support initialState

Author: timche

src/index.js

@@ -1,5 +1,19 @@
-export default (...reducers) => (prevState, value, ...args) =>
-  reducers.reduce(
-    (newState, reducer) => reducer(newState, value, ...args),
-    prevState
-  );
+export default (...args) => {
+  const initialState =
+    typeof args[args.length - 1] !== 'function' && args.pop();
+  const reducers = args;
+
+  if (typeof initialState === 'undefined') {
+    throw new TypeError(
+      'The initial state may not be undefined. If you do not want to set a value for this reducer, you can use null instead of undefined.'
+    );
+  }
+
+  return (prevState, value, ...args) =>
+    typeof prevState === 'undefined'
+      ? initialState
+      : reducers.reduce(
+          (newState, reducer) => reducer(newState, value, ...args),
+          prevState
+        );
+};

test/index.test.js

@@ -1,5 +1,24 @@
 import reduceReducers from '../src';
 
+test('returns initialState', () => {
+  const initialState = { A: 0, B: 0 };
+  const reducer = reduceReducers(
+    (state, payload) => ({
+      ...state,
+      A: state.A + payload
+    }),
+    initialState
+  );
+
+  expect(reducer()).toEqual(initialState);
+});
+
+test('throws an error if initialState is undefined', () => {
+  expect(() => {
+    reduceReducers(undefined);
+  }).toThrow();
+});
+
 test('combines multiple reducers into a single reducer', () => {
   const reducer = reduceReducers(
     (state, payload) => ({ ...state, A: state.A + payload }),