Bun

Bun

Overview

Bun is an all-in-one JavaScript/TypeScript runtime that replaces Node.js, npm, webpack, and Jest with a single binary. It provides native TypeScript support, a high-performance HTTP server (Bun.serve()), a fast package manager, a bundler, and a Jest-compatible test runner with dramatically faster performance.

Instructions

• When creating HTTP servers, use Bun.serve() which handles 100K+ req/s with built-in WebSocket support, TLS, and streaming responses.
• When managing packages, use bun install (10-30x faster than npm), bun add, bun remove, and prefer bun.lock (text format) for readable git diffs.
• When bundling, use Bun.build() with appropriate target ("browser", "bun", "node"), enable code splitting with splitting: true, and configure tree shaking.
• When writing tests, use bun test with Jest-compatible API (describe, it, expect), snapshot testing, mocking with mock.module(), and --coverage for code coverage.
• When doing file I/O, prefer Bun.file() and Bun.write() over Node.js fs for significantly faster file operations, and use Bun.Glob for pattern matching.
• When handling authentication, use Bun.password.hash() and Bun.password.verify() for bcrypt/argon2 instead of npm packages.
• When migrating from Node.js, replace node with bun in scripts, keep package.json unchanged, and note that most npm packages work without modifications. Use bun:sqlite for embedded databases instead of SQLite npm packages.

Examples

Example 1: Build a high-performance API server

User request: "Create a REST API using Bun's built-in HTTP server"

Actions:

1. Create server with Bun.serve() and route handler
2. Parse JSON bodies with request.json() and return Response objects
3. Add WebSocket upgrade for real-time features
4. Use Bun.password for auth and bun:sqlite for data storage

Output: A fast API server using only Bun built-ins with no external dependencies.

Example 2: Migrate a Node.js project to Bun

User request: "Switch my Express project from Node.js to Bun"

Actions:

1. Replace npm install with bun install in CI and local setup
2. Update package.json scripts to use bun run instead of node
3. Replace dotenv with Bun's built-in .env loading
4. Switch test runner from Jest to bun test with same test files

Output: A Bun-powered project with faster installs, startup, and test execution.

Guidelines

• Use Bun.serve() for new HTTP servers; it is significantly faster than Express on Bun.
• Prefer Bun.file() and Bun.write() over Node.js fs for file operations.
• Use bun:sqlite for local data instead of adding SQLite npm packages.
• Use Bun.password for auth instead of bcrypt/argon2 npm packages for zero native dependencies.
• Keep bun.lock (text format) in git for readable diffs.
• Test with bun test instead of Jest for the same API with dramatically faster execution.
• When targeting browsers, use Bun.build() with target: "browser".