worker.ts

export interface Env {
  ASSETS: Fetcher;
}

const SPA_FALLBACK_STATUS = 404;

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const url = new URL(request.url);

    try {
      let response = await env.ASSETS.fetch(request);

      if (shouldHandleAsSpa(request, response, url)) {
        const indexRequest = new Request(new URL("/index.html", url.origin), request);
        response = await env.ASSETS.fetch(indexRequest);
      }

      return response;
    } catch (error) {
      console.error("Asset handling failed", error);
      return new Response("Internal Server Error", { status: 500 });
    }
  },
};

function shouldHandleAsSpa(request: Request, response: Response, url: URL): boolean {
  if (response.status !== SPA_FALLBACK_STATUS || request.method !== "GET") {
    return false;
  }

  const accept = request.headers.get("accept");
  const wantsHTML = accept?.includes("text/html") ?? false;
  const looksLikeAsset = url.pathname.includes(".");

  return wantsHTML && !looksLikeAsset;
}