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