From 7b3315e27b6ce92346dee34c6d25e541523db6f9 Mon Sep 17 00:00:00 2001
From: Erik Michaelson <erik.michaelson@gmail.com>
Date: Tue, 7 Jun 2016 21:12:30 -0400
Subject: [PATCH] =?UTF-8?q?Add=20optional=20=E2=80=9Coptions=E2=80=9D=20pa?=
 =?UTF-8?q?ram=20to=20combineReducers=20and=20add=20example=20test=20using?=
 =?UTF-8?q?=20immutable-js?=
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---
 package.json                 |  1 +
 src/combineReducers.js       | 27 ++++++++++++++++++++++-----
 test/combineReducers.spec.js | 22 ++++++++++++++++++++++
 3 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/package.json b/package.json
index 2c0803da72..b4981f86f0 100644
--- a/package.json
+++ b/package.json
@@ -101,6 +101,7 @@
     "expect": "^1.8.0",
     "gitbook-cli": "^0.3.4",
     "glob": "^6.0.4",
+    "immutable": "3.8.1",
     "isparta": "^4.0.0",
     "mocha": "^2.2.5",
     "rimraf": "^2.3.4",
diff --git a/src/combineReducers.js b/src/combineReducers.js
index ad2d48439e..d4fddb1a6b 100644
--- a/src/combineReducers.js
+++ b/src/combineReducers.js
@@ -87,10 +87,26 @@ function assertReducerSanity(reducers) {
  * if the state passed to them was undefined, and the current state for any
  * unrecognized action.
  *
+ * @param {Object} [options={
+ *  create: (obj) => obj,
+ *  get: (state, key) => state[key],
+ *  set: (state, key, value) => {
+ *    state[key] = value
+ *    return state
+ *  }
+ * }] An optional object for configuring how the top-level state is managed.
+ *
  * @returns {Function} A reducer function that invokes every reducer inside the
  * passed object, and builds a state object with the same shape.
  */
-export default function combineReducers(reducers) {
+export default function combineReducers(reducers, options = {
+  create: (obj) => obj,
+  get: (state, key) => state[key],
+  set: (state, key, stateForKey) => {
+    state[key] = stateForKey
+    return state
+  },
+}) {
   var reducerKeys = Object.keys(reducers)
   var finalReducers = {}
   for (var i = 0; i < reducerKeys.length; i++) {
@@ -99,6 +115,7 @@ export default function combineReducers(reducers) {
       finalReducers[key] = reducers[key]
     }
   }
+
   var finalReducerKeys = Object.keys(finalReducers)
 
   var sanityError
@@ -108,7 +125,7 @@ export default function combineReducers(reducers) {
     sanityError = e
   }
 
-  return function combination(state = {}, action) {
+  return function combination(state = options.create({}), action) {
     if (sanityError) {
       throw sanityError
     }
@@ -121,17 +138,17 @@ export default function combineReducers(reducers) {
     }
 
     var hasChanged = false
-    var nextState = {}
+    var nextState = options.create({})
     for (var i = 0; i < finalReducerKeys.length; i++) {
       var key = finalReducerKeys[i]
       var reducer = finalReducers[key]
-      var previousStateForKey = state[key]
+      var previousStateForKey = options.get(state, key)
       var nextStateForKey = reducer(previousStateForKey, action)
       if (typeof nextStateForKey === 'undefined') {
         var errorMessage = getUndefinedStateErrorMessage(key, action)
         throw new Error(errorMessage)
       }
-      nextState[key] = nextStateForKey
+      nextState = options.set(nextState, key, nextStateForKey)
       hasChanged = hasChanged || nextStateForKey !== previousStateForKey
     }
     return hasChanged ? nextState : state
diff --git a/test/combineReducers.spec.js b/test/combineReducers.spec.js
index d861ba371f..c8e83b58ee 100644
--- a/test/combineReducers.spec.js
+++ b/test/combineReducers.spec.js
@@ -1,4 +1,5 @@
 import expect from 'expect'
+import Immutable from 'immutable'
 import { combineReducers } from '../src'
 import createStore, { ActionTypes } from '../src/createStore'
 
@@ -18,6 +19,27 @@ describe('Utils', () => {
       expect(s2).toEqual({ counter: 1, stack: [ 'a' ] })
     })
 
+    it('accepts options that configure how the combined reducer state is managed', () => {
+      const reducer = combineReducers({
+        counter: (state = 0, action) =>
+        action.type === 'increment' ? state + 1 : state,
+        stack: (state = [], action) =>
+        action.type === 'push' ? [ ...state, action.value ] : state
+      }, {
+        create: (obj) =>
+          (obj instanceof Immutable.Map) ? obj : new Immutable.Map(obj),
+        get: (state, key) => state.get(key),
+        set: (state, key, value) => state.set(key, value)
+      })
+
+      const s1 = reducer(undefined, { type: 'increment' })
+      expect(s1.get('counter')).toEqual(1)
+      expect(s1.get('stack')).toEqual([])
+      const s2 = reducer(s1, { type: 'push', value: 'a' })
+      expect(s2.get("counter")).toEqual(1)
+      expect(s2.get("stack")).toEqual([ 'a' ])
+    })
+
     it('ignores all props which are not a function', () => {
       const reducer = combineReducers({
         fake: true,