fix replaceReducer type (reduxjs#3507)

* fix replaceReducer type

* remove extraneous type

* Update the JSDoc so IDEs don't complain.

* new test for replaceReducers


Co-authored-by: Tim Dorr <[email protected]>
Former-commit-id: 65ca503

Author: cellog

index.d.ts.REMOVED.git-id

@@ -1 +1 @@
-50e8d018faee8887ed82337fcd1993ad07df0968
+2fc2af4c9eb4ea9e5952cc465881eed93434efcd

src/createStore.js.REMOVED.git-id

@@ -1 +1 @@
-cef9a2ecaeff319d4986af24babf1c79ec9912c6
+57c57d7636d7bc54dd85411f400e868fdd912ed6

test/replaceReducers.spec.ts

@@ -0,0 +1,18 @@
+import { createStore, combineReducers } from '..'
+
+describe('replaceReducers test', () => {
+  it('returns the original store', () => {
+    const nextReducer = combineReducers({
+      foo: (state = 1, _action) => state,
+      bar: (state = 2, _action) => state
+    })
+    const store = createStore((state, action) => {
+      if (state === undefined) return 5
+      return action
+    })
+
+    const nextStore = store.replaceReducer(nextReducer)
+
+    expect(nextStore).toBe(store)
+  })
+})