_middleware.js

function parseAcceptHeader(acceptHeader) {
  // generated by Google AI "parse accept header javascript"
  if (!acceptHeader) {
    return [];
  }

  return acceptHeader.split(',')
    .map(item => item.trim())
    .map(item => {
      const parts = item.split(';');
      const value = parts[0].trim();
      let quality = 1;

      if (parts.length > 1) {
        const q = parts[1].trim();
        if (q.startsWith('q=')) {
          quality = parseFloat(q.substring(2));
        }
      }

      return { value, quality };
    })
    .sort((a, b) => b.quality - a.quality);
    // From:
    // text/html, application/xhtml+xml, application/xml;q=0.9, */*;q=0.8
    // Expected output:
    // [
    //   { value: 'text/html', quality: 1 },
    //   { value: 'application/xhtml+xml', quality: 1 },
    //   { value: 'application/xml', quality: 0.9 },
    //   { value: '*/*', quality: 0.8 }
    // ]
}

const typeToExt = {
  "text/turtle": "ttl",
  "application/ld+json": "jsonld"
};

export async function onRequest(context) {
  try {
    const parsedAccept = parseAcceptHeader(context.request.headers.get('Accept'));
    const accept = parsedAccept[0].value;

    // if we have a mapping for this extension, send it. Otherwise fall through.
    if(Object.keys(typeToExt).indexOf(accept) > -1) {
      const ext = typeToExt[accept] || `html`;
      const rewrittenUrl = context.request.url + '.' + ext;
      const asset = await context.env.ASSETS.fetch(rewrittenUrl);
      const response = new Response(asset.body, asset);
      return response;
    }
    return context.next();
  } catch (err) {
    return new Response(`${err.message}\n${err.stack}`, { status: 500 });
  }
}