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';
+
+// UNDEF = undefined;
+const UNDEF = void 0;
 
 export default function composeStores(stores) {
   const finalStores = pick(stores, (val) => typeof val === 'function');
   return function Composition(atom = {}, action) {
-    return mapValues(finalStores, (store, key) =>
-      store(atom[key], action)
-    );
+    return mapValues(finalStores, (store, key) => {
+      const state = store(atom[key], action);
+      if (state === UNDEF) {
+        throw new Error(`Store ${key} returns undefined. By default store should return original state.`);
+      }
+      return state;
+    });
   };
 }

test/composeStores.spec.js

@@ -1,6 +1,9 @@
 import expect from 'expect';
 import { composeStores } from '../src';
 
+// UNDEF = undefined;
+const UNDEF = void 0;
+
 describe('Utils', () => {
   describe('composeStores', () => {
     it('should return a store that maps state keys to reducer functions', () => {
@@ -27,5 +30,29 @@ describe('Utils', () => {
 
       expect(Object.keys(store({}, {type: 'push'}))).toEqual(['stack']);
     });
+    it('should throw an error if undefined return from store', () => {
+      const store = composeStores({
+        stack: (state = []) => state,
+        bad: (state = [], action) => {
+          if (action.type === 'something') {
+            return state;
+          }
+        }
+      });
+      expect(() => store({}, {type: '@@testType'})).toThrow();
+    });
+    it('should throw an error if undefined return not by default', () => {
+      const store = composeStores({
+        stack: (state = []) => state,
+        bad: (state = 1, action) => {
+          if (action.type === 'something') {
+            return UNDEF;
+          }
+          return state;
+        }
+      });
+      expect(store({}, {type: '@@testType'})).toEqual({stack: [], bad: 1});
+      expect(() => store({}, {type: 'something'})).toThrow();
+    });
   });
 });