add third argument state to combineReducer. add some documentation

docs/api/combineReducers.md

@@ -13,7 +13,7 @@ rootReducer = combineReducers({potato: potatoReducer, tomato: tomatoReducer})
 // This would produce the following state object
 {
   potato: {
-    // ... potatoes, and other state managed by the potatoReducer ... 
+    // ... potatoes, and other state managed by the potatoReducer ...
   },
   tomato: {
     // ... tomatoes, and other state managed by the tomatoReducer, maybe some nice sauce? ...
@@ -32,6 +32,7 @@ A popular convention is to name reducers after the state slices they manage, so
 #### Arguments
 
 1. `reducers` (*Object*): An object whose values correspond to different reducing functions that need to be combined into one. See the notes below for some rules every passed reducer must follow.
+1. `state` (*Object*) (Optional): Will make the entire state available in the reducer.
 
 > Earlier documentation suggested the use of the ES6 `import * as reducers` syntax to obtain the reducers object. This was the source of a lot of confusion, which is why we now recommend exporting a single reducer obtained using `combineReducers()` from `reducers/index.js` instead. An example is included below.
 

docs/recipes/reducers/BeyondCombineReducers.md

@@ -49,7 +49,7 @@ function someSpecialActionCreator() {
     return (dispatch, getState) => {
         const state = getState();
         const dataFromB = selectImportantDataFromB(state);
-        
+
         dispatch({
             type : "SOME_SPECIAL_ACTION",
             payload : {
@@ -69,7 +69,7 @@ A third approach would be to use the reducer generated by `combineReducers` to h
 const combinedReducer = combineReducers({
     a : sliceReducerA,
     b : sliceReducerB
-}); 
+});
 
 function crossSliceReducer(state, action) {
     switch(action.type) {
@@ -91,6 +91,23 @@ function rootReducer(state, action) {
 }
 ```
 
+A fourth approach would be to pass the slice-reducers state as a third argument. This method is suitable for testability.
+
+```js
+
+
+function sliceReducer(state, action, entireState) {
+    switch(action.type) {
+        case "SOME_SPECIAL_ACTION" : {
+            const stateA = entireState.a
+            return {...state, doSomething(stateA)}    
+        }
+        default : return state;
+    }
+}
+
+```
+
 As it turns out, there's a useful utility called [reduce-reducers](https://github.com/acdlite/reduce-reducers) that can make that process easier.  It simply takes multiple reducers and runs `reduce()` on them, passing the intermediate state values to the next reducer in line:
 
 ```js
@@ -116,4 +133,3 @@ const rootReducer = combineReducers({
 Note that `combineReducers` doesn't know or care that there's anything special about the reducer function that's responsible for managing `a`.  We didn't need to modify `combineReducers` to specifically know how to undo things - we just built up the pieces we needed into a new composed function.  
 
 Also, while `combineReducers` is the one reducer utility function that's built into Redux, there's a wide variety of third-party reducer utilities that have published for reuse.  The [Redux Addons Catalog](https://github.com/markerikson/redux-ecosystem-links) lists many of the third-party utilities that are available.  Or, if none of the published utilities solve your use case, you can always write a function yourself that does just exactly what you need.
-

src/combineReducers.js

@@ -168,7 +168,7 @@ export default function combineReducers(reducers) {
       const key = finalReducerKeys[i]
       const reducer = finalReducers[key]
       const previousStateForKey = state[key]
-      const nextStateForKey = reducer(previousStateForKey, action)
+      const nextStateForKey = reducer(previousStateForKey, action, state)
       if (typeof nextStateForKey === 'undefined') {
         const errorMessage = getUndefinedStateErrorMessage(key, action)
         throw new Error(errorMessage)