name	Bun Fullstack dev server
description	Build fullstack applications with Bun's integrated dev server that bundles frontend assets and handles API routes

Fullstack dev server

Build fullstack applications with Bun's integrated dev server that bundles frontend assets and handles API routes

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

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}`);

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.

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().

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:

<!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:

<!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.

```ts title="src/backend.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} import dashboard from "../public/dashboard.html"; import { serve } from "bun";

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


```tsx title="src/frontend.tsx" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
import { createRoot } from 'react-dom/client';
import App from './app';

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

<!DOCTYPE html>
<html>
  <head>
    <title>Dashboard</title>
    <link rel="stylesheet" href="../src/styles.css" />
  </head>
  <body>
    <div id="root"></div>
    <script type="module" src="../src/frontend.tsx"></script>
  </body>
</html>

import { useState } from "react";

export default function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h1>Dashboard</h1>
      <button onClick={() => setCount(count + 1)}>Count: {count}</button>
    </div>
  );
}

Development Mode

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

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().

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.

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.

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

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:

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:

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

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

You can use TailwindCSS by installing and adding the tailwindcss package and bun-plugin-tailwind plugin.

bun add tailwindcss bun-plugin-tailwind

[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:

<!doctype html>
<html>
  <head>
    <!-- [!code ++] -->
    <link rel="stylesheet" href="tailwindcss" />
  </head>
  <!-- the rest of your HTML... -->
</html>

Alternatively, you can import TailwindCSS in your CSS file:

@import "tailwindcss";

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

<!doctype html>
<html>
  <head>
    <!-- [!code ++] -->
    <link rel="stylesheet" href="./style.css" />
  </head>
  <!-- the rest of your HTML... -->
</html>

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:

[serve.static]
plugins = ["./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.

Inline Environment Variables

Bun can replace process.env.* references in your frontend JavaScript and TypeScript with their actual values at build time. Configure the env option in your bunfig.toml:

[serve.static]
env = "PUBLIC_*"  # only inline env vars starting with PUBLIC_ (recommended)
# env = "inline"  # inline all environment variables
# env = "disable" # disable env var replacement (default)

This only works with literal `process.env.FOO` references, not `import.meta.env` or indirect access like `const env = process.env; env.FOO`.

If an environment variable is not set, you may see runtime errors like ReferenceError: process is not defined in the browser.

See the HTML & static sites documentation for more details on build-time configuration and examples.

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

* Transpiles TypeScript, JSX, and TSX in ` * Processes CSS imports and `` tags * Concatenates CSS files * Rewrites url and asset paths to include content-addressable hashes in URLs

```html title="index.html" icon="file-code" theme={"theme":{"light":"github-light","dark":"dracula"}}
<link rel="stylesheet" href="./styles.css" />
```

* 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 * Combines all `

Complete Example

Here's a complete fullstack application example:

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}`);

<!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>

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

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

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>
  );
}

* {
  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

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

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

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

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

# 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

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

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

// 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

// 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.

name	Bun Fullstack dev server
description	Build fullstack applications with Bun's integrated dev server that bundles frontend assets and handles API routes