Merge pull request #4638 from exuanbo/patch-1

fix(types/store): Unexpectedly narrowed return type of function `Store['getState']`

Author: EskiMojo14

src/createStore.ts

@@ -5,7 +5,8 @@ import {
   StoreEnhancer,
   Dispatch,
   Observer,
-  ListenerCallback
+  ListenerCallback,
+  UnknownIfNonSpecific
 } from './types/store'
 import { Action } from './types/actions'
 import { Reducer } from './types/reducers'
@@ -46,7 +47,7 @@ export function createStore<
 >(
   reducer: Reducer<S, A>,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
 /**
  * @deprecated
  *
@@ -82,7 +83,7 @@ export function createStore<
   reducer: Reducer<S, A, PreloadedState>,
   preloadedState?: PreloadedState | undefined,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
 export function createStore<
   S,
   A extends Action,
@@ -93,7 +94,7 @@ export function createStore<
   reducer: Reducer<S, A, PreloadedState>,
   preloadedState?: PreloadedState | StoreEnhancer<Ext, StateExt> | undefined,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext {
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext {
   if (typeof reducer !== 'function') {
     throw new Error(
       `Expected the root reducer to be a function. Instead, received: '${kindOf(
@@ -432,7 +433,7 @@ export function legacy_createStore<
 >(
   reducer: Reducer<S, A>,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
 /**
  * Creates a Redux store that holds the state tree.
  *
@@ -473,7 +474,7 @@ export function legacy_createStore<
   reducer: Reducer<S, A, PreloadedState>,
   preloadedState?: PreloadedState | undefined,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
 export function legacy_createStore<
   S,
   A extends Action,
@@ -484,6 +485,6 @@ export function legacy_createStore<
   reducer: Reducer<S, A>,
   preloadedState?: PreloadedState | StoreEnhancer<Ext, StateExt> | undefined,
   enhancer?: StoreEnhancer<Ext, StateExt>
-): Store<S, A, StateExt> & Ext {
+): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext {
   return createStore(reducer, preloadedState as any, enhancer)
 }

src/types/store.ts

@@ -81,7 +81,7 @@ export type Observer<T> = {
 export interface Store<
   S = any,
   A extends Action = UnknownAction,
-  StateExt extends {} = {}
+  StateExt extends unknown = unknown
 > {
   /**
    * Dispatches an action. It is the only way to trigger a state change.
@@ -164,6 +164,8 @@ export interface Store<
   [Symbol.observable](): Observable<S & StateExt>
 }
 
+export type UnknownIfNonSpecific<T> = {} extends T ? unknown : T
+
 /**
  * A store creator is a function that creates a Redux store. Like with
  * dispatching function, we must distinguish the base store creator,
@@ -180,7 +182,7 @@ export interface StoreCreator {
   <S, A extends Action, Ext extends {} = {}, StateExt extends {} = {}>(
     reducer: Reducer<S, A>,
     enhancer?: StoreEnhancer<Ext, StateExt>
-  ): Store<S, A, StateExt> & Ext
+  ): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
   <
     S,
     A extends Action,
@@ -191,7 +193,7 @@ export interface StoreCreator {
     reducer: Reducer<S, A, PreloadedState>,
     preloadedState?: PreloadedState | undefined,
     enhancer?: StoreEnhancer<Ext>
-  ): Store<S, A, StateExt> & Ext
+  ): Store<S, A, UnknownIfNonSpecific<StateExt>> & Ext
 }
 
 /**

test/typescript/store.ts

@@ -61,6 +61,11 @@ const funcWithStore = (store: Store<State, DerivedAction>) => {}
 
 const store: Store<State> = createStore(reducer)
 
+// test that nullable state is preserved
+const nullableStore = createStore((): string | null => null)
+
+expectTypeOf(nullableStore.getState()).toEqualTypeOf<string | null>()
+
 // ensure that an array-based state works
 const arrayReducer = (state: any[] = []) => state || []
 const storeWithArrayState: Store<any[]> = createStore(arrayReducer)