consistency

Author: ben.durrant

packages/toolkit/src/configureStore.ts

@@ -44,15 +44,13 @@ export interface ConfigureStoreOptions<
   A extends Action = AnyAction,
   M extends Middlewares<S> = Middlewares<S>,
   E extends Enhancers = Enhancers,
-  PreloadedState = S
+  P = S
 > {
   /**
    * A single reducer function that will be used as the root reducer, or an
    * object of slice reducers that will be passed to `combineReducers()`.
    */
-  reducer:
-    | Reducer<S, A, PreloadedState>
-    | ReducersMapObject<S, A, PreloadedState>
+  reducer: Reducer<S, A, P> | ReducersMapObject<S, A, P>
 
   /**
    * An array of Redux middleware to install. If not supplied, defaults to
@@ -78,16 +76,8 @@ export interface ConfigureStoreOptions<
    * function (either directly or indirectly by passing an object as `reducer`),
    * this must be an object with the same shape as the reducer map keys.
    */
-  /*
-  Not 100% correct but the best approximation we can get:
-  - if S is a `CombinedState` applying a second `CombinedState` on it does not change anything.
-  - if it is not, there could be two cases:
-    - `ReducersMapObject<S, A>` is being passed in. In this case, we will call `combineReducers` on it and `CombinedState<S>` is correct
-    - `Reducer<S, A>` is being passed in. In this case, actually `CombinedState<S>` is wrong and `S` would be correct.
-    As we cannot distinguish between those two cases without adding another generic parameter,
-    we just make the pragmatic assumption that the latter almost never happens.
-  */
-  preloadedState?: PreloadedState
+  // we infer here, and instead complain if the reducer doesn't match
+  preloadedState?: P
 
   /**
    * The store enhancers to apply. See Redux's `createStore()`.
@@ -143,10 +133,8 @@ export function configureStore<
   A extends Action = AnyAction,
   M extends Middlewares<S> = [ThunkMiddlewareFor<S>],
   E extends Enhancers = [StoreEnhancer],
-  PreloadedState = S
->(
-  options: ConfigureStoreOptions<S, A, M, E, PreloadedState>
-): EnhancedStore<S, A, M, E> {
+  P = S
+>(options: ConfigureStoreOptions<S, A, M, E, P>): EnhancedStore<S, A, M, E> {
   const curriedGetDefaultMiddleware = curryGetDefaultMiddleware<S>()
 
   const {
@@ -157,16 +145,12 @@ export function configureStore<
     enhancers = undefined,
   } = options || {}
 
-  let rootReducer: Reducer<S, A, PreloadedState>
+  let rootReducer: Reducer<S, A, P>
 
   if (typeof reducer === 'function') {
     rootReducer = reducer
   } else if (isPlainObject(reducer)) {
-    rootReducer = combineReducers(reducer) as unknown as Reducer<
-      S,
-      A,
-      PreloadedState
-    >
+    rootReducer = combineReducers(reducer) as unknown as Reducer<S, A, P>
   } else {
     throw new Error(
       '"reducer" is a required argument, and must be a function or an object of functions that can be passed to combineReducers'