for和forEach循环不适用于对象数组

Question

I have array of custom objects, I want to iterate with shorten loop, but this doesn't work, each time it returns "0".
for(var i =0;i<array.length;i++) works like a charm

Here is my class

var Product = (function () {
    var nextId = 0;
    return function Product(name, quantity, price, picture) {
        this.id = nextId++;
        this.name = name;
        this.quantityInStock = quantity;
        this.price = price;
        this.picture = picture;
    }
})();

var Cart = (function () {
    var nextId = 0;
    return function Cart(productList) {
        this.id = nextId++;
        this.productList = (productList === undefined || !productList instanceof Array) ? [] : productList;
    }
})();
Cart.prototype.addProduct = function (product) {
    (product instanceof Product) ? this.productList.push(product) : console.log("Invalid object type");
};
Cart.prototype.calculateLumpSum = function () {
    var sum = 0;
    this.productList.forEach(function (product) { // doesn't work as well as for in loop
        sum += product.price;
    });
    return sum;
};

Please help where is my fault, again simple for loop works well

EDIT

Neither for in nor forEach loop works with this array, in debugger it shows that for example

for(var product in productList) {

}

product in this case equals to "0" but array is correct and has added items.

Answer 1

I assume this piece of code:

Cart.prototype.calculateLumpSum = function () {
    var sum = 0;
    for(var product in this.productList) {
        sum += product.price;
    }
    return sum;
};

This works perfectly, if product.price exists and has value.