Bun React SSR

Build custom server-rendered React applications with Bun.

Quick Start

# Initialize project
mkdir my-ssr-app && cd my-ssr-app
bun init

# Install dependencies
bun add react react-dom
bun add -D @types/react @types/react-dom

Basic SSR Setup

Server Entry

// src/server.tsx
import { renderToString } from "react-dom/server";
import App from "./App";

Bun.serve({
  port: 3000,
  async fetch(req) {
    const url = new URL(req.url);

    // Serve static files
    if (url.pathname.startsWith("/static/")) {
      const file = Bun.file(`./public${url.pathname}`);
      if (await file.exists()) {
        return new Response(file);
      }
    }

    // Render React app
    const html = renderToString(<App url={url.pathname} />);

    return new Response(
      `<!DOCTYPE html>
      <html>
        <head>
          <meta charset="utf-8">
          <meta name="viewport" content="width=device-width, initial-scale=1">
          <title>React SSR</title>
        </head>
        <body>
          <div id="root">${html}</div>
          <script src="/static/client.js"></script>
        </body>
      </html>`,
      {
        headers: { "Content-Type": "text/html" },
      }
    );
  },
});

console.log("Server running on http://localhost:3000");

Client Entry

// src/client.tsx
import { hydrateRoot } from "react-dom/client";
import App from "./App";

hydrateRoot(
  document.getElementById("root")!,
  <App url={window.location.pathname} />
);

React App

// src/App.tsx
interface AppProps {
  url: string;
}

export default function App({ url }: AppProps) {
  return (
    <div>
      <h1>React SSR with Bun</h1>
      <p>Current path: {url}</p>
      <button onClick={() => alert("Hydrated!")}>Click me</button>
    </div>
  );
}

Build Client Bundle

// build.ts
await Bun.build({
  entrypoints: ["./src/client.tsx"],
  outdir: "./public/static",
  target: "browser",
  minify: true,
  splitting: true,
});

# Build client
bun run build.ts

# Start server
bun run src/server.tsx

Streaming SSR

// src/server-streaming.tsx
import { renderToReadableStream } from "react-dom/server";
import App from "./App";

Bun.serve({
  port: 3000,
  async fetch(req) {
    const url = new URL(req.url);

    const stream = await renderToReadableStream(
      <App url={url.pathname} />,
      {
        bootstrapScripts: ["/static/client.js"],
        onError(error) {
          console.error(error);
        },
      }
    );

    // Wait for shell to be ready (Suspense boundaries)
    await stream.allReady;

    return new Response(stream, {
      headers: { "Content-Type": "text/html" },
    });
  },
});

With Suspense

// src/App.tsx
import { Suspense } from "react";

function SlowComponent() {
  // This would be a data fetching component
  return <div>Loaded!</div>;
}

export default function App({ url }: { url: string }) {
  return (
    <html>
      <head>
        <title>Streaming SSR</title>
      </head>
      <body>
        <div id="root">
          <h1>Fast Shell</h1>
          <Suspense fallback={<div>Loading...</div>}>
            <SlowComponent />
          </Suspense>
        </div>
      </body>
    </html>
  );
}

Data Fetching

Server-Side Data

// src/server.tsx
import { renderToString } from "react-dom/server";
import { Database } from "bun:sqlite";
import App from "./App";

const db = new Database("data.sqlite");

Bun.serve({
  async fetch(req) {
    const url = new URL(req.url);

    // Fetch data server-side
    const users = db.query("SELECT * FROM users").all();

    const html = renderToString(
      <App url={url.pathname} initialData={{ users }} />
    );

    return new Response(
      `<!DOCTYPE html>
      <html>
        <head><title>SSR</title></head>
        <body>
          <div id="root">${html}</div>
          <script>
            window.__INITIAL_DATA__ = ${JSON.stringify({ users })};
          </script>
          <script src="/static/client.js"></script>
        </body>
      </html>`,
      { headers: { "Content-Type": "text/html" } }
    );
  },
});

Client Hydration

// src/client.tsx
import { hydrateRoot } from "react-dom/client";
import App from "./App";

const initialData = (window as any).__INITIAL_DATA__;

hydrateRoot(
  document.getElementById("root")!,
  <App url={window.location.pathname} initialData={initialData} />
);

Routing

Simple Router

// src/Router.tsx
import { useState, useEffect } from "react";

interface Route {
  path: string;
  component: React.ComponentType;
}

interface RouterProps {
  routes: Route[];
  initialPath: string;
}

export function Router({ routes, initialPath }: RouterProps) {
  const [path, setPath] = useState(initialPath);

  useEffect(() => {
    const handlePopState = () => setPath(window.location.pathname);
    window.addEventListener("popstate", handlePopState);
    return () => window.removeEventListener("popstate", handlePopState);
  }, []);

  const route = routes.find((r) => r.path === path);
  const Component = route?.component || NotFound;

  return <Component />;
}

export function Link({ href, children }: { href: string; children: React.ReactNode }) {
  const handleClick = (e: React.MouseEvent) => {
    e.preventDefault();
    window.history.pushState({}, "", href);
    window.dispatchEvent(new PopStateEvent("popstate"));
  };

  return <a href={href} onClick={handleClick}>{children}</a>;
}

function NotFound() {
  return <h1>404 - Not Found</h1>;
}

CSS Handling

Inline Styles

const html = renderToString(<App />);

return new Response(
  `<!DOCTYPE html>
  <html>
    <head>
      <style>${await Bun.file("./src/styles.css").text()}</style>
    </head>
    <body>
      <div id="root">${html}</div>
    </body>
  </html>`,
  { headers: { "Content-Type": "text/html" } }
);

External Stylesheet

// Build CSS
await Bun.build({
  entrypoints: ["./src/styles.css"],
  outdir: "./public/static",
});

// Link in HTML
`<link rel="stylesheet" href="/static/styles.css">`

Development Setup

Hot Reload Development

// dev.ts
import { watch } from "fs";

const srcDir = "./src";
let serverProcess: Subprocess | null = null;

async function startServer() {
  serverProcess?.kill();
  serverProcess = Bun.spawn(["bun", "run", "src/server.tsx"], {
    stdout: "inherit",
    stderr: "inherit",
  });
}

// Watch for changes
watch(srcDir, { recursive: true }, async (event, filename) => {
  console.log(`Change detected: ${filename}`);
  await startServer();
});

await startServer();
console.log("Dev server watching...");

Production Build

// build-prod.ts

// Build client
await Bun.build({
  entrypoints: ["./src/client.tsx"],
  outdir: "./dist/public/static",
  target: "browser",
  minify: true,
  splitting: true,
  sourcemap: "external",
});

// Build server
await Bun.build({
  entrypoints: ["./src/server.tsx"],
  outdir: "./dist",
  target: "bun",
  minify: true,
});

console.log("Build complete!");

Common Errors

Error	Cause	Fix
`Hydration mismatch`	Server/client HTML differs	Check initial state
`document is not defined`	SSR accessing DOM	Guard with typeof window
`Cannot use hooks`	Hooks outside component	Check component structure
`Flash of unstyled content`	CSS not loaded	Inline critical CSS

Performance Tips

Use streaming SSR for faster TTFB
Inline critical CSS above the fold
Code split client bundle
Cache rendered HTML for static pages
Use Suspense for progressive loading

When to Load References

Load references/streaming-patterns.md when:

Complex Suspense boundaries
Selective hydration
Progressive enhancement

Load references/caching.md when:

HTML caching strategies
CDN integration
Edge rendering

Bun React SSR

Bun React SSR

Quick Start

Basic SSR Setup

Server Entry

Client Entry

React App

Build Client Bundle

Streaming SSR

With Suspense

Data Fetching

Server-Side Data

Client Hydration

Routing

Simple Router

CSS Handling

Inline Styles

External Stylesheet

Development Setup

Hot Reload Development

Production Build

Common Errors

Performance Tips

When to Load References

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