Bun Hot Reloading

Bun Hot Reloading

Bun provides built-in hot reloading for faster development cycles.

Watch Mode vs Hot Mode

Feature	--watch	--hot
Behavior	Restart process	Reload modules
State	Lost on reload	Preserved
Speed	~20ms restart	Instant reload
Use case	Any file type	Bun.serve HTTP

Watch Mode (--watch)

Restarts the entire process when files change.

# Basic watch mode
bun --watch run src/index.ts

# Watch specific script
bun --watch run dev

# Watch with test runner
bun --watch test

package.json Scripts

{
  "scripts": {
    "dev": "bun --watch run src/index.ts",
    "dev:server": "bun --watch run src/server.ts",
    "test:watch": "bun --watch test"
  }
}

Watch Behavior

• Watches imported files automatically
• Triggers on any .ts, .tsx, .js, .jsx change
• Also watches .json imports
• Restarts with fresh state

Hot Mode (--hot)

Reloads modules in-place without restarting the process.

bun --hot run src/server.ts

HTTP Server Hot Reload

// src/server.ts
let counter = 0; // State preserved across hot reloads