added unit testing, and started implementing unit tests...phew

node_modules/async/groupBySeries.js

@@ -0,0 +1,37 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+
+var _doLimit = require('./internal/doLimit');
+
+var _doLimit2 = _interopRequireDefault(_doLimit);
+
+var _groupByLimit = require('./groupByLimit');
+
+var _groupByLimit2 = _interopRequireDefault(_groupByLimit);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+/**
+ * The same as [`groupBy`]{@link module:Collections.groupBy} but runs only a single async operation at a time.
+ *
+ * @name groupBySeries
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.groupBy]{@link module:Collections.groupBy}
+ * @category Collection
+ * @param {Array|Iterable|Object} coll - A collection to iterate over.
+ * @param {number} limit - The maximum number of async operations at a time.
+ * @param {AsyncFunction} iteratee - An async function to apply to each item in
+ * `coll`.
+ * The iteratee should complete with a `key` to group the value under.
+ * Invoked with (value, callback).
+ * @param {Function} [callback] - A callback which is called when all `iteratee`
+ * functions have finished, or an error occurs. Result is an `Object` whoses
+ * properties are arrays of values which returned the corresponding key.
+ */
+exports.default = (0, _doLimit2.default)(_groupByLimit2.default, 1);
+module.exports = exports['default'];