Warns when type of reducers passed is invalid (not a function) to avoid silent failure

Author: jekku

src/combineReducers.js

@@ -106,8 +106,11 @@ export default function combineReducers(reducers) {
     const key = reducerKeys[i]
 
     if (process.env.NODE_ENV !== 'production') {
-      if (typeof reducers[key] === 'undefined') {
+      const reducerUnderInspection = reducers[key]
+      if (typeof reducerUnderInspection === 'undefined') {
         warning(`No reducer provided for key "${key}"`)
+      } else if (typeof reducerUnderInspection !== 'function') {
+        warning(`Reducer provided for "${key}" is not a function. Received a ${typeof key}.`)
       }
     }
 

test/combineReducers.spec.js

@@ -18,14 +18,30 @@ 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.mock.calls[0][0]).toMatch(
+        /Reducer provided for "fake" is not a function/
+      )
+
+      expect(spy.mock.calls[1][0]).toMatch(
+          /Reducer provided for "broken" is not a function/
+      )
+
+      expect(spy.mock.calls[2][0]).toMatch(
+          /Reducer provided for "another" is not a function/
+      )
+
       expect(
         Object.keys(reducer({ }, { type: 'push' }))
       ).toEqual([ 'stack' ])

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)