Refactor evolve and evolveWithMetadata to avoid code duplication and a potential bug for amounts

[shraddha38]

Summary:

The evolve and evolveWithMetadata functions in ShoppingCart have some code duplication and lack safety for new event types.

Suggested Improvements

1. Removing duplication - Reduce duplication in event handling logic and ensure safety by adding a default case in switch statements

Same logic is used in evolve and evolveMetatadata fn, so we can extract it as following -

const updateCart = (state: ShoppingCart, productItem: PricedProductItem): ShoppingCart => ({
  productItems: [...state.productItems, productItem],
  totalAmount: state.totalAmount + productItem.price * productItem.quantity,
});

2. Bug- Prevent discounts from causing negative amounts

Current

 case 'DiscountApplied':
      return {
        ...state,
        totalAmount: state.totalAmount * (1 - data.percent / 100),
      };

Suggestion

 case 'DiscountApplied':
      return {
        ...state,
        totalAmount: Math.max(state.totalAmount * (1 - data.percent / 100), 0),
      };

Next Steps

1. Need review from maintainers on this issue.
2. After that I can create a Pr for the changes.

[oskardudycz]

@shraddha38 thank you for checking Emmett and the suggestion.

The code you linked is made for tests purposes, to cover specific use cases (so with and without metadata). It’s more related to fit as an example use case, so sharing the code between them would be blurring that. I’d prefer to keep it as it is.

Yet, I’d be happy to take the small PR with suggestion to use Max to avoid negative amount.

[shraddha38]

@oskardudycz Thanks, totally aligned to keep the code as is. Added a small pr for the change of max to avoid a negative amount for discounts. #201