Preserve nullable store state type by avoiding intersection with {}

Author: EskiMojo14

packages/toolkit/src/configureStore.ts

@@ -26,6 +26,7 @@ import type {
   ExtractDispatchExtensions,
   ExtractStoreExtensions,
   ExtractStateExtensions,
+  UnknownIfNonSpecific,
 } from './tsHelpers'
 import type { Tuple } from './utils'
 import type { GetDefaultEnhancers } from './getDefaultEnhancers'
@@ -103,7 +104,8 @@ export type EnhancedStore<
   S = any,
   A extends Action = UnknownAction,
   E extends Enhancers = Enhancers
-> = ExtractStoreExtensions<E> & Store<S & ExtractStateExtensions<E>, A>
+> = ExtractStoreExtensions<E> &
+  Store<S, A, UnknownIfNonSpecific<ExtractStateExtensions<E>>>
 
 /**
  * A friendly abstraction over the standard Redux `createStore()` function.

packages/toolkit/src/tests/configureStore.typetest.ts

@@ -13,7 +13,7 @@ import type { PayloadAction, ConfigureStoreOptions } from '@reduxjs/toolkit'
 import { configureStore, createSlice, Tuple } from '@reduxjs/toolkit'
 import type { ThunkMiddleware, ThunkAction, ThunkDispatch } from 'redux-thunk'
 import { thunk } from 'redux-thunk'
-import { expectNotAny, expectType } from './helpers'
+import { expectExactType, expectNotAny, expectType } from './helpers'
 
 const _anyMiddleware: any = () => () => () => {}
 
@@ -138,6 +138,17 @@ const _anyMiddleware: any = () => () => () => {}
   })
 }
 
+/**
+ * Test: nullable state is preserved
+ */
+
+{
+  const store = configureStore({
+    reducer: (): string | null => null,
+  })
+  expectExactType<string | null>(null)(store.getState())
+}
+
 /*
  * Test: configureStore() accepts store Tuple, but not plain array
  */

packages/toolkit/src/tsHelpers.ts

@@ -205,3 +205,5 @@ export type Id<T> = { [K in keyof T]: T[K] } & {}
 export type Tail<T extends any[]> = T extends [any, ...infer Tail]
   ? Tail
   : never
+
+export type UnknownIfNonSpecific<T> = {} extends T ? unknown : T