Mapping array of many-to-many entities to map with faster access

manyToManyMap.js

function manyToManyMap(source, first, second) {
    'use strict';
    var result = {}, i, item;
    for (i = 0; i < source.length; i += 1) {
        item = source[i];
        if (!result[item[first]]) {
            result[item[first]] = {};
        }
        result[item[first]][item[second]] = item;
    }
    return result;
}

test.js

/*http://jsperf.com/many-to-many-mapping*/

/*global manyToManyMap*/
/*jslint devel: true */

var testArray = [
    {first_id: 1, second_id: 1},
    {first_id: 2, second_id: 2},
    {first_id: 1, second_id: 2},
    {first_id: 2, second_id: 1}
];

var testMap = manyToManyMap(testArray, 'first_id', 'second_id');

console.log(testMap[1][1]);
console.log(testMap[1][2]);
console.log(testMap[2][1]);
console.log(testMap[2][2]);
/*
Object {first_id: 1, second_id: 1}
Object {first_id: 1, second_id: 2}
Object {first_id: 2, second_id: 1}
Object {first_id: 2, second_id: 2}
*/

withUnderscoreOrLodash.js

/*jslint nomen: true */
/*global _*/

function manyToManyMap(source, first, second) {
    'use strict';
    return _.each(_.groupBy(source, first), function (group, key, collection) {
        collection[key] = _.indexBy(group, second);
    });
}