Tracking specific fields in an array instead of the whole object

[salmanbabri]

For example, if I've an invoice with a list of products in it, like this:-

type Invoice = {
    id: number;
    products: Array<Product>;
}

type Product = {
    id: number;
    quantity: number;
    price: number;
    notes: string;
}

Now, let's say I wanted to calculate the total cost using quantity and price. Is it possible to watch only these specific fields? Or my only options Is to watch the whole products array?