Checking that the return value of every store function inside compose…

package.json

@@ -50,17 +50,20 @@
     "react": "^0.13.0",
     "react-hot-loader": "^1.2.7",
     "rimraf": "^2.3.4",
+    "sinon": "^1.15.3",
     "webpack": "^1.9.6",
     "webpack-dev-server": "^1.8.2"
   },
   "dependencies": {
     "invariant": "^2.0.0"
   },
   "npmName": "redux",
-  "npmFileMap": [{
-    "basePath": "/dist/",
-    "files": [
-      "*.js"
-    ]
-  }]
+  "npmFileMap": [
+    {
+      "basePath": "/dist/",
+      "files": [
+        "*.js"
+      ]
+    }
+  ]
 }

src/utils/composeStores.js

@@ -3,6 +3,18 @@ import pick from '../utils/pick';
 
 export default function composeStores(stores) {
   const finalStores = pick(stores, (val) => typeof val === 'function');
+
+  const dummyAction = {};
+  const dummyStore = {};
+
+  if (process.env.NODE_ENV !== 'production') {
+    Object.keys(finalStores).forEach(key => {
+      if (finalStores[key](dummyStore, dummyAction) !== dummyStore) {
+        console.warn(`Your ${key} Store must return the state given to it for any unknown actions.`);
+      }
+    });
+  }
+
   return function Composition(atom = {}, action) {
     return mapValues(finalStores, (store, key) =>
       store(atom[key], action)

test/composeStores.spec.js

@@ -1,7 +1,19 @@
 import expect from 'expect';
 import { composeStores } from '../src';
+import sinon from 'sinon';
 
 describe('Utils', () => {
+
+  var sandbox;
+  beforeEach(function() {
+    sandbox = sinon.sandbox.create();
+    sandbox.stub(console, 'warn');
+  });
+
+  afterEach(function() {
+    sandbox.restore();
+  });
+
   describe('composeStores', () => {
     it('should return a store that maps state keys to reducer functions', () => {
       const store = composeStores({
@@ -27,5 +39,42 @@ describe('Utils', () => {
 
       expect(Object.keys(store({}, {type: 'push'}))).toEqual(['stack']);
     });
+
+    it('should check that return value of every reducer is the state given to it for any unknown actions #1', () => {
+
+      const store = composeStores({
+        counter: (state = 0, action) =>
+          action.type === 'increment' ? state + 1 : state,
+        stack: (state = [], action) =>
+          action.type === 'push' ? [...state, action.value] : state,
+        some: (state = initialState, action) => {}
+      });
+
+      sinon.assert.calledOnce(console.warn);
+
+    });
+
+    it('should check that return value of every reducer is the state given to it for any unknown actions #2', () => {
+
+      const store = composeStores({
+        someOther: (state = initialState, action) => 'somevalue'
+      });
+      sinon.assert.calledOnce(console.warn);
+
+    });
+
+    it('should check that return value of every reducer is the state given to it for any unknown actions #3', () => {
+
+      const store = composeStores({
+        counter: (state = 0, action) =>
+          action.type === 'increment' ? state + 1 : state,
+        stack: (state = [], action) =>
+          action.type === 'push' ? [...state, action.value] : state,
+      });
+      sinon.assert.notCalled(console.warn);
+
+    });
+
+
   });
 });