@reduxjs/toolkit

Updates the middleware and enhancers options of configureStore to require callbacks

Updates the packaging for better ESM/CJS compatibility and modernizes the build output

Includes all changes to Redux core 5.0, Reselect 5.0, and Redux Thunk 3.0

Updates RTKQ default subscription behavior

Has many additional TS tweaks and improvements

It wasn't clear what the syntax for declaring a thunk inside should look like.

Thunks have access to getState and dispatch, but the RootState and AppDispatch types are normally inferred from the store, which in turn infers it from the slice state types. Declaring thunks inside createSlice would cause circular type inference errors, as the store needs the slice types but the slice needs the store types. We weren't willing to ship an API that would work okay for our JS users but not for our TS users, especially since we want people to use TS with RTK.

You can't do synchronous conditional imports in ES modules, and there's no good way to make the createAsyncThunk import optional. Either createSlice always depends on it (and adds that to the bundle size), or it can't use createAsyncThunk at all.

In order to create async thunks with createSlice, you specifically need to set up a custom version of createSlice that has access to createAsyncThunk.

You can declare thunks inside of createSlice.reducers, by using a "creator callback" syntax for the reducers field that is similar to the build callback syntax in RTK Query's createApi (using typed functions to create fields in an object). Doing this does look a bit different than the existing "object" syntax for the reducers field, but is still fairly similar.