Infer action types from combineReducers (#3411)

* Infer action types from combineReducers

This change allows for `combineReducers` to completely infer both the state and action types for its returned reducer.

From experience with large TypeScript projects, it's common to see that the action type is not explicitly specified, which results in `AnyAction` in the resulting reducer type. Unfortunately, this will propagate through the type inference for `createStore` resulting in `dispatch` being very weakly typed. This change alone causes a chain reaction of a more correctly (and strongly) typed project with regards to Redux.

* Fix formatting issues.

index.d.ts

@@ -90,12 +90,13 @@ export type ReducersMapObject<S = any, A extends Action = Action> = {
  * @returns A reducer function that invokes every reducer inside the passed
  *   object, and builds a state object with the same shape.
  */
-export function combineReducers<S>(
-  reducers: ReducersMapObject<S, any>
-): Reducer<S>
-export function combineReducers<S, A extends Action = AnyAction>(
-  reducers: ReducersMapObject<S, A>
-): Reducer<S, A>
+export function combineReducers<T extends ReducersMapObject<any, any>>(
+  reducers: T
+): Reducer<InferStateType<T>, InferActionTypes<InferReducerTypes<T>>>
+
+type InferActionTypes<R> = R extends Reducer<any, infer A> ? A : AnyAction
+type InferReducerTypes<T> = T extends Record<any, infer R> ? R : Reducer
+type InferStateType<T> = T extends ReducersMapObject<infer S, any> ? S : never
 
 /* store */
 

test/typescript/reducers.ts

@@ -42,7 +42,7 @@ function simple() {
   // Combined reducer also accepts any action.
   const combined = combineReducers({ sub: reducer })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
+  let cs = combined(undefined, { type: 'init' })
   cs = combined(cs, { type: 'INCREMENT', count: 10 })
 
   // Combined reducer's state is strictly checked.
@@ -110,17 +110,18 @@ function discriminated() {
   // typings:expect-error
   s = reducer(s, { type: 'SOME_OTHER_TYPE', someField: 'value' })
 
-  // Combined reducer accepts any action by default which allows to include
-  // third-party reducers without the need to add their actions to the union.
-  const combined = combineReducers({ sub: reducer })
+  // Combined reducer accepts a union actions types accepted each reducer,
+  // which can be very permissive for unknown third-party reducers.
+  const combined = combineReducers({
+    sub: reducer,
+    unknown: (state => state) as Reducer
+  })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
-  cs = combined(cs, { type: 'SOME_OTHER_TYPE' })
+  let cs = combined(undefined, { type: 'init' })
+  cs = combined(cs, { type: 'SOME_OTHER_TYPE', someField: 'value' })
 
   // Combined reducer can be made to only accept known actions.
-  const strictCombined = combineReducers<{ sub: State }, MyAction>({
-    sub: reducer
-  })
+  const strictCombined = combineReducers({ sub: reducer })
 
   strictCombined(cs, { type: 'INCREMENT' })
   // typings:expect-error
@@ -179,7 +180,7 @@ function typeGuards() {
 
   const combined = combineReducers({ sub: reducer })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
+  let cs = combined(undefined, { type: 'init' })
   cs = combined(cs, { type: 'INCREMENT', count: 10 })
 }