code: function (indexName, key) {
      return new Promise((resolve, reject) => {
        if (index[indexName]) {
          resolve(index[indexName][key] || null);
        } else {
          debug("opening index %s", indexName);
          storage.openIndex(indexName).then(index => {
            index[indexName] = index;
            resolve(index[indexName][key] || null);
          });
        }
      });
    }
docs: Returns an item from the index.
  @param {String} indexName Name of the index to query. Cannot be null.
  @param {String} key Unique key of the required item. Cannot be null.