Have combineReducers return same object if nothing changes

Fixes #853

Author: aaronjensen

src/utils/combineReducers.js

@@ -113,13 +113,16 @@ export default function combineReducers(reducers) {
       throw sanityError;
     }
 
+    var hasChanged = false;
     var finalState = mapValues(finalReducers, (reducer, key) => {
-      var newState = reducer(state[key], action);
-      if (typeof newState === 'undefined') {
+      var previousStateForKey = state[key];
+      var nextStateForKey = reducer(previousStateForKey, action);
+      if (typeof nextStateForKey === 'undefined') {
         var errorMessage = getUndefinedStateErrorMessage(key, action);
         throw new Error(errorMessage);
       }
-      return newState;
+      hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
+      return nextStateForKey;
     });
 
     if (process.env.NODE_ENV !== 'production') {
@@ -129,6 +132,6 @@ export default function combineReducers(reducers) {
       }
     }
 
-    return finalState;
+    return hasChanged ? finalState : state;
   };
 }

test/utils/combineReducers.spec.js

@@ -112,6 +112,45 @@ describe('Utils', () => {
       expect(reducer({counter: 0}, { type: increment }).counter).toEqual(1);
     });
 
+    it('should maintain referential equality if the reducers it is combining do', () => {
+      const reducer = combineReducers({
+        child1(state = {}) {
+          return state;
+        },
+        child2(state = {}) {
+          return state;
+        },
+        child3(state = {}) {
+          return state;
+        }
+      });
+
+      const initialState = reducer(undefined, '@@INIT');
+      expect(reducer(initialState, { type: 'FOO' })).toBe(initialState);
+    });
+
+    it('should not have referential equality if one of the reducers changes something', () => {
+      const reducer = combineReducers({
+        child1(state = {}) {
+          return state;
+        },
+        child2(state = { count: 0 }, action) {
+          switch (action.type) {
+          case 'increment':
+            return { count: state.count + 1 };
+          default:
+            return state;
+          }
+        },
+        child3(state = {}) {
+          return state;
+        }
+      });
+
+      const initialState = reducer(undefined, '@@INIT');
+      expect(reducer(initialState, { type: 'increment' })).toNotBe(initialState);
+    });
+
     it('should throw an error on first call if a reducer attempts to handle a private action', () => {
       const reducer = combineReducers({
         counter(state, action) {