Warns when type of reducers passed is invalid to avoid silent failure

src/combineReducers.js

@@ -108,6 +108,10 @@ export default function combineReducers(reducers) {
     if (process.env.NODE_ENV !== 'production') {
       if (typeof reducers[key] === 'undefined') {
         warning(`No reducer provided for key "${key}"`)
+      } else if (key === '__esModule') {
+        warning(`Passing a whole ES Module to combine reducers is discouraged.`)
+      } else if (typeof reducers[key] !== 'function') {
+        warning(`Reducer provided for "${key}" is not a function. Received type: ${typeof key}.`)
       }
     }
 

test/combineReducers.spec.js

@@ -1,6 +1,7 @@
 /* eslint-disable no-console */
 import { combineReducers } from '../src'
 import createStore, { ActionTypes } from '../src/createStore'
+import * as reducers from './helpers/reducers'
 
 describe('Utils', () => {
   describe('combineReducers', () => {
@@ -18,17 +19,39 @@ describe('Utils', () => {
       expect(s2).toEqual({ counter: 1, stack: [ 'a' ] })
     })
 
-    it('ignores all props which are not a function', () => {
+    it('warns on props which are not a function and excludes them', () => {
+      const preSpy = console.error
+      const spy = jest.fn()
+      console.error = spy
+
       const reducer = combineReducers({
         fake: true,
         broken: 'string',
         another: { nested: 'object' },
         stack: (state = []) => state
       })
 
+      expect(spy).toHaveBeenLastCalledWith('Reducer provided for "another" is not a function. Received type: string.')
+      expect(spy).toHaveBeenCalledTimes(3)
+
       expect(
         Object.keys(reducer({ }, { type: 'push' }))
       ).toEqual([ 'stack' ])
+
+      spy.mockClear()
+      console.error = preSpy
+    })
+
+    it('warns if a module imported with * syntax is passed', () => {
+      const preSpy = console.error
+      const spy = jest.fn()
+      console.error = spy
+
+      combineReducers(reducers)
+      expect(spy).toHaveBeenCalledWith(`Passing a whole ES Module to combine reducers is discouraged.`)
+
+      spy.mockClear()
+      console.error = preSpy
     })
 
     it('warns if a reducer prop is undefined', () => {
@@ -38,15 +61,11 @@ describe('Utils', () => {
 
       let isNotDefined
       combineReducers({ isNotDefined })
-      expect(spy.mock.calls[0][0]).toMatch(
-        /No reducer provided for key "isNotDefined"/
-      )
+      expect(spy).toHaveBeenCalledWith('No reducer provided for key "isNotDefined"')
 
       spy.mockClear()
       combineReducers({ thing: undefined })
-      expect(spy.mock.calls[0][0]).toMatch(
-        /No reducer provided for key "thing"/
-      )
+      expect(spy).toHaveBeenCalledWith('No reducer provided for key "thing"')
 
       spy.mockClear()
       console.error = preSpy

test/createStore.spec.js

@@ -6,7 +6,7 @@ import $$observable from 'symbol-observable'
 
 describe('createStore', () => {
   it('exposes the public API', () => {
-    const store = createStore(combineReducers(reducers))
+    const store = createStore(reducers.todos)
     const methods = Object.keys(store)
 
     expect(methods.length).toBe(4)