feat(redux): add counter reducer

src/common/redux/reducers/counter.js

@@ -0,0 +1,18 @@
+import { INCREMENT_COUNTER, DECREMENT_COUNTER } from '../actions/CounterActions'
+
+const initialState = 0
+
+const counter = (state = initialState, action) => {
+  switch (action.type) {
+    case INCREMENT_COUNTER:
+      return state + 1
+
+    case DECREMENT_COUNTER:
+      return state - 1
+
+    default:
+      return state
+  }
+}
+
+export default counter

src/common/redux/reducers.js → src/common/redux/reducers/index.js

@@ -1,8 +1,11 @@
 import { combineReducers } from 'redux'
 import { routerReducer } from 'react-router-redux'
 
+import counter from './counter'
+
 const rootReducer = combineReducers({
-  routing: routerReducer,
+  counter,
+  routing: routerReducer
 })
 
 export default rootReducer