Example of how you could use the Cache API to create a generic async key/value store. Something like this could be used over localStorage because it allows for more space and is available to workers. Working example: https://jsfiddle.net/ccnokes/790xaw53/

cache-based-async-storage.js

// Working example: https://jsfiddle.net/ccnokes/790xaw53/

class Storage {
  constructor(key = location.origin + '/cache-key-v1') {
    this.KEY = key;
  }

  get(id) {
    return caches.open(this.KEY)
      .then(cache => cache.match(id))
      .then(res => {
      	if (res) {
          return res.json();
        } else {
          return null;
        }
      });
  }
  
  set(id, val) {
    return caches.open(this.KEY)
    	.then(cache => cache.put(id, new Response(JSON.stringify(val))));
  }
  
  delete(id) {
    return caches.open(this.KEY)
    	.then(cache => cache.delete(id))
  }
  
  deleteAll() {
    return caches.delete(this.KEY);
  }
}