From 001a1979372dbd9cf431805f439a179eb05e20be Mon Sep 17 00:00:00 2001
From: gupta <guptanub@amazon.com>
Date: Wed, 31 Jul 2019 15:40:02 +0530
Subject: [PATCH] Fixed combineReducers changeDetection logic(#3488)

---
 src/combineReducers.js       |  2 ++
 test/combineReducers.spec.js | 51 ++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/src/combineReducers.js b/src/combineReducers.js
index 05c9d56513..bdbcfb2291 100644
--- a/src/combineReducers.js
+++ b/src/combineReducers.js
@@ -173,6 +173,8 @@ export default function combineReducers(reducers) {
       nextState[key] = nextStateForKey
       hasChanged = hasChanged || nextStateForKey !== previousStateForKey
     }
+    hasChanged =
+      hasChanged || finalReducerKeys.length !== Object.keys(state).length
     return hasChanged ? nextState : state
   }
 }
diff --git a/test/combineReducers.spec.js b/test/combineReducers.spec.js
index 321010a598..f00aff9976 100644
--- a/test/combineReducers.spec.js
+++ b/test/combineReducers.spec.js
@@ -281,5 +281,56 @@ describe('Utils', () => {
       spy.mockClear()
       console.error = preSpy
     })
+
+    describe('With Replace Reducers', function() {
+      const foo = (state = {}) => state
+      const bar = (state = {}) => state
+      const ACTION = { type: 'ACTION' }
+
+      it('should return an updated state when additional reducers are passed to combineReducers', function() {
+        const originalCompositeReducer = combineReducers({ foo })
+        const store = createStore(originalCompositeReducer)
+        store.dispatch(ACTION)
+        const initialState = store.getState()
+        store.replaceReducer(combineReducers({ foo, bar }))
+        store.dispatch(ACTION)
+        const nextState = store.getState()
+        expect(nextState).not.toBe(initialState)
+      })
+
+      it('should return an updated state when reducers passed to combineReducers are changed', function() {
+        const baz = (state = {}) => state
+
+        const originalCompositeReducer = combineReducers({ foo, bar })
+        const store = createStore(originalCompositeReducer)
+        store.dispatch(ACTION)
+        const initialState = store.getState()
+        store.replaceReducer(combineReducers({ baz, bar }))
+        store.dispatch(ACTION)
+        const nextState = store.getState()
+        expect(nextState).not.toBe(initialState)
+      })
+
+      it('should return the same state when reducers passed to combineReducers not changed', function() {
+        const originalCompositeReducer = combineReducers({ foo, bar })
+        const store = createStore(originalCompositeReducer)
+        store.dispatch(ACTION)
+        const initialState = store.getState()
+        store.replaceReducer(combineReducers({ foo, bar }))
+        store.dispatch(ACTION)
+        const nextState = store.getState()
+        expect(nextState).toBe(initialState)
+      })
+
+      it('should return an updated state when one of more reducers passed to the combineReducers are removed', function() {
+        const originalCompositeReducer = combineReducers({ foo, bar })
+        const store = createStore(originalCompositeReducer)
+        store.dispatch(ACTION)
+        const initialState = store.getState()
+        store.replaceReducer(combineReducers({ bar }))
+        const nextState = store.getState()
+        expect(nextState).not.toBe(initialState)
+      })
+    })
   })
 })