Add a warning for undefined properties passed to combineReducers (#1789)

* Add warning to combineReducers for undefined properties

* Test that it warns for undefined reducer props

* Duh remove only

* add some newlines

src/combineReducers.js

@@ -102,6 +102,13 @@ export default function combineReducers(reducers) {
   var finalReducers = {}
   for (var i = 0; i < reducerKeys.length; i++) {
     var key = reducerKeys[i]
+
+    if (process.env.NODE_ENV !== 'production') {
+      if (typeof reducers[key] === 'undefined') {
+        warning(`No reducer provided for key "${key}"`)
+      }
+    }
+
     if (typeof reducers[key] === 'function') {
       finalReducers[key] = reducers[key]
     }

test/combineReducers.spec.js

@@ -31,6 +31,24 @@ describe('Utils', () => {
       ).toEqual([ 'stack' ])
     })
 
+    it('warns if a reducer prop is undefined', () => {
+      const spy = expect.spyOn(console, 'error')
+
+      let isNotDefined
+      combineReducers({ isNotDefined })
+      expect(spy.calls[0].arguments[0]).toMatch(
+        /No reducer provided for key "isNotDefined"/
+      )
+
+      spy.reset()
+      combineReducers({ thing: undefined })
+      expect(spy.calls[0].arguments[0]).toMatch(
+        /No reducer provided for key "thing"/
+      )
+
+      spy.restore()
+    })
+
     it('throws an error if a reducer returns undefined handling an action', () => {
       const reducer = combineReducers({
         counter(state = 0, action) {