cachedFetchJson.js

// use Cache API and cache expire 2 weeks
async function cachedFetchJson(uri,seconds)
{
  const expire = seconds || (14 * 24 * 3600);

  const today = Math.ceil(new Date().getTime() / 1000);
  const cache = await caches.open('v1');
  let response = await cache.match(uri);
  if(!response || ((parseInt(response.headers.get('cache-timestamp') || -1) + expire <= today)))
  {
    await addex(cache,uri);
    response = await cache.match(uri);
  }

  return response.json();
}

// add cache cloned response
async function addex(cache,url)
{
  return fetch(url).then(async (response) => {
    if (!response.ok) {
      throw new TypeError("bad response status");
    }

    const headers = new Headers(response.headers);
    headers.append('Cache-Timestamp',Math.ceil(new Date().getTime() / 1000));
    const blob = await response.blob();

    return cache.put(
      url,
      new Response(blob, {
        status: response.status,
        statusText: "",
        headers
      })
    );
  });
}

// no using Cache API
async function fetchJson(uri)
{
  const response = await fetch(uri);
  if(!response.ok)
    throw new Error('invalid response')

  return response.json();
}