cloudflare worker early hints implementation

worker.js

export default {
  async fetch(request, env) {
    switch (request.method) {
      case 'HEAD':
      case 'GET':
        const url = new URL(request.url);
        const key = decodeURIComponent(url.pathname);

        // replace suffix / after concating cause raw key still needed for regex
        let objKey = key.replace(/\/+$/gm, '');
        // regex to detect path that doesnt has extension
        if (!/[^\/]+\.[^\/.]+$/.test(key)) {
          objKey += '/index.html';
        }

        // MY_BUCKET is r2 binding
        let object = await env.MY_BUCKET.get(objKey, opts);
        if (object === null)
          return new Response(`404 Not Found`, { status: 404 });

        const headers = new Headers();
        object.writeHttpMetadata(headers);
        headers.set('etag', object.httpEtag);
        headers.set('Content-Encoding', 'gzip');

        if (objKey.endsWith('.html')) {
          const text = await object.text();
          const links = extractEarlyHintsAssets(host, text);
          for (const l of links) {
            let v = `<${l.url}>; rel=${l.isPreload ? 'preload' : 'preconnect'}`;
            if (l.isPreload) v = `${v} as=${l.as}`;
            headers.append('link', v);
          }
          return new Response(text, {
            headers,
          });
        }

        return new Response(object.body, {
          headers,
        });
      default:
        return new Response('Method Not Allowed', {
          status: 405,
          headers: {
            Allow: 'GET, HEAD',
          },
        });
    }
  },
};

const extractEarlyHintsAssets = (host, body) => {
  const reg = new RegExp(`^(?:/|https?://${host.replaceAll('.', '.')}).*$`);
  const s = new Set();
  return [
    ...body.matchAll(
      /<link.+href="([^"]+)".*>|<script.+src="([^"]+)".*>|<img.+src="([^"]+)".*>/gm,
    ),
  ]
    .flatMap((match) => match.slice(1))
    .map((url) => {
      if (!url) return { url: '' };

      url = url.split('?')[0];
      let as = '';
      if (url.endsWith('.css')) as = 'style';
      else if (url.endsWith('.js')) as = 'script';
      const isPreload = as != '' && reg.test(url);
      if (!isPreload) {
        try {
          const u = new URL(url);
          url = `${u.protocol}//${u.host}`;
        } catch (err) {
          url = '';
        }
      } else {
        url = url.replace(`https://${host}`, '');
      }
      return { isPreload, url, as };
    })
    .filter((link) => {
      let found = true;
      if (s.has(link.url)) found = false;
      else s.add(link.url);
      return link.url && link.url !== `https://${host}` && found;
    });
};