Add check that reducer returns current state for unknown actions

src/createStore.js

@@ -7,7 +7,7 @@ import isPlainObject from './utils/isPlainObject'
  * Do not reference these action types directly in your code.
  */
 export var ActionTypes = {
-  INIT: '@@redux/INIT'
+  INIT: '@@redux/INIT_' + Math.random().toString(36).substring(7).split('').join('.')
 }
 
 /**

src/utils/combineReducers.js

@@ -65,13 +65,11 @@ function assertReducerSanity(reducers) {
       )
     }
 
-    var type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
-    if (typeof reducer(undefined, { type }) === 'undefined') {
+    var randomState = Math.random().toString(36).substring(7).split('').join('.')
+    if (reducer(randomState, { type: ActionTypes.INIT }) !== randomState) {
       throw new Error(
-        `Reducer "${key}" returned undefined when probed with a random type. ` +
-        `Don't try to handle ${ActionTypes.INIT} or other actions in "redux/*" ` +
-        `namespace. They are considered private. Instead, you must return the ` +
-        `current state for any unknown actions, unless it is undefined, ` +
+        `Reducer "${key}" did not return the current state when probed with a random type. ` +
+        `You must return the current state for any unknown actions, unless it is undefined, ` +
         `in which case you must return the initial state, regardless of the ` +
         `action type. The initial state may not be undefined.`
       )

test/utils/combineReducers.spec.js

@@ -1,6 +1,6 @@
 import expect from 'expect'
 import { combineReducers } from '../../src'
-import createStore, { ActionTypes } from '../../src/createStore'
+import createStore from '../../src/createStore'
 
 describe('Utils', () => {
   describe('combineReducers', () => {
@@ -151,24 +151,22 @@ describe('Utils', () => {
       expect(reducer(initialState, { type: 'increment' })).toNotBe(initialState)
     })
 
-    it('throws an error on first call if a reducer attempts to handle a private action', () => {
+    it('throws an error if reducer does not return current state for all unknown action types', () => {
       const reducer = combineReducers({
         counter(state, action) {
           switch (action.type) {
             case 'increment':
               return state + 1
             case 'decrement':
               return state - 1
-            // Never do this in your code:
-            case ActionTypes.INIT:
-              return 0
             default:
-              return undefined
+              return 0
           }
         }
       })
+
       expect(() => reducer()).toThrow(
-        /"counter".*private/
+        /"counter".*probed/
       )
     })