"Globals" in service workers via the cache and Proxies

example.js

getGlobal('flags').then(flags => {
  flags['debug'] = true;
  flags['email'] = 'xxx@example.com';
});

// sometime later

getGlobal('flags').then(flags => {
  console.log(JSON.stringify(flags)); // -> {"debug":true,"email":"xxx@example.com"}
});

getGlobal.js

function getGlobal(name) {
  return caches.open('globals').then(cache => {
    function put(obj) {
      let req = new Request(name);
      let res = new Response(JSON.stringify(obj));
      return cache.put(req, res).then(() => obj);
    }
    return cache.match(new Request(name)).then(r => {
      return r ? r.json() : put({});
    }).then(obj => {
      return new Proxy(obj, {
        "set": (obj, k, v) => {
          obj[k] = v;
          put(obj);
          return obj;
        },
        "deleteProperty": (obj, k) => {
          delete obj[k];
          put(obj);
          return true;
        }
      });
    });
  });
}