You must study this commit for the sake of Mark-san

src/components/PostList.js

@@ -9,7 +9,10 @@ class PostList extends React.Component {
         this.props.fetchPosts();
     }
     renderList() {
-        return this.props.posts.map(post => {
+        const {posts} = this.props
+        const postKeys = Object.keys(posts.entities)
+        return postKeys.map(id => {
+            const post = posts.entities[id]
             return (
                 <div className="item" key={post.id}>
                     <div className="content">

src/reducers/postsReducer.js

@@ -1,7 +1,18 @@
-export default (state = [], action) => {
+const initialState = {
+    entities: {},
+};
+
+const normalizeEntities = (entities, payloadData) => {
+    const entitiesMap = {}
+    payloadData.forEach(post => entitiesMap[post.id] = post)
+    return {...entities, ...entitiesMap}
+}
+
+export default (state = initialState, action) => {
     switch (action.type) {
         case 'FETCH_POSTS':
-            return action.payload;
+            const mergedEntities = normalizeEntities(state.entities, action.payload)
+            return {...state, ...{entities: mergedEntities}}
         case 'FETCH_POST':
             return action.payload;
         default: