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

export default {
  port: 3000,
  fetch(req: Request) {
    counter++;
    return new Response(`Request #${counter}`);
  },
};

bun --hot run src/server.ts

When you modify server.ts, the module reloads instantly while counter keeps its value.

Bun.serve with Hot Reload

// src/server.ts
const server = Bun.serve({
  port: 3000,
  fetch(req) {
    return new Response("Hello!");
  },
});

// Hot reload handler
if (import.meta.hot) {
  import.meta.hot.accept(() => {
    console.log("Hot reload!");
  });
}

console.log(`Server running on port ${server.port}`);

import.meta.hot API

// Check if hot reload is available
if (import.meta.hot) {
  // Accept updates to this module
  import.meta.hot.accept();

  // Accept with callback
  import.meta.hot.accept((newModule) => {
    console.log("Module updated:", newModule);
  });

  // Cleanup before reload
  import.meta.hot.dispose(() => {
    // Close connections, clear intervals, etc.
    clearInterval(myInterval);
  });

  // Decline hot reload (force full restart)
  import.meta.hot.decline();

  // Invalidate this module (trigger parent reload)
  import.meta.hot.invalidate();
}