Fullstack dev server

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

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

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";

Pass the imported HTML files as routes 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.

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

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

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().

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:

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

Advanced Development Configuration

To echo console logs from the browser to the terminal, 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,
  },
});

Bun sends the logs over the existing HMR WebSocket connection.

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 help 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 fullstack 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 bundles 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

If you’d rather not add a build step, set development: false in Bun.serve(). This:

Enables in-memory caching of bundled assets. Bun bundles assets lazily on the first request to an .html file and caches the result in memory until the server restarts.
Enables Cache-Control and ETag headers
Minifies 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

To use TailwindCSS, install the tailwindcss package and the bun-plugin-tailwind plugin.

terminal

bun add tailwindcss bun-plugin-tailwind

bunfig.toml

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

You can now use TailwindCSS utility classes in your HTML and CSS files. Import tailwindcss somewhere in your project:

index.html

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

Alternatively, you can import TailwindCSS in your CSS file:

style.css

@import "tailwindcss";

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

index.html

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

Custom Plugins

Any JS file or module that exports a valid bundler plugin object (an object with a name and a setup field) can be placed in 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 lazily resolves and loads each plugin and uses them to bundle your routes.

Plugins live in bunfig.toml so that the bun build CLI, once it supports them, can know statically which plugins are in use. These plugins work in Bun.build()’s JS API, but not yet in the CLI.

Inline Environment Variables

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

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 HTML & static sites for 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

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

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.

​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

​Inline Environment Variables

​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

Inline Environment Variables

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