Bun etag middlware

etag.ts

type RequestHandler = (req: Request) => Promise<Response>;

export const retainedHeaders = [
  "cache-control",
  "content-location",
  "date",
  "etag",
  "expires",
  "vary",
];

function create304Response(response: Response): Response {
  for (const header of response.headers.keys()) {
    if (!retainedHeaders.includes(header)) {
      response.headers.delete(header);
    }
  }
  return new Response(undefined, {
    status: 304,
    // headers should be kept
    // https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304
    headers: response.headers,
  });
}

function etagMatches(etag: string, ifNoneMatch: string | null): boolean {
  if (ifNoneMatch === null) {
    return false;
  }
  return ifNoneMatch.split(", ").includes(etag);
}

export function middleware(handle: RequestHandler): RequestHandler {
  return async (req) => {
    const ifNoneMatch = req.headers.get("If-None-Match");

    // https://github.com/bogeychan/elysia-etag/blob/master/src/index.ts
    // https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304
    const isGetOrHead = req.method === "GET" || req.method === "HEAD";
    if (ifNoneMatch !== null && !isGetOrHead) {
      return new Response(undefined, { status: 412 });
    }

    const response = await handle(req);

    if (response.body === null) {
      return response;
    }

    const headerEtag = response.headers.get("ETag");
    if (headerEtag !== null) {
      if (etagMatches(headerEtag, ifNoneMatch)) {
        return create304Response(response);
      }
      return response;
    }

    const mime = response.headers.get("Content-Type")?.split(";")[0];
    if (mime === undefined) {
      return response;
    }

    // what else?
    if (mime === "text/event-stream") {
      return response;
    }

    const body = await response.arrayBuffer();
    const etag = Bun.hash(body).toString();

    if (etagMatches(etag, ifNoneMatch)) {
      return create304Response(response);
    }
    const newResponse = new Response(body, response);
    newResponse.headers.set("ETag", etag);

    return newResponse;
  };
}