Cache POST requests

async function sha256(message) {
  // encode as UTF-8
  const msgBuffer = new TextEncoder().encode(message);

  // hash the message
  const hashBuffer = await crypto.subtle.digest('SHA-256', msgBuffer);

  // convert bytes to hex string
  return [...new Uint8Array(hashBuffer)].map(b => b.toString(16).padStart(2, '0')).join('');
}

async function handlePostRequest(event) {
  const request = event.request;
  const body = await request.clone().text();

  // Hash the request body to use it as a part of the cache key
  const hash = await sha256(body);
  const cacheUrl = new URL(request.url);

  // Store the URL in cache by prepending the body's hash
  cacheUrl.pathname = '/posts' + cacheUrl.pathname + hash;

  // Convert to a GET to be able to cache
  const cacheKey = new Request(cacheUrl.toString(), {
    headers: request.headers,
    method: 'GET',
  });

  const cache = caches.default;

  // Find the cache key in the cache
  let response = await cache.match(cacheKey);

  // Otherwise, fetch response to POST request from origin
  if (!response) {
    response = await fetch(request);
    event.waitUntil(cache.put(cacheKey, response.clone()));
  }
  return response;
}

addEventListener('fetch', event => {
  try {
    const request = event.request;
    if (request.method.toUpperCase() === 'POST') return event.respondWith(handlePostRequest(event));
    return event.respondWith(fetch(request));
  } catch (e) {
    return event.respondWith(new Response('Error thrown ' + e.message));
  }
});