;
                    self._createStores(db, oldVersion);
                }; 
                return reqAsPromise(req).then(function() {
                    self._database = req.result;
                });
            }

            openTxn(mode, storeName) {
                const txn = this._database.transaction([storeName], mode);
                txn.addEventListener("complete", () => {
                    console.info(`transaction ${mode} for ${storeName} completed`);
                });
                