code: function(type, key, opt_childKey) {
      return (
        /**
         * @param {Object} item
         * @return {any}
         */
        function(item) {
          if (opt_childKey !== undefined) {
            item = item[opt_childKey];
          }
          return item[key];
        });
    }
docs: Factory for creating simple getter functions for extractors.
  If the value is in a child property, the opt_childKey must be defined.
  The type parameter is used by closure to type the returned function.
  @param {any} type An object of the expected result type.
  @param {string} key Key used for retrieving the value.
  @param {string=} opt_childKey Key of a child object.
  @return {function(Object): any} Getter function.