--- /dev/null
+// Inspired by Underscore's groupBy:
+// http://documentcloud.github.com/underscore/#groupBy
+
+'use strict';
+
+var callable = require('../../object/valid-callable')
+ , value = require('../../object/valid-value')
+
+ , forEach = Array.prototype.forEach, apply = Function.prototype.apply;
+
+module.exports = function (cb/*, thisArg*/) {
+ var r;
+
+ (value(this) && callable(cb));
+
+ r = {};
+ forEach.call(this, function (v) {
+ var key = apply.call(cb, this, arguments);
+ if (!r.hasOwnProperty(key)) r[key] = [];
+ r[key].push(v);
+ }, arguments[1]);
+ return r;
+};