Cloudflare Bundler Apps

This skill teaches how to build apps that deploy cleanly through the space deploy pipeline. Every project committed to a space is built by @cloudflare/worker-bundler (createApp when there are static assets, createWorker when there are none) and served on a Dynamic Worker via WorkerLoader. Get the conventions right and the preview "just works". Get them wrong and you get a blank page, a 500, or a Build failed error.

How the pipeline works

When deploy_space(branch) is called, the space DO:

1. Reads every file from the branch's working tree (skipping .git/).
2. Parses wrangler.json / wrangler.jsonc / wrangler.toml for main, compatibility_date, compatibility_flags, and [assets].
3. If [assets].directory is set, files under that directory become static assets served host-side by handleAssetRequest. Everything else is built into a Worker via createApp (with server: <main>).
4. If no assets directory is configured, only createWorker({ entryPoint: <main> }) is run. The output is loaded as a Dynamic Worker; all requests go to the Worker.
5. Previews are served at /space/:name/preview/:branch/*. Responses with content-type: text/html are run through HTMLRewriter, which prefixes root-relative src / href / action attributes with the preview base path. JS-side fetches and dynamic imports are NOT rewritten.

Practical consequences:

• You can rely on root-relative src="/foo.js" and href="/style.css" in HTML. They will be rewritten to the preview path automatically.
• You cannot rely on root-relative paths inside JS strings: fetch("/api/x"), new URL("/foo", location.origin), dynamic import("/lib.js"). These hit the wrong path under the preview prefix. Use relative paths (./api/x, import.meta.url) or read the base path from <base href> / a meta tag injected at build time.
• The <base href> tag is also rewritten if present — using it lets all relative URLs resolve against the preview path.

Project layout

A typical full-stack space project:

/
├── wrangler.json          # required for non-trivial setups
├── package.json           # npm deps (text-only packages, see limits below)
├── src/
│   └── index.ts           # server entry: export default { fetch }
└── public/                # static assets directory (configurable)
    ├── index.html
    ├── app.js             # compiled, no JSX
    └── styles.css

A static-only SPA can skip src/ entirely — just wrangler.json + public/ is enough as long as main points to a minimal pass-through worker or you accept that all requests fall through assets.

Wrangler config

Minimum viable wrangler.json:

{
  "main": "src/index.ts",
  "compatibility_date": "2025-04-01",
  "assets": {
    "directory": "./public",
    "html_handling": "auto-trailing-slash",
    "not_found_handling": "single-page-application"
  }
}

Notes:

• main is required when there's any server code. The bundler also auto-detects src/index.ts, src/index.js, index.ts, index.js if missing.
• compatibility_date defaults to 2025-04-01 if omitted. Set it explicitly for newer features.
• Add "compatibility_flags": ["nodejs_compat"] only if you actually need Node built-ins.
• assets.directory is the only way to ship static files. Files outside this directory are bundled into the Worker or ignored — they will not be reachable via URL.
• html_handling: "auto-trailing-slash" is usually what you want for multi-page sites; SPAs should also set not_found_handling: "single-page-application" so deep links return index.html.

TOML works too (wrangler.toml), but the parser only handles top-level scalar fields plus an [assets] table — no inline tables, no env overrides. Prefer JSON.

Server entry: one Durable Object class named App

Your entire backend is one Durable Object. Your main module exports
a class named App extending DurableObject. The platform loads it as
a Cloudflare Durable Object
Facet
of the host SpaceDO and forwards every non-asset request — including
WebSocket upgrades — to your App.fetch(request).

Do not export default { fetch }. A default fetch handler will be
ignored. There is no separate worker; the DO is the worker.

// src/index.ts
import { DurableObject } from "cloudflare:workers";

export class App extends DurableObject {
  async fetch(request: Request): Promise<Response> {
    const url = new URL(request.url);

    // Lazy schema init on the first request. Idempotent.
    this.ctx.storage.sql.exec(`
      CREATE TABLE IF NOT EXISTS notes (
        id INTEGER PRIMARY KEY AUTOINCREMENT,
        body TEXT NOT NULL
      )
    `);

    if (url.pathname === "/api/notes" && request.method === "POST") {
      const { body } = await request.json<{ body: string }>();
      this.ctx.storage.sql.exec(`INSERT INTO notes (body) VALUES (?)`, body);
      return new Response(null, { status: 201 });
    }
    if (url.pathname === "/api/notes") {
      const rows = this.ctx.storage.sql
        .exec(`SELECT id, body FROM notes ORDER BY id DESC`)
        .toArray();
      return Response.json(rows);
    }
    return new Response("Not found", { status: 404 });
  }
}

Or with Hono inside the DO:

import { DurableObject } from "cloudflare:workers";
import { Hono } from "hono";

export class App extends DurableObject {
  private app = new Hono()
    .get("/api/hello", (c) => c.json({ msg: "hi" }))
    .post("/api/notes", async (c) => {
      const { body } = await c.req.json<{ body: string }>();
      this.ctx.storage.sql.exec(`INSERT INTO notes (body) VALUES (?)`, body);
      return c.json({ ok: true });
    });

  async fetch(request: Request) {
    return this.app.fetch(request);
  }
}

When [assets] is configured, static files take priority — your fetch
only sees requests that didn't match an asset. For a SPA, put
index.html in public/ and App.fetch only handles /api/*.

State and storage

State lives on this.ctx.storage. No env.DB exists; you do not need
to declare any binding. The DO's storage is a per-(space, branch)
SQLite database that survives redeploys.

• this.ctx.storage.sql.exec(sql, ...params) — full SQLite. Returns a
  cursor with .toArray(), .one(), .columnNames, etc.
• this.ctx.storage.kv.get(key) / .put(key, value) — simple KV
  backed by SQLite.
• this.ctx.storage.transactionSync(() => { ... }) — atomic batch
  inside the same DO instance.

// SQL
this.ctx.storage.sql.exec(`CREATE TABLE IF NOT EXISTS counter (n INTEGER)`);
const cur = this.ctx.storage.sql.exec(`SELECT n FROM counter LIMIT 1`).toArray();
const n = (cur[0]?.n as number) ?? 0;

// KV
const last = this.ctx.storage.kv.get<number>("last_seen") ?? 0;
this.ctx.storage.kv.put("last_seen", Date.now());

Use the DO's storage for all persistent state — users, sessions,
content, settings, anything that should survive a reload. Do not use
localStorage or sessionStorage for primary data; they are
per-browser, per-origin, and lost on incognito or any other browser.

WebSockets

The platform forwards Upgrade: websocket requests transparently into
App.fetch. Use the standard Durable Object hibernation API:

export class App extends DurableObject {
  async fetch(request: Request) {
    const upgrade = request.headers.get("Upgrade");
    if (upgrade === "websocket") {
      const { 0: client, 1: server } = new WebSocketPair();
      this.ctx.acceptWebSocket(server);
      return new Response(null, { status: 101, webSocket: client });
    }
    return new Response("not found", { status: 404 });
  }

  webSocketMessage(ws: WebSocket, message: string | ArrayBuffer) {
    // Broadcast to every connected client (chat-style fan-out).
    for (const peer of this.ctx.getWebSockets()) {
      peer.send(typeof message === "string" ? message : new Uint8Array(message));
    }
  }

  webSocketClose(_ws: WebSocket, _code: number, _reason: string, _wasClean: boolean) {
    // Cleanup if needed.
  }
}

acceptWebSocket + webSocketMessage is the right pattern for
multi-connection use cases (chat rooms, multiplayer, presence) because
the DO can hibernate between messages without losing connections.

Per-entity grouping (chat rooms, per-user records)

You get one App DO per (space, branch) — a single instance, not a
namespace. Do not call idFromName or try to declare a DO binding in
wrangler.json; both are rejected. For multi-room / multi-user state,
partition rows with a column in the App's SQLite:

CREATE TABLE messages (
  room_id TEXT NOT NULL,
  user_id TEXT NOT NULL,
  body TEXT NOT NULL,
  ts INTEGER NOT NULL DEFAULT (strftime('%s','now') * 1000)
);
CREATE INDEX messages_by_room ON messages (room_id, ts);

Then route by room_id inside App.fetch. SQLite is fast enough that
sharing the same table across rooms is fine for prototyping; if you
later need stronger isolation, attach room_id to a tag and use
acceptWebSocket(ws, [room_id]) so getWebSockets(room_id) returns
only that room's sockets.

What you cannot declare

• durable_objects.bindings in wrangler.json. The platform
  extracts your App class automatically. Adding bindings fails the
  deploy with a clear error.
• new_sqlite_classes / migrations. Same reason. Your App's
  storage is set up by the platform.
• d1_databases, r2_buckets, kv_namespaces. Not available.
  Everything goes in this.ctx.storage.

Static asset rules (read this twice)

This is where most preview failures happen. The browser eventually runs your assets — the bundler does not transform them. So:

NEVER ship JSX in a <script type="module"> (or any other script tag). Browsers cannot parse JSX.

This page will be blank with a SyntaxError: Unexpected token '<':

<script type="module">
  import React from "https://esm.sh/react@18";
  const App = () => <div>hi</div>;   // ← browser dies here
</script>

Fix one of these three ways:

1. Pre-compile — write JSX in a build step (Vite/esbuild/tsup) and emit plain JS into public/. Best for production.
2. React.createElement by hand — works without a build step but is verbose.
3. @babel/standalone — only for prototypes. Load Babel before the script and use type="text/babel". The modern @babel/preset-react defaults to the automatic JSX runtime, which emits import { jsx } from "react/jsx-runtime". Browsers will reject that bare specifier unless your importmap maps it. Always ship a complete importmap alongside the Babel script:

html <script type="importmap"> { "imports": { "react": "https://esm.sh/react@18.3.1", "react/jsx-runtime": "https://esm.sh/react@18.3.1/jsx-runtime", "react-dom": "https://esm.sh/react-dom@18.3.1", "react-dom/client": "https://esm.sh/react-dom@18.3.1/client" } } </script> <script src="https://unpkg.com/@babel/standalone/babel.min.js"></script> <script type="text/babel" data-type="module" data-presets="react"> import React from "react"; import { createRoot } from "react-dom/client"; const App = () => <div>hi</div>; createRoot(document.getElementById("root")).render(<App />); </script>

data-type="module" is required for import to work inside the Babel script. Import React from the importmap key ("react") — not from a hard-coded esm.sh URL — so your code and Babel's emitted react/jsx-runtime import resolve to the same React instance.

If you really cannot ship react/jsx-runtime in the importmap, force Babel to the legacy classic runtime instead so it emits React.createElement calls and never touches react/jsx-runtime:

html <script type="text/babel" data-type="module" data-presets="react" data-plugins='[["transform-react-jsx", { "runtime": "classic" }]]' > import React from "https://esm.sh/react@18.3.1"; /* must be in scope */ const App = () => <div>hi</div>; </script>

The classic runtime requires React to be in lexical scope (because <div> becomes React.createElement("div")). The automatic runtime (default) does not, but needs react/jsx-runtime resolvable.

Symptom-to-fix index

Other asset gotchas

• Binary assets are not extracted from npm packages. Fonts, images, and .wasm shipped via npm tarballs will be missing. Put binaries directly in public/ instead.
• Paths in JS need care under preview: prefer import.meta.url, relative paths, or a runtime base detected from document.baseURI. Don't hard-code /api/... in client code; use ./api/... or read a base from a <meta> tag.
• Trailing slashes matter for auto-trailing-slash mode: /about serves about.html, /about/ serves about/index.html. Pick one shape and link consistently.
• No CSS preprocessors at runtime — ship .css, not .scss. Compile first if you need Sass.
• Importmaps work since they're inline JSON. Use them to avoid bundling React/etc. for prototypes — but watch the dual-React trap below.

The dual-React trap (the #1 cause of "blank page + useContext is null`)

esm.sh bundles a package's peer dependencies into the package itself unless you tell it otherwise. So this importmap looks correct but ships two copies of React — one for your app, another nested inside framer-motion:

<!-- BROKEN: framer-motion has its own React inside, hooks crash with `Cannot read properties of null (reading 'useContext')` -->
<script type="importmap">
{
  "imports": {
    "react": "https://esm.sh/react@18.2.0",
    "react-dom": "https://esm.sh/react-dom@18.2.0/client",
    "framer-motion": "https://esm.sh/framer-motion@11"
  }
}
</script>

The symptom is exactly: blank background + console errors like

TypeError: Cannot read properties of null (reading 'useContext')
  at u (SwitchLayoutGroupContext.mjs:1:1)

because the nested React instance has no provider in the tree.

Fix: append ?external=react,react-dom (and any other shared peers) to every esm.sh URL that has React as a peer dep. Pin the exact same React version everywhere too.

<!-- CORRECT: one React shared across the whole importmap -->
<script type="importmap">
{
  "imports": {
    "react": "https://esm.sh/react@18.3.1",
    "react/jsx-runtime": "https://esm.sh/react@18.3.1/jsx-runtime",
    "react-dom": "https://esm.sh/react-dom@18.3.1",
    "react-dom/client": "https://esm.sh/react-dom@18.3.1/client",
    "framer-motion": "https://esm.sh/framer-motion@11?external=react,react-dom",
    "lucide-react": "https://esm.sh/lucide-react@0.400.0?external=react,react-dom"
  }
}
</script>

Apply ?external=... to every React-consuming dep (UI libraries, icon sets, animation libs, headless components). Same rule for vue, solid-js, etc. if you swap the framework.

If a project needs more than two or three of these packages, stop using the importmap path — switch to option 1 (pre-compile with Vite/esbuild) so the bundler can dedupe React for you.

npm dependencies

The bundler installs deps from npm at build time. Limits to respect:

• Flat node_modules. No two versions of the same package can coexist — peer-dep conflicts will pick one and break the other. Keep dep graphs shallow.
• Text-only files are extracted from tarballs. .js, .ts, .json, .css, .md work. .wasm, .node, native binaries, fonts, images do not.
• No PAX tar headers — packages whose internal paths exceed 100 chars may have those files silently dropped. Avoid deeply-nested monorepo packages.
• No build scripts run — postinstall, prepare, etc. are ignored. Packages that compile native code or run codegen at install time will not work.
• cloudflare:* imports are always external and resolved by the runtime (cloudflare:workers, etc.). Don't add them to package.json.

Safe and well-tested deps: hono, zod, itty-router, nanoid, valibot, @hono/zod-validator. Avoid anything that needs node-native modules unless you set compatibility_flags: ["nodejs_compat"] and the package is pure JS under that flag.

Project shapes

Pre-flight checklist before deploy_space

Run through every item — most "preview is broken" reports trace back to one of these.

• wrangler.json exists at repo root with main and (if static assets exist) [assets].directory.
• Every browser-loaded .js / .mjs / <script type="module"> is plain JS, no JSX, or wrapped with @babel/standalone + type="text/babel".
• If you use React via importmap (Babel-standalone or pre-compiled with the automatic JSX runtime), the importmap maps all of react, react/jsx-runtime, react-dom, and react-dom/client — same version on every entry. Missing react/jsx-runtime produces Failed to resolve module specifier "react/jsx-runtime" and a blank page.
• The <script type="importmap"> tag appears before any <script type="module"> (or type="text/babel" data-type="module") that depends on the mapped specifiers.
• Static files live under the configured assets.directory (default suggestion: public/).
• HTML uses root-relative paths (/foo.js) — they get rewritten. JS uses relative paths (./foo).
• No binary assets are imported from npm packages. Binaries live in public/.
• package.json deps are pure-JS, no native modules, no install scripts.
• SPA routing? Set not_found_handling: "single-page-application".
• Server entry exports class App extends DurableObject from your main module (no export default { fetch }).
• No durable_objects / d1_databases / kv_namespaces / r2_buckets blocks in wrangler.json.
• compatibility_date is set if you use APIs newer than the default.

If any of these are off, fix them in the working tree, commit, and redeploy. The preview will pick up the new build on the next deploy_space call (the dynamic worker is keyed by commit hash, so old builds are not reused).

Minimal end-to-end example

A working SPA with a tiny API:

wrangler.json
package.json
src/index.ts
public/index.html
public/app.js
public/style.css

wrangler.json:

{
  "main": "src/index.ts",
  "compatibility_date": "2025-04-01",
  "assets": {
    "directory": "./public",
    "html_handling": "auto-trailing-slash",
    "not_found_handling": "single-page-application"
  }
}

package.json:

{ "name": "demo", "type": "module", "dependencies": { "hono": "^4.6.0" } }

src/index.ts:

import { DurableObject } from "cloudflare:workers";
import { Hono } from "hono";

export class App extends DurableObject {
  private app = new Hono()
    .get("/api/time", (c) => c.json({ now: new Date().toISOString() }))
    .post("/api/visit", async (c) => {
      this.ctx.storage.sql.exec(
        `CREATE TABLE IF NOT EXISTS visits (id INTEGER PRIMARY KEY AUTOINCREMENT, ts INTEGER)`
      );
      this.ctx.storage.sql.exec(`INSERT INTO visits (ts) VALUES (?)`, Date.now());
      const cnt = this.ctx.storage.sql
        .exec(`SELECT COUNT(*) AS c FROM visits`)
        .one().c as number;
      return c.json({ count: cnt });
    });

  async fetch(request: Request) {
    return this.app.fetch(request);
  }
}

public/index.html:

<!doctype html>
<html>
<head>
  <link rel="stylesheet" href="/style.css">
</head>
<body>
  <div id="root">Loading…</div>
  <script type="module" src="/app.js"></script>
</body>
</html>

public/app.js:

const res = await fetch("./api/time");
const data = await res.json();
document.getElementById("root").textContent = data.now;

// Increment + display visit counter (server state lives in App.ctx.storage.sql)
const v = await fetch("./api/visit", { method: "POST" }).then((r) => r.json());
document.getElementById("root").textContent += `  ·  visits: ${v.count}`;

Note ./api/time (relative) in JS, /style.css (root-relative) in HTML. The HTML href is rewritten by the preview; the JS fetch resolves against the document's base URL.

Commit this, call deploy_space("main"), and the preview at /space/<name>/preview/main/ will render the timestamp and an incrementing visit counter persisted in the App DO's SQLite.