Throw error on undefined value from store function

src/utils/composeStores.js

@@ -1,11 +1,18 @@
-import mapValues from '../utils/mapValues';
-import pick from '../utils/pick';
+import mapValues from './mapValues';
+import pick from './pick';
 
 export default function composeStores(stores) {
   const finalStores = pick(stores, (val) => typeof val === 'function');
+
+  if (process.env.NODE_ENV !== 'production') {
+    Object.keys(finalStores).forEach(function(key) {
+      if (finalStores[key](undefined, '@@TEST') === undefined) {
+        throw new Error(`Store ${key} returns undefined. By default store should return original state.`);
+      }
+    });
+  }
+
   return function Composition(atom = {}, action) {
-    return mapValues(finalStores, (store, key) =>
-      store(atom[key], action)
-    );
+    return mapValues(finalStores, (store, key) => store(atom[key], action));
   };
 }

test/composeStores.spec.js

@@ -27,5 +27,15 @@ describe('Utils', () => {
 
       expect(Object.keys(store({}, {type: 'push'}))).toEqual(['stack']);
     });
+    it('should throw an error if undefined return from store', () => {
+      expect(() => composeStores({
+        stack: (state = []) => state,
+        bad: (state= [], action) => {
+          if (action === 'something') {
+            return state;
+          }
+        }
+      })).toThrow();
+    });
   });
 });