Map array data

mapBy.js

/* 
Create a mapped object by any identifier
example: 
var a = [{id:1, name:'a'},{id:2, name:'b'},{id:3, name:'c'}];
a.mapBy('id') 
output: [
    {1:{id:1,name:'a'}},
    {2:{id:2,name:'b'}},
    {3:{id:3,name:'c'}}
]
a.mapBy('id', true) 
output: [
    {1:{name:'a'}},
    {2:{name:'b'}},
    {3:{name:'c'}}
]
*/

var a = [
        {
            "id": "PROMO255-4",
            "programId": "1",
            "name": "Promo cardSet # 255-4",
            "cardQuantity": 450,
            "loadedCards": [
                {"quantity": 10, "amount": 55},
                {"quantity": 20, "amount": 100}
            ]   
        },
        {
            "id": "PROMO257-2",
            "programId": "1",
            "name": "Promo cardSet # 257-1",
            "cardQuantity": 67,
            "loadedCards": [
                {"quantity": 40, "amount": 25},
                {"quantity": 18, "amount": 85},
                {"quantity": 5, "amount": 50}
            ]   
        }
    ];

Array.prototype.mapBy = function (id, arr) {
	var b = arr.reduce(function(map, obj){
		map[obj[id]] = obj;
    return map;
	},{});
    return b;
};

console.log(a.mapBy('id', a));