code: function has(key) {
      if (options.cacheValues) {
        return Promise.resolve((cachedData[key] !== undefined));
      } else if (options.cacheKeys) {
        return Promise.resolve((cachedKeys.indexOf(key) !== -1));
      } else {
        return readData().then(function(data) {
          return (data[key] !== undefined);
        });
      }
    }
docs: Implements checking for if a value for a key exists
  
  @param {Key} key Key to see if there is a value for
  
  @returns {Promise} A promise that will resolve to a Boolean value of
  whether or not a value for the given key exists