Fullstack dev server

To get started, import HTML files and pass them to the routes option in Bun.serve().

https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240

app.ts

import {serve} from 'bun';
import dashboard from './dashboard.html';
import homepage from './index.html';

const server = serve({
	routes: {
		// ** HTML imports **
		// Bundle & route index.html to "/". This uses HTMLRewriter to scan
		// the HTML for `<script>` and `<link>` tags, runs Bun's JavaScript
		// & CSS bundler on them, transpiles any TypeScript, JSX, and TSX,
		// downlevels CSS with Bun's CSS parser and serves the result.
		'/': homepage,
		// Bundle & route dashboard.html to "/dashboard"
		'/dashboard': dashboard,

		// ** API endpoints ** (Bun v1.2.3+ required)
		'/api/users': {
			async GET(req) {
				const users = await sql`SELECT * FROM users`;
				return Response.json(users);
			},
			async POST(req) {
				const {name, email} = await req.json();
				const [user] = await sql`INSERT INTO users (name, email) VALUES (${name}, ${email})`;
				return Response.json(user);
			},
		},
		'/api/users/:id': async req => {
			const {id} = req.params;
			const [user] = await sql`SELECT * FROM users WHERE id = ${id}`;
			return Response.json(user);
		},
	},

	// Enable development mode for:
	// - Detailed error messages
	// - Hot reloading (Bun v1.2.3+ required)
	development: true,
});

console.log(`Listening on ${server.url}`);

terminal

bun run app.ts

HTML Routes

HTML Imports as Routes

The web starts with HTML, and so does Bun’s fullstack dev server. To specify entrypoints to your frontend, import HTML files into your JavaScript/TypeScript/TSX/JSX files.

app.ts

import dashboard from './dashboard.html';
import homepage from './index.html';

These HTML files are used as routes in Bun’s dev server you can pass to Bun.serve().

app.ts

Bun.serve({
	routes: {
		'/': homepage,
		'/dashboard': dashboard,
	},

	fetch(req) {
		// ... api requests
	},
});

When you make a request to /dashboard or /, Bun automatically bundles the <script> and <link> tags in the HTML files, exposes them as static routes, and serves the result.

HTML Processing Example

An index.html file like this:

index.html

<!DOCTYPE html>
<html>
	<head>
		<title>Home</title>
		<link rel="stylesheet" href="./reset.css" />
		<link rel="stylesheet" href="./styles.css" />
	</head>
	<body>
		<div id="root"></div>
		<script type="module" src="./sentry-and-preloads.ts"></script>
		<script type="module" src="./my-app.tsx"></script>
	</body>
</html>

Becomes something like this:

index.html

<!DOCTYPE html>
<html>
	<head>
		<title>Home</title>
		<link rel="stylesheet" href="/index-[hash].css" />
	</head>
	<body>
		<div id="root"></div>
		<script type="module" src="/index-[hash].js"></script>
	</body>
</html>

React Integration

To use React in your client-side code, import react-dom/client and render your app.

src/backend.ts
src/frontend.tsx
public/dashboard.html
src/app.tsx

src/backend.ts

import dashboard from "../public/dashboard.html";
import { serve } from "bun";

serve({
	routes: {
		"/": dashboard,
	},

	async fetch(req) {
		// ...api requests
		return new Response("hello world");
	},
});

Development Mode

When building locally, enable development mode by setting development: true in Bun.serve().

src/backend.ts

import homepage from "./index.html";
import dashboard from "./dashboard.html";

Bun.serve({
  routes: {
    "/": homepage,
    "/dashboard": dashboard,
  },

  development: true,

  fetch(req) {
    // ... api requests
  },
});

Development Mode Features

When development is true, Bun will:

Include the SourceMap header in the response so that devtools can show the original source code
Disable minification
Re-bundle assets on each request to a .html file
Enable hot module reloading (unless hmr: false is set)
Echo console logs from browser to terminal

Advanced Development Configuration

Bun.serve() supports echoing console logs from the browser to the terminal. To enable this, pass console: true in the development object in Bun.serve().

src/backend.ts

import homepage from './index.html';

Bun.serve({
	// development can also be an object.
	development: {
		// Enable Hot Module Reloading
		hmr: true,

		// Echo console logs from the browser to the terminal
		console: true,
	},

	routes: {
		'/': homepage,
	},
});

When console: true is set, Bun will stream console logs from the browser to the terminal. This reuses the existing WebSocket connection from HMR to send the logs.

Development vs Production

Feature	Development	Production
Source maps	✅ Enabled	❌ Disabled
Minification	❌ Disabled	✅ Enabled
Hot reloading	✅ Enabled	❌ Disabled
Asset bundling	🔄 On each request	💾 Cached
Console logging	🖥️ Browser → Terminal	❌ Disabled
Error details	📝 Detailed	🔒 Minimal

Production Mode

Hot reloading and development: true helps you iterate quickly, but in production, your server should be as fast as possible and have as few external dependencies as possible.

Ahead of Time Bundling (Recommended)

As of Bun v1.2.17, you can use Bun.build or bun build to bundle your full-stack application ahead of time.

terminal

bun build --target=bun --production --outdir=dist ./src/index.ts

When Bun’s bundler sees an HTML import from server-side code, it will bundle the referenced JavaScript/TypeScript/TSX/JSX and CSS files into a manifest object that Bun.serve() can use to serve the assets.

src/backend.ts

import {serve} from 'bun';
import index from './index.html';

serve({
	routes: {'/': index},
});

Runtime Bundling

When adding a build step is too complicated, you can set development: false in Bun.serve(). This will:

Enable in-memory caching of bundled assets. Bun will bundle assets lazily on the first request to an .html file, and cache the result in memory until the server restarts.
Enable Cache-Control headers and ETag headers
Minify JavaScript/TypeScript/TSX/JSX files

src/backend.ts

import {serve} from 'bun';
import homepage from './index.html';

serve({
	routes: {
		'/': homepage,
	},

	// Production mode
	development: false,
});

API Routes

HTTP Method Handlers

Define API endpoints with HTTP method handlers:

src/backend.ts

import {serve} from 'bun';

serve({
	routes: {
		'/api/users': {
			async GET(req) {
				// Handle GET requests
				const users = await getUsers();
				return Response.json(users);
			},

			async POST(req) {
				// Handle POST requests
				const userData = await req.json();
				const user = await createUser(userData);
				return Response.json(user, {status: 201});
			},

			async PUT(req) {
				// Handle PUT requests
				const userData = await req.json();
				const user = await updateUser(userData);
				return Response.json(user);
			},

			async DELETE(req) {
				// Handle DELETE requests
				await deleteUser(req.params.id);
				return new Response(null, {status: 204});
			},
		},
	},
});

Dynamic Routes

Use URL parameters in your routes:

src/backend.ts

serve({
	routes: {
		// Single parameter
		'/api/users/:id': async req => {
			const {id} = req.params;
			const user = await getUserById(id);
			return Response.json(user);
		},

		// Multiple parameters
		'/api/users/:userId/posts/:postId': async req => {
			const {userId, postId} = req.params;
			const post = await getPostByUser(userId, postId);
			return Response.json(post);
		},

		// Wildcard routes
		'/api/files/*': async req => {
			const filePath = req.params['*'];
			const file = await getFile(filePath);
			return new Response(file);
		},
	},
});

Request Handling

src/backend.ts

serve({
	routes: {
		'/api/data': {
			async POST(req) {
				// Parse JSON body
				const body = await req.json();

				// Access headers
				const auth = req.headers.get('Authorization');

				// Access URL parameters
				const {id} = req.params;

				// Access query parameters
				const url = new URL(req.url);
				const page = url.searchParams.get('page') || '1';

				// Return response
				return Response.json({
					message: 'Data processed',
					page: parseInt(page),
					authenticated: !!auth,
				});
			},
		},
	},
});

Plugins

Bun’s bundler plugins are also supported when bundling static routes. To configure plugins for Bun.serve, add a plugins array in the [serve.static] section of your bunfig.toml.

TailwindCSS Plugin

For example, enable TailwindCSS on your routes by installing and adding the bun-plugin-tailwind plugin:

terminal

bun add bun-plugin-tailwind

bunfig.toml

[serve.static]
plugins = ["bun-plugin-tailwind"]

This will allow you to use TailwindCSS utility classes in your HTML and CSS files. All you need to do is import tailwindcss somewhere:

index.html

<!doctype html>
<html>
	<head>
		<title>Home</title>
		<link rel="stylesheet" href="tailwindcss" />
	</head>
	<body>
		<div class="bg-blue-500 text-white p-4">
			<h1 class="text-2xl font-bold">Hello World</h1>
		</div>
	</body>
</html>

Or in your CSS:

style.css

@import 'tailwindcss';

.custom-class {
	@apply bg-red-500 text-white;
}

Custom Plugins

Any JS file or module which exports a valid bundler plugin object (essentially an object with a name and setup field) can be placed inside the plugins array:

bunfig.toml

[serve.static]
plugins = ["./my-plugin-implementation.ts"]

my-plugin-implementation.ts

import type {BunPlugin} from 'bun';

const myPlugin: BunPlugin = {
	name: 'my-custom-plugin',
	setup(build) {
		// Plugin implementation
		build.onLoad({filter: /\.custom$/}, async args => {
			const text = await Bun.file(args.path).text();
			return {
				contents: `export default ${JSON.stringify(text)};`,
				loader: 'js',
			};
		});
	},
};

export default myPlugin;

Bun will lazily resolve and load each plugin and use them to bundle your routes.

This is currently in bunfig.toml to make it possible to know statically which plugins are in use when we eventually integrate this with the bun build CLI. These plugins work in Bun.build()’s JS API, but are not yet supported in the CLI.

How It Works

Bun uses HTMLRewriter to scan for <script> and <link> tags in HTML files, uses them as entrypoints for Bun’s bundler, generates an optimized bundle for the JavaScript/TypeScript/TSX/JSX and CSS files, and serves the result.

Processing Pipeline

1. <script> Processing

Transpiles TypeScript, JSX, and TSX in <script> tags
Bundles imported dependencies
Generates sourcemaps for debugging
Minifies when development is not true in Bun.serve()

index.html

<script type="module" src="./counter.tsx"></script>

2. <link> Processing

Processes CSS imports and <link> tags
Concatenates CSS files
Rewrites url and asset paths to include content-addressable hashes in URLs

index.html

<link rel="stylesheet" href="./styles.css" />

3. <img> & Asset Processing

Links to assets are rewritten to include content-addressable hashes in URLs
Small assets in CSS files are inlined into data: URLs, reducing the total number of HTTP requests sent over the wire

4. HTML Rewriting

Combines all <script> tags into a single <script> tag with a content-addressable hash in the URL
Combines all <link> tags into a single <link> tag with a content-addressable hash in the URL
Outputs a new HTML file

5. Serving

All the output files from the bundler are exposed as static routes, using the same mechanism internally as when you pass a Response object to static in Bun.serve().
This works similarly to how Bun.build processes HTML files.

Complete Example

Here’s a complete fullstack application example:

server.ts

import {serve} from 'bun';
import {Database} from 'bun:sqlite';
import homepage from './public/index.html';
import dashboard from './public/dashboard.html';

// Initialize database
const db = new Database('app.db');
db.exec(`
  CREATE TABLE IF NOT EXISTS users (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL,
    email TEXT UNIQUE NOT NULL,
    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
  )
`);

const server = serve({
	routes: {
		// Frontend routes
		'/': homepage,
		'/dashboard': dashboard,

		// API routes
		'/api/users': {
			async GET() {
				const users = db.query('SELECT * FROM users').all();
				return Response.json(users);
			},

			async POST(req) {
				const {name, email} = await req.json();

				try {
					const result = db
						.query('INSERT INTO users (name, email) VALUES (?, ?) RETURNING *')
						.get(name, email);

					return Response.json(result, {status: 201});
				} catch (error) {
					return Response.json({error: 'Email already exists'}, {status: 400});
				}
			},
		},

		'/api/users/:id': {
			async GET(req) {
				const {id} = req.params;
				const user = db.query('SELECT * FROM users WHERE id = ?').get(id);

				if (!user) {
					return Response.json({error: 'User not found'}, {status: 404});
				}

				return Response.json(user);
			},

			async DELETE(req) {
				const {id} = req.params;
				const result = db.query('DELETE FROM users WHERE id = ?').run(id);

				if (result.changes === 0) {
					return Response.json({error: 'User not found'}, {status: 404});
				}

				return new Response(null, {status: 204});
			},
		},

		// Health check endpoint
		'/api/health': {
			GET() {
				return Response.json({
					status: 'ok',
					timestamp: new Date().toISOString(),
				});
			},
		},
	},

	// Enable development mode
	development: {
		hmr: true,
		console: true,
	},

	// Fallback for unmatched routes
	fetch(req) {
		return new Response('Not Found', {status: 404});
	},
});

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

public/index.html

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<meta name="viewport" content="width=device-width, initial-scale=1" />
		<title>Fullstack Bun App</title>
		<link rel="stylesheet" href="../src/styles.css" />
	</head>
	<body>
		<div id="root"></div>
		<script type="module" src="../src/main.tsx"></script>
	</body>
</html>

src/main.tsx

import {createRoot} from 'react-dom/client';
import {App} from './App';

const container = document.getElementById('root')!;
const root = createRoot(container);
root.render(<App />);

src/App.tsx

import {useState, useEffect} from 'react';

interface User {
	id: number;
	name: string;
	email: string;
	created_at: string;
}

export function App() {
	const [users, setUsers] = useState<User[]>([]);
	const [name, setName] = useState('');
	const [email, setEmail] = useState('');
	const [loading, setLoading] = useState(false);

	const fetchUsers = async () => {
		const response = await fetch('/api/users');
		const data = await response.json();
		setUsers(data);
	};

	const createUser = async (e: React.FormEvent) => {
		e.preventDefault();
		setLoading(true);

		try {
			const response = await fetch('/api/users', {
				method: 'POST',
				headers: {'Content-Type': 'application/json'},
				body: JSON.stringify({name, email}),
			});

			if (response.ok) {
				setName('');
				setEmail('');
				await fetchUsers();
			} else {
				const error = await response.json();
				alert(error.error);
			}
		} catch (error) {
			alert('Failed to create user');
		} finally {
			setLoading(false);
		}
	};

	const deleteUser = async (id: number) => {
		if (!confirm('Are you sure?')) return;

		try {
			const response = await fetch(`/api/users/${id}`, {
				method: 'DELETE',
			});

			if (response.ok) {
				await fetchUsers();
			}
		} catch (error) {
			alert('Failed to delete user');
		}
	};

	useEffect(() => {
		fetchUsers();
	}, []);

	return (
		<div className="container">
			<h1>User Management</h1>

			<form onSubmit={createUser} className="form">
				<input
					type="text"
					placeholder="Name"
					value={name}
					onChange={e => setName(e.target.value)}
					required
				/>
				<input
					type="email"
					placeholder="Email"
					value={email}
					onChange={e => setEmail(e.target.value)}
					required
				/>
				<button type="submit" disabled={loading}>
					{loading ? 'Creating...' : 'Create User'}
				</button>
			</form>

			<div className="users">
				<h2>Users ({users.length})</h2>
				{users.map(user => (
					<div key={user.id} className="user-card">
						<div>
							<strong>{user.name}</strong>
							<br />
							<span>{user.email}</span>
						</div>
						<button onClick={() => deleteUser(user.id)} className="delete-btn">
							Delete
						</button>
					</div>
				))}
			</div>
		</div>
	);
}

src/styles.css

* {
	margin: 0;
	padding: 0;
	box-sizing: border-box;
}

body {
	font-family: -apple-system, BlinkMacSystemFont, sans-serif;
	background: #f5f5f5;
	color: #333;
}

.container {
	max-width: 800px;
	margin: 0 auto;
	padding: 2rem;
}

h1 {
	color: #2563eb;
	margin-bottom: 2rem;
}

.form {
	background: white;
	padding: 1.5rem;
	border-radius: 8px;
	box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
	margin-bottom: 2rem;
	display: flex;
	gap: 1rem;
	flex-wrap: wrap;
}

.form input {
	flex: 1;
	min-width: 200px;
	padding: 0.75rem;
	border: 1px solid #ddd;
	border-radius: 4px;
}

.form button {
	padding: 0.75rem 1.5rem;
	background: #2563eb;
	color: white;
	border: none;
	border-radius: 4px;
	cursor: pointer;
}

.form button:hover {
	background: #1d4ed8;
}

.form button:disabled {
	opacity: 0.5;
	cursor: not-allowed;
}

.users {
	background: white;
	padding: 1.5rem;
	border-radius: 8px;
	box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.user-card {
	display: flex;
	justify-content: space-between;
	align-items: center;
	padding: 1rem;
	border-bottom: 1px solid #eee;
}

.user-card:last-child {
	border-bottom: none;
}

.delete-btn {
	padding: 0.5rem 1rem;
	background: #dc2626;
	color: white;
	border: none;
	border-radius: 4px;
	cursor: pointer;
}

.delete-btn:hover {
	background: #b91c1c;
}

Best Practices

Project Structure

my-app/
├── src/
│   ├── components/
│   │   ├── Header.tsx
│   │   └── UserList.tsx
│   ├── styles/
│   │   ├── globals.css
│   │   └── components.css
│   ├── utils/
│   │   └── api.ts
│   ├── App.tsx
│   └── main.tsx
├── public/
│   ├── index.html
│   ├── dashboard.html
│   └── favicon.ico
├── server/
│   ├── routes/
│   │   ├── users.ts
│   │   └── auth.ts
│   ├── db/
│   │   └── schema.sql
│   └── index.ts
├── bunfig.toml
└── package.json

Environment-Based Configuration

server/config.ts

export const config = {
	development: process.env.NODE_ENV !== 'production',
	port: process.env.PORT || 3000,
	database: {
		url: process.env.DATABASE_URL || './dev.db',
	},
	cors: {
		origin: process.env.CORS_ORIGIN || '*',
	},
};

Error Handling

server/middleware.ts

export function errorHandler(error: Error, req: Request) {
	console.error('Server error:', error);

	if (process.env.NODE_ENV === 'production') {
		return Response.json({error: 'Internal server error'}, {status: 500});
	}

	return Response.json(
		{
			error: error.message,
			stack: error.stack,
		},
		{status: 500},
	);
}

API Response Helpers

server/utils.ts

export function json(data: any, status = 200) {
	return Response.json(data, {status});
}

export function error(message: string, status = 400) {
	return Response.json({error: message}, {status});
}

export function notFound(message = 'Not found') {
	return error(message, 404);
}

export function unauthorized(message = 'Unauthorized') {
	return error(message, 401);
}

Type Safety

types/api.ts

export interface User {
	id: number;
	name: string;
	email: string;
	created_at: string;
}

export interface CreateUserRequest {
	name: string;
	email: string;
}

export interface ApiResponse<T> {
	data?: T;
	error?: string;
}

Deployment

Production Build

terminal

# Build for production
bun build --target=bun --production --outdir=dist ./server/index.ts

# Run production server
NODE_ENV=production bun dist/index.js

Docker Deployment

Dockerfile

FROM oven/bun:1 as base
WORKDIR /usr/src/app

# Install dependencies
COPY package.json bun.lockb ./
RUN bun install --frozen-lockfile

# Copy source code
COPY . .

# Build application
RUN bun build --target=bun --production --outdir=dist ./server/index.ts

# Production stage
FROM oven/bun:1-slim
WORKDIR /usr/src/app
COPY --from=base /usr/src/app/dist ./
COPY --from=base /usr/src/app/public ./public

EXPOSE 3000
CMD ["bun", "index.js"]

Environment Variables

.env.production

NODE_ENV=production
PORT=3000
DATABASE_URL=postgresql://user:pass@localhost:5432/myapp
CORS_ORIGIN=https://myapp.com

Migration from Other Frameworks

From Express + Webpack

server.ts

// Before (Express + Webpack)
app.use(express.static('dist'));
app.get('/api/users', (req, res) => {
	res.json(users);
});

// After (Bun fullstack)
serve({
	routes: {
		'/': homepage, // Replaces express.static
		'/api/users': {
			GET() {
				return Response.json(users);
			},
		},
	},
});

From Next.js API Routes

server.ts

// Before (Next.js)
export default function handler(req, res) {
  if (req.method === 'GET') {
    res.json(users);
  }
}

// After (Bun)
"/api/users": {
  GET() { return Response.json(users); }
}

Limitations and Future Plans

Current Limitations

bun build CLI integration is not yet available for fullstack apps
Auto-discovery of API routes is not implemented
Server-side rendering (SSR) is not built-in

Planned Features

Integration with bun build CLI
File-based routing for API endpoints
Built-in SSR support
Enhanced plugin ecosystem

This is a work in progress. Features and APIs may change as Bun continues to evolve.

Core

Development Server

Asset Processing

Single File Executable

Extensions

Optimization

Migration

​HTML Routes

​HTML Imports as Routes

​HTML Processing Example

​React Integration

​Development Mode

​Development Mode Features

​Advanced Development Configuration

​Development vs Production

​Production Mode

​Ahead of Time Bundling (Recommended)

​Runtime Bundling

​API Routes

​HTTP Method Handlers

​Dynamic Routes

​Request Handling

​Plugins

​TailwindCSS Plugin

​Custom Plugins

​How It Works

​Processing Pipeline

​Complete Example

​Best Practices

​Project Structure

​Environment-Based Configuration

​Error Handling

​API Response Helpers

​Type Safety

​Deployment

​Production Build

​Docker Deployment

​Environment Variables

​Migration from Other Frameworks

​From Express + Webpack

​From Next.js API Routes

​Limitations and Future Plans

​Current Limitations

​Planned Features

HTML Routes

HTML Imports as Routes

HTML Processing Example

React Integration

Development Mode

Development Mode Features

Advanced Development Configuration

Development vs Production

Production Mode

Ahead of Time Bundling (Recommended)

Runtime Bundling

API Routes

HTTP Method Handlers

Dynamic Routes

Request Handling

Plugins

TailwindCSS Plugin

Custom Plugins

How It Works

Processing Pipeline

Complete Example

Best Practices

Project Structure

Environment-Based Configuration

Error Handling

API Response Helpers

Type Safety

Deployment

Production Build

Docker Deployment

Environment Variables

Migration from Other Frameworks

From Express + Webpack

From Next.js API Routes

Limitations and Future Plans

Current Limitations

Planned Features