemented in Rust (migrated from Zig). Not 100% Node-compatible — some native addons, less-common `node:` internal APIs, and packages that depend on Node-specific internals can still fail; verify before depending on it in production.
- **Package manager**: Bun includes a package manager; installation performance depends on the project, network conditions, and cache state. Lockfile is `bun.lock` (text) by default in current Bun; older versions used `bun.lockb` (binary) — Bun still reads that format for migration, but new projects should use `bun.lock`.
- **Bundler**: Built-in bundler and transpiler for apps and libraries.
- **Test runner**: Built-in `bun test` with a 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.

**Package and workspace commands**: `bun add <pkg>` / `bun remove <pkg>` / `bun update [pkg]` manage dependencies; `bun outdated` lists stale deps. `bun pm ls` lists project dependencies and resolved versions using the lockfile; `--all` includes transitive dependencies. It is not an integrity check of installed file contents. [`bun audit`](https://bun.com/docs/pm/cli/audit) queries registry advisories using package names and versions from `bun.lock`; it does not modify `package.json`, `bun.lock`, or `node_modules`. Review approved destinations before sending private package metadata. Packages whose scoped registry lacks an advisory endpoint are skipped and do not affect the exit code. `bun audit fix` upgrades and installs dependencies, changing the lockfile and installed packages and sometimes direct version pins; review it as a modifying install operation, including applicable lifecycle scripts. `bun dedupe` collapses duplicate installs, and `bun prune` removes packages no longer referenced. Monorepos use a root `workspaces` array in `package.json` (same convention as npm/yarn); run a script in one workspace with `bun run --filter <pkg-name> <script>`.

**Built-in APIs**: reach for these before adding a dependency — `Bun.file` / `Bun.write` for file I/O, `bun:sqlite` for an embedded SQLite database, `Bun.serve` for an HTTP/WebSocket server, [`Bun.sql`](https://bun.com/docs/runtime/sql) for SQL databases, [`Bun.redis`](https://bun.com/docs/runtime/redis) for Redis, and [`Bun.S3Client`](https://bun.com/docs/runtime/s3) for S3-compatible object storage. See the [1.4 release notes](https://bun.com/blog/bun-v1.4) for the rest of what's new.

**Vercel**: Set `bunVersion: "1.4.x"` in `vercel.json` to use Bun 1.4 (Rust runtime). See [Vercel's Bun runtime docs](https://vercel.com/docs/functions/runtimes/bun). Build: `bun run build` or `bun build ./src/index.ts --outdir=dist`. Install: `bun install --frozen-lockfile` for reproducible deploys.

**Reference**: [Bun 1.4 release notes](https://bun.com/blog/bun-v1.4).

## Examples

### Run and install

```bash
# Install dependencies (creates/updates bun.lock)
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
```

For CI on Bun 1.4, available options include `bun test --changed[=<ref>]` for affected tests, `bun test --isolate` for per-file globals, `bun test --parallel[=<n>]` for worker processes, and `bun test --shard=<n>/<count>` for CI partitioning. `--isolate` creates a fresh JavaScript global for each file in the same process and cleans up documented per-file resources; it is not a process or security sandbox. `--parallel` enables it by default. `--timings=<file>` reads prior durations for scheduling; `--update-timings` records durations. Example: `bun test --parallel --timings=./test-timings.json --update-timings`. Check the installed version's help and documentation before choosing timing-file paths for a multi-shard workflow.

```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`) 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.


---
Source attribution: affaan-m
Original project: https://github.com/affaan-m/everything-claude-code
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2026 Gabriel Moreira

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: affaan-m
Original project: https://github.com/affaan-m/everything-claude-code
Attribution basis: Upstream repository explicitly recorded in package mirror path

Fields of work: Development & Engineering, Product Management
Roles: Backend Developer, DevOps / SRE, Frontend Developer, Full-Stack Developer, Product Manager
Specialties: Backend, Cross Platform, Frontend
Tools: Bun, Vercel
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 959e7bca9b4c59859defa44bef657dfb1358c00f35b8454c2b73f8eb28476c11