code: function clear(dbid) {
    return new Promise((resolve, reject) => {
      return instance[dbid].then(db => {
        const req = db.transaction(db.name, 'readwrite')
            .objectStore(db.name).clear();
          req.onsuccess = () => resolve();
          req.onerror = event => reject(event);
      });
    });
  }
docs: Clear database
  @param {string} dbid - database ID