Throw error on undefined value from reducer function

src/utils/composeReducers.js

@@ -1,12 +1,18 @@
 import mapValues from '../utils/mapValues';
 import pick from '../utils/pick';
+import invariant from 'invariant';
 
 export default function composeReducers(reducers) {
   const finalReducers = pick(reducers, (val) => typeof val === 'function');
 
   return function composition(state = {}, action) {
-    return mapValues(finalReducers, (store, key) =>
-      store(state[key], action)
-    );
+    return mapValues(finalReducers, (reducer, key) => {
+      const newState = reducer(state[key], action);
+      invariant(
+          typeof newState !== 'undefined',
+          `Reducer ${key} returns undefined. By default reducer should return original state.`
+      );
+      return newState;
+    });
   };
 }

test/composeReducers.spec.js

@@ -29,5 +29,30 @@ describe('Utils', () => {
         Object.keys(reducer({}, { type: 'push' }))
       ).toEqual(['stack']);
     });
+
+    it('should throw an error if undefined return from reducer', () => {
+      const reducer = composeReducers({
+        stack: (state = []) => state,
+        bad: (state = [], action) => {
+          if (action.type === 'something') {
+            return state;
+          }
+        }
+      });
+      expect(() => reducer({}, {type: '@@testType'})).toThrow();
+    });
+
+    it('should throw an error if undefined return not by default', () => {
+      const reducer = composeReducers({
+        stack: (state = []) => state,
+        bad: (state = 1, action) => {
+          if (action.type !== 'something') {
+            return state;
+          }
+        }
+      });
+      expect(reducer({}, {type: '@@testType'})).toEqual({stack: [], bad: 1});
+      expect(() => reducer({}, {type: 'something'})).toThrow();
+    });
   });
 });