Do not reinvent the wheel. Outsource utility functions.

package.json

@@ -62,7 +62,10 @@
   },
   "dependencies": {
     "invariant": "^2.0.0",
-    "warning": "^2.0.0"
+    "warning": "^2.0.0",
+    "lodash.isplainobject": "^3.2.0",
+    "lodash.mapvalues": "^3.0.1",
+    "lodash.pick": "^3.1.0"
   },
   "npmName": "redux",
   "npmFileMap": [

src/createStore.js

@@ -1,5 +1,5 @@
 import invariant from 'invariant';
-import isPlainObject from './utils/isPlainObject';
+import isPlainObject from 'lodash.isplainobject';
 
 /**
  * These are private action types reserved by Redux.

src/utils/bindActionCreators.js

@@ -1,5 +1,5 @@
 import invariant from 'invariant';
-import mapValues from '../utils/mapValues';
+import mapValues from 'lodash.mapvalues';
 
 function bindActionCreator(actionCreator, dispatch) {
   return (...args) => dispatch(actionCreator(...args));

src/utils/combineReducers.js

@@ -1,7 +1,7 @@
 import { ActionTypes } from '../createStore';
-import isPlainObject from '../utils/isPlainObject';
-import mapValues from '../utils/mapValues';
-import pick from '../utils/pick';
+import isPlainObject from 'lodash.isplainobject';
+import mapValues from 'lodash.mapvalues';
+import pick from 'lodash.pick';
 import invariant from 'invariant';
 import warning from 'warning';