function readTable(table, idxName) {
  let open = indexedDB.open("books", 1);

  return new Promise(resolve => {
    open.onsuccess = evt => {
      let db = open.result;
      let tran = db.transaction(table);
      let objStore = tran.objectStore(table);
      let idx = objStore.index(idxName);
      let tranCursor = idx.openCursor();

      let result = [];

      tranCursor.onsuccess = evt => {
        let cursor = evt.target.result;
        if (!cursor) return resolve(result);

        let item = cursor.value;
        result.push(item);
        cursor.continue();
      };
    };
  });
}