My world changed when I started thinking in a strongly typed way (not just using string and number) and representing state changes as different types rather than interpreting than through property values. Once you do that then your code becomes mapping from one type to the next.

For example, if `type ShoppingCart = EmptyCart | LoadedCart` then `addToCart` is just a map from `ShoppingCart` to `LoadedCart`. It makes invalid states impossible to represent and flows become clearer. Add in good FP and composition becomes easier.