groupBy.js

var groupBy = function(xs, key) {
	return xs.reduce(function(rv, x) {
		(rv[x[key]] = rv[x[key]] || []).push(x);
		return rv;
	}, {});
};

// Usage
// console.log(groupBy(['one', 'two', 'three'], 'length'));


/**
 * Javascript -
 *   Array multiple groupBy function
 *   Forked from http://codereview.stackexchange.com/questions/37028/grouping-elements-in-array-by-multiple-properties
 *
 * Usage:
 *   var list = [
 *     { foo: "bar", id: 1  },
 *     { foo: "barbar", id: 2  },
 *     { foo: "bar", id: 1  },
 *     { foo: "bar", id: 3  },
 *     { foo: "barbar", id: 2  },
 *   ];
 *
 *   var newList = list.groupBy(function (item) {
 *     return [item.id];
 *   });
 */
if ( ! Array.prototype.groupBy) {
	Array.prototype.groupBy = function (f)
	{
		var groups = {};
		this.forEach(function(o) {
			var group = JSON.stringify(f(o));
			groups[group] = groups[group] || [];
			groups[group].push(o);
		});

		return Object.keys(groups).map(function (group) {
			return groups[group];
		});
	};
}

/*
 * Details: implementation for functional programming
 * http://www.datchley.name/getting-functional-with-javascript-part-2/
 * /