Building a Reducer Method to add an Item to the Shopping Cart

Reducer methods in RTK's createSlice method take two arguments. The state and an action of type PayloadAction. PayloadAction is a generic type that requires you to pass in the specific type of the payload you are expecting. In our case it's going to be PayloadAction<string>.

One thing that's really impressive about RTK is that it builds its reducers using immer. Immer allows you to deal with immutable data in a seemingly mutable way. While it appears that we're directly modifying our cart state here, if you hover the state variable in your IDE you'll see it's actually of type WritableDraft<CartState>, which means it's a draft of your state. Any changes made there will captured by immer and converted into a single new state object that will replace your current one after your redux method is done running. It's pretty clever and much easier to write than the old redux way of having to make long copies of everything to avoid mutating the redux state directly!