Creating the CartProduct service

The cart item service also extracts the logic of the CartProduct model. To create this service, follow these steps:

1. Create a file called CartProductService.js in the service folder.

2. Remove the addUnit and removeUnit methods from the CartProduct model.

3. Update the service with these methods:

   var CartProductService = (function() {
   
     var addUnit = function (cartItem) {
       var u = cartItem.units();
       var _stock =  cartItem.product.stock();
       if (_stock === 0) {
         return;
       }
       cartItem.units(u+1);
       cartItem.product.stock(--_stock);
     };
   
     var removeUnit = function (cartItem) {
       var u =  cartItem.units();
       var _stock =  cartItem.product.stock();
       if (u === 0) {
         return;
       }
       cartItem.units(u-1);
       cartItem.product.stock(++_stock);
     };
   
     return {
       addUnit:addUnit,
       removeUnit:removeUnit
     };
   })();