--- /dev/null
+var createAggregator = require('./_createAggregator');
+
+/** Used for built-in method references. */
+var objectProto = Object.prototype;
+
+/** Used to check objects for own properties. */
+var hasOwnProperty = objectProto.hasOwnProperty;
+
+/**
+ * Creates an object composed of keys generated from the results of running
+ * each element of `collection` through `iteratee`. The corresponding value
+ * of each key is an array of elements responsible for generating the key.
+ * The iteratee is invoked with one argument: (value).
+ *
+ * @static
+ * @memberOf _
+ * @category Collection
+ * @param {Array|Object} collection The collection to iterate over.
+ * @param {Function|Object|string} [iteratee=_.identity] The iteratee to transform keys.
+ * @returns {Object} Returns the composed aggregate object.
+ * @example
+ *
+ * _.groupBy([6.1, 4.2, 6.3], Math.floor);
+ * // => { '4': [4.2], '6': [6.1, 6.3] }
+ *
+ * // The `_.property` iteratee shorthand.
+ * _.groupBy(['one', 'two', 'three'], 'length');
+ * // => { '3': ['one', 'two'], '5': ['three'] }
+ */
+var groupBy = createAggregator(function(result, value, key) {
+ if (hasOwnProperty.call(result, key)) {
+ result[key].push(value);
+ } else {
+ result[key] = [value];
+ }
+});
+
+module.exports = groupBy;