# Bun Runtime

Use Bun as runtime, package manager, bundler and test runner, with migration notes.

- Repository: https://github.com/affaan-m/ECC (294725 stars)
- Category: Engineering
- License: MIT
- Last updated: 2026-10-30
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add affaan-m/ECC --skill bun-runtime -g -a claude-code -y`
Codex: `npx skills add affaan-m/ECC --skill bun-runtime -g -a codex -y`
Cursor: `npx skills add affaan-m/ECC --skill bun-runtime -g -a cursor -y`
Gemini CLI: `npx skills add affaan-m/ECC --skill bun-runtime -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add affaan-m/ECC`

## Description

Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# 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.

The rest: https://github.com/affaan-m/ECC/blob/HEAD/.agents/skills/bun-runtime/SKILL.md