underscore.js extended groupBy with value selector

underscore_groupby_with_selector.js

// The code below will extend underscore.js adding the groupByAndMap function.
// groupByAndMap works like the regular groupBy, but it accepts an optional third "selector" argument.
// The 'selector' is used to select values inside each group.
// As usual in underscore, you can either pass a function or a string with the name of an attribute you want.
// groupByAndMap is useful when you don't want the key to be present in the values inside the group.
// Also, it completely covers the groupBy functionality because the last argument is optional.
// If you want, you can replace groupBy instead of defining a new function.

_.mixin({
	groupByAndMap: function(list, keySelector, valueSelector) {
		function lookupIterator(val) {
			return _.isFunction(val) ? val : function(obj) { return obj[val]; };
		};

		var keySelectorFunc = lookupIterator(keySelector);
		var valueSelectorFunc = valueSelector ? lookupIterator(valueSelector) : _.identity;

		return _.foldl(list, function(result, value) {
		  var key = keySelectorFunc(value);
		  (result[key] || (result[key] = [])).push(valueSelectorFunc(value));
		  return result;
		}, {});
	}
});

// And here is a qunit test that you can run to verify that the function works

test('collections: groupBy with selector', function() {
  var list = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"];

  var selected = _.groupByAndMap(list, 'length', function(num){ return num.substr(0, 2); });
  deepEqual(selected, {
    '3': ['on', 'tw', 'si', 'te'],
    '4': ['fo', 'fi', 'ni'],
    '5': ['th', 'se', 'ei']
  });

  var lens = _.groupByAndMap(list, function(num){ return num.indexOf('f'); }, 'length');
  deepEqual(lens, {
    '-1': [3, 3, 5, 3, 5, 5, 4, 3],
    '0': [4, 4],
  });
});

Great!! thank you