pass initial state when state is undefined and value defined

src/index.js

@@ -9,11 +9,19 @@ export default (...args) => {
     );
   }
 
-  return (prevState, value, ...args) =>
-    typeof (prevState === 'undefined') && initialState
-      ? initialState
-      : reducers.reduce(
-          (newState, reducer) => reducer(newState, value, ...args),
-          prevState
-        );
+  return (prevState, value, ...args) => {
+    const prevStateIsUndefined = typeof (prevState === 'undefined');
+    const valueIsUndefined = typeof value === 'undefined';
+
+    if (prevStateIsUndefined && valueIsUndefined && initialState) {
+      return initialState;
+    }
+
+    return reducers.reduce(
+      (newState, reducer) => reducer(newState, value, ...args),
+      prevStateIsUndefined && !valueIsUndefined && initialState
+        ? initialState
+        : prevState
+    );
+  };
 };

test/index.test.js

@@ -13,6 +13,19 @@ test('returns initialState', () => {
   expect(reducer()).toEqual(initialState);
 });
 
+test('passes `initialState` when state is `undefined` and value is defined', () => {
+  const initialState = { A: 0, B: 0 };
+  const reducer = reduceReducers(
+    (state, payload) => ({
+      ...state,
+      A: state.A + payload
+    }),
+    initialState
+  );
+
+  expect(reducer(undefined, 1)).toEqual({ A: 1, B: 0 });
+});
+
 test('throws an error if initialState is undefined', () => {
   expect(() => {
     reduceReducers(undefined);