promisify() {
    return new Promise((resolve, reject) => {
      this.request.onsuccess = event => {
        resolve(event.target.result);
      };
      this.request.onerror = event => {
        reject(event.target.error);
      };
    });
  }
/**
   * [promisify turns the IDBCursor into a promise]
   * @return {Promise<IdbRequest>} [Promise that resolves on success or rejects on error]
   */
[promisify turns the IDBCursor into a promise]
@return {Promise} [Promise that resolves on success or rejects on error]
[ promisify turns the IDBCursor into a promise ] @return { Promise } [ Promise that resolves on success or rejects on error ]
promisify() {
    return new Promise((resolve, reject) => {
      this.request.onsuccess = event => {
        resolve(event.target.result);
      };
      this.request.onerror = event => {
        reject(event.target.error);
      };
    });
  }
promisify ( ) { return new Promise ( ( resolve , reject ) => { this . request . onsuccess = event => { resolve ( event . target . result ) ; } ; this . request . onerror = event => { reject ( event . target . error ) ; } ; } ) ; }
[promisify turns the IDBCursor into a promise]
 @return {Promise<IdbRequest>} [Promise that resolves on success or rejects on error]
[ promisify turns the IDBCursor into a promise ] @return { Promise<IdbRequest > } [ Promise that resolves on success or rejects on error ]