fix(Store): Handling of @ngrx/store/update-reducers Action on Startup leads to strange behaviour for initialStates

modules/store/spec/store.spec.ts

@@ -296,7 +296,7 @@ describe('ngRx Store', () => {
       store.removeReducer(key);
       store.dispatch({ type: INCREMENT });
       store.take(1).subscribe(val => {
-        expect(val.counter4).toBeUndefined();
+        expect(val.counter4).toBe(0);
       });
     });
   });

modules/store/spec/utils.spec.ts

@@ -37,6 +37,18 @@ describe(`Store utils`, () => {
     it(`should return original state if nothing changed`, () => {
       expect(combination(initialState, { type: '' })).toBe(initialState);
     });
+
+    it(`should return full initial state of the root when action '@ngrx/store/update-reducers' is running with no initial state`, () => {
+      const initialStateForReducer2Only = { reducer2: { y: 'bar' } };
+      const combinationForReducer1 = combineReducers(
+        { reducer1 },
+        initialStateForReducer2Only as any
+      );
+      const updateAction1 = { type: 'state1', payload: 'foo' };
+      expect(combinationForReducer1(undefined, updateAction1)).toEqual(
+        initialState
+      );
+    });
   });
 
   describe(`omit()`, () => {

modules/store/src/reducer_manager.ts

@@ -72,7 +72,10 @@ export class ReducerManager extends BehaviorSubject<ActionReducer<any, any>>
 
   private updateReducers(key: string) {
     this.next(this.reducerFactory(this.reducers, this.initialState));
-    this.dispatcher.next(<Action & {feature: string}>{ type: UPDATE, feature: key });
+    this.dispatcher.next(<Action & { feature: string }>{
+      type: UPDATE,
+      feature: key,
+    });
   }
 
   ngOnDestroy() {

modules/store/src/utils.ts

@@ -39,7 +39,7 @@ export function combineReducers(
       nextState[key] = nextStateForKey;
       hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
     }
-    return hasChanged ? nextState : state;
+    return hasChanged ? { ...state, ...nextState } : state;
   };
 }