[рони]

Romann,

var id = "1"; // id товара
  var name = "item 2"; // переменная имени
  var x = 1;
  var price = 500;


function Cart(id, name, price, x) {
    var foo = localStorage.getItem("foo");
    foo = foo ? JSON.parse(foo) : {};
    foo.sum = function(id) {
        return id in this ? this[id][2] * this[id][3] : 0
    };
    Object.defineProperty(foo, "sum", {
        enumerable: false
    });
    Object.defineProperty(foo, "total", {
        enumerable: false,
        get: function() {
            return Object.keys(this).reduce(function(s, a) {
                return s + foo.sum(a)
            }, 0)
        }
    });
    if (id != void 0) {
        var item = foo[id];
        var text = "обновляем товар";
        if (!(id in foo)) {
            foo[id] = [id, name, price, 0];
            text = "добавляем новый товар"
        }
        foo[id][3] += x;
        localStorage.setItem("foo", JSON.stringify(foo));
        alert(text)
    }
    return foo
};
var foo = Cart(id,name,price,x);
console.log(foo.sum(1),foo.total)