JavaScript Shopping Cart: Updating Quantity and Adding New Items

This code finds the index of an object in an array called 'cart' that has an 'id' property equal to the 'id' property of the 'obj' parameter. If the object is found, it increases the 'num' property of that object by the 'num' property of the current object. If the object is not found, it adds the current object to the end of the 'cart' array.

In other words, this code is updating a shopping cart. If the same product (identified by its ID) is already in the cart, it increases the quantity. If it's a new product, it adds it to the cart.