Commented full solution (#35620)

* Include a default case for messageReducer

* Commented full solution

guide/english/certifications/front-end-libraries/react-and-redux/extract-local-state-into-redux/index.md

@@ -53,6 +53,8 @@ const connect = ReactRedux.connect;
 class Presentational extends React.Component {
   constructor(props) {
     super(props);
+    
+    // Remove property 'messages' from Presentational's local state
     this.state = {
       input: ''
     }
@@ -65,11 +67,13 @@ class Presentational extends React.Component {
     });
   }
   submitMessage() {
-     this.props.submitNewMessage(this.state.input);
-     this.setState({
-       input: ''
-     });
-    
+  
+    // Call 'submitNewMessage', which has been mapped to Presentational's props, with a new message;
+    // meanwhile, remove the 'messages' property from the object returned by this.setState().
+    this.props.submitNewMessage(this.state.input);
+    this.setState({
+      input: ''
+    });
   }
   render() {
     return (
@@ -80,6 +84,9 @@ class Presentational extends React.Component {
           onChange={this.handleChange}/><br/>
         <button onClick={this.submitMessage}>Submit</button>
         <ul>
+           {/* The messages state is mapped to Presentational's props; therefore, when rendering,
+               you should access the messages state through props, instead of Presentational's
+               local state. */}
           {this.props.messages.map( (message, idx) => {
               return (
                  <li key={idx}>{message}</li>