Bun runtime

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