Fullstack Dev Server – Bundler

As of Bun v1.1.44, we've added initial support for bundling frontend apps directly in Bun's HTTP server: Bun.serve(). Run your frontend and backend in the same app with no extra steps.

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

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

Bun.serve({
  // Add HTML imports to `static`
  static: {
    // Bundle & route index.html to "/"
    "/": homepage,
    // Bundle & route dashboard.html to "/dashboard"
    "/dashboard": dashboard,
  },

  // Enable development mode for:
  // - Detailed error messages
  // - Rebuild on request
  development: true,

  // Handle API requests
  async fetch(req) {
    // ...your API code
    if (req.url.endsWith("/api/users")) {
      const users = await Bun.sql`SELECT * FROM users`;
      return Response.json(users);
    }

    // Return 404 for unmatched routes
    return new Response("Not Found", { status: 404 });
  },
});

You'll need to run your app with bun --experimental-html to enable this feature:

bun --experimental-html run app.ts

HTML imports are 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({
  static: {
    "/": 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.

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>

How to use with React

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/styles.css

src/app.tsx

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

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

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

src/frontend.tsx
import "./styles.css";
import { createRoot } from "react-dom/client";
import { App } from "./app.tsx";

document.addEventListener("DOMContentLoaded", () => {
  const root = createRoot(document.getElementById("root"));
  root.render(<App />);
});

public/dashboard.html
<!DOCTYPE html>
<html>
  <head>
    <title>Dashboard</title>
  </head>
  <body>
    <div id="root"></div>
    <script type="module" src="../src/frontend.tsx"></script>
  </body>
</html>

src/styles.css
body {
  background-color: red;
}

src/app.tsx
export function App() {
  return <div>Hello World</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({
  static: {
    "/": homepage,
    "/dashboard": dashboard,
  }

  development: true,

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

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

Production mode

When serving your app in production, set development: false in Bun.serve().

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.
Enables Cache-Control headers and ETag headers
Minifies JavaScript/TypeScript/TSX/JSX files

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.

<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()
```
<script type="module" src="./counter.tsx"></script>
```
<link> processing
- Processes CSS imports and <link> tags
- Concatenates CSS files
- Rewrites url and asset paths to include content-addressable hashes in URLs
```
<link rel="stylesheet" href="./styles.css" />
```
<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
Rewrite HTML
- 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
Serve
- 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.

This is a work in progress

Client-side hot reloading isn't wired up yet. It will be in the future.
This doesn't support bun build yet. It also will in the future.
We haven't figured out plugins yet. This probably will live in bunfig.toml with the same API as in Bun.build otherwise.

HTML imports are routes

How to use with React

Development mode

Production mode

How this works

This is a work in progress