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Bun Runtime

@secondsky/claude-skills
9
0

Use for Bun runtime, bunfig.toml, watch/hot modes, env vars, CLI flags, and module resolution.

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SKILL.md

name Bun Runtime
description Use for Bun runtime, bunfig.toml, watch/hot modes, env vars, CLI flags, and module resolution.
version 1.0.0

Bun Runtime

Bun is a fast all-in-one JavaScript runtime built on JavaScriptCore (Safari's engine). It provides 4x faster startup than Node.js on Linux.

Quick Start

# Run a file
bun run index.ts
bun index.ts  # shorthand

# Run with watch mode
bun --watch run index.ts

# Run package.json script
bun run dev

# Run with hot reloading
bun --hot run server.ts

Core CLI Flags

Flag Purpose
--watch Restart on file changes
--hot Hot module replacement (preserves state)
--smol Reduce memory usage (slower GC)
--inspect Enable debugger
--preload Load modules before execution
--env-file Load specific .env file
-e, --eval Evaluate code string

Running Files

Bun transpiles TypeScript and JSX on-the-fly:

bun run index.js
bun run index.ts
bun run index.jsx
bun run index.tsx

Important: Put Bun flags immediately after bun:

bun --watch run dev    # Correct
bun run dev --watch    # Wrong - flag passed to script

Package.json Scripts

# Run script
bun run dev
bun dev  # shorthand (if no Bun command conflicts)

# List available scripts
bun run

# Run with Bun instead of Node
bun run --bun vite

Bun respects lifecycle hooks (preclean, postclean, etc.).

Watch Mode vs Hot Reloading

Mode Flag Behavior
Watch --watch Full process restart on changes
Hot --hot Replace modules, preserve state 
# Watch mode - full restart
bun --watch run server.ts

# Hot reloading - preserves connections/state
bun --hot run server.ts

Environment Variables

Bun automatically loads .env files:

# Loads automatically: .env, .env.local, .env.development
bun run index.ts

# Specify env file
bun --env-file .env.production run index.ts

# Disable auto-loading
# In bunfig.toml: env = false

Access in code:

const apiKey = process.env.API_KEY;
const bunEnv = Bun.env.NODE_ENV;

Globals Available

Global Source Notes
Bun Bun Main API object
Buffer Node.js Binary data
process Node.js Process info
fetch Web HTTP requests
Request/Response Web HTTP types
WebSocket Web WebSocket client
crypto Web Cryptography
console Web Logging
__dirname Node.js Current directory
__filename Node.js Current file

Preload Scripts

Load modules before your main script:

bun --preload ./setup.ts run index.ts

Or in bunfig.toml:

preload = ["./setup.ts"]

Use cases: polyfills, global setup, instrumentation.

Stdin Execution

# Pipe code to Bun
echo "console.log('Hello')" | bun run -

# Redirect file
bun run - < script.js

Workspaces & Monorepos

# Run script in specific packages
bun run --filter 'pkg-*' build

# Run in all workspaces
bun run --filter '*' test

Debugging

# Start debugger
bun --inspect run index.ts

# Wait for debugger connection
bun --inspect-wait run index.ts

# Break on first line
bun --inspect-brk run index.ts

Connect via Chrome DevTools or VS Code.

Common Errors

Error Cause Fix
Cannot find module Missing dependency Run bun install
Top-level await Using await outside async Wrap in async function or use .mts
--watch not working Flag in wrong position Put flag before run

When to Load References

Load references/bunfig.md when:

  • Configuring bunfig.toml
  • Setting up test configuration
  • Configuring package manager behavior
  • Setting JSX options

Load references/cli-flags.md when:

  • Need complete CLI flag reference
  • Configuring advanced runtime options
  • Setting up debugging

Load references/module-resolution.md when:

  • Troubleshooting import errors
  • Configuring path aliases
  • Understanding Bun's resolution algorithm