db.js

function Storage(name) {
  this.ready = new Promise((resolve, reject) => {
    var request = indexedDB.open(location.origin);

    request.onupgradeneeded = e => {
      this.db = e.target.result;
      this.db.createObjectStore('store');
    };

    request.onsuccess = e => {
      this.db = e.target.result;
      resolve();
    };

    request.onerror = e => {
      this.db = e.target.result;
      reject(e);
    };
  });
}

Storage.prototype.get = function(key) {
  return this.ready.then(() => {
    return new Promise((resolve, reject) => {
      var request = this.getStore().get(key);
      request.onsuccess = e => resolve(e.target.result);
      request.onerror = reject;
    });
  });
};

Storage.prototype.getStore = function() {
  return this.db
    .transaction(['store'], 'readwrite')
    .objectStore('store');
};

Storage.prototype.set = function(key, value) {
  return this.ready.then(() => {
    return new Promise((resolve, reject) => {
      var request = this.getStore().put(value, key);
      request.onsuccess = resolve;
      request.onerror = reject;
    });
  });
};

Storage.prototype.reset = function() {
  indexedDB.deleteDatabase(location.origin);
};