Recursively Prune State on store.replaceReducer

src/createStore.js

@@ -66,6 +66,27 @@ export default function createStore(reducer, preloadedState, enhancer) {
     }
   }
 
+  function pruneState(state, currentShape, nextShape) {
+    if (![state, currentShape, nextShape].every(isPlainObject)) {
+      return state
+    }
+    let hasChanged = false
+    const stateKeys = Object.keys(state)
+    const nextState = {...state}
+    for (let i = 0; i < stateKeys.length; i++) {
+      const key = stateKeys[i]
+      if (key in currentShape) {
+        if (key in nextShape) {
+          nextState[key] = pruneState(state[key], currentShape[key], nextShape[key])
+        } else {
+          delete nextState[key]
+        }
+      }
+      hasChanged = hasChanged || nextState[key] !== state[key]
+    }
+    return hasChanged ? nextState : state
+  }
+
   /**
    * Reads the state tree managed by the store.
    *
@@ -196,6 +217,12 @@ export default function createStore(reducer, preloadedState, enhancer) {
       throw new Error('Expected the nextReducer to be a function.')
     }
 
+    if (process.env.NODE_ENV !== 'production') {
+      const currentStateShape = currentReducer(undefined, { type: ActionTypes.INIT })
+      const nextStateShape = nextReducer(undefined, { type: ActionTypes.INIT })
+      currentState = pruneState(currentState, currentStateShape, nextStateShape)
+    }
+
     currentReducer = nextReducer
     dispatch({ type: ActionTypes.INIT })
   }

test/createStore.spec.js

@@ -734,4 +734,30 @@ describe('createStore', () => {
       expect(results).toEqual([ { foo: 0, bar: 0, fromRx: true }, { foo: 1, bar: 0, fromRx: true } ])
     })
   })
+
+  it('does not log an error if parts of the current state will be ignored by a nextReducer using combineReducers', () => {
+    const originalConsoleError = console.error
+    console.error = jest.fn()
+
+    const store = createStore(
+      combineReducers({
+        x: (s=0, a) => s,
+        y: combineReducers({
+          z: (s=0, a) => s,
+          w: (s=0, a) => s,
+        }),
+      })
+    )
+
+    store.replaceReducer(
+      combineReducers({
+        y: combineReducers({
+          z: (s=0, a) => s,
+        }),
+      })
+    )
+
+    expect(console.error.mock.calls.length).toBe(0)
+    console.error = originalConsoleError
+  })
 })