Chap03n -> Understanding and enabling strict mode

QandA/frontend/src/App.tsx

@@ -6,6 +6,7 @@ function App() {
   return (
     <div className="App">
       <header className="App-header">
+        <ProblemComponent />
         <img src={logo} className="App-logo" alt="logo" />
         <p>
           Edit <code>src/App.tsx</code> and save to reload.
@@ -23,4 +24,10 @@ function App() {
   );
 }
 
+class ProblemComponent extends React.Component {
+  render() {
+    return <div ref="div" />;
+  }
+}
+
 export default App;

QandA/frontend/src/index.tsx

@@ -8,10 +8,16 @@ ReactDOM.render(
   <React.StrictMode>
     <App />
   </React.StrictMode>,
-  document.getElementById('root')
+  document.getElementById('root'),
 );
 
 // If you want to start measuring performance in your app, pass a function
 // to log results (for example: reportWebVitals(console.log))
 // or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals
 reportWebVitals();
+
+class ProblemComponent extends React.Component {
+  render() {
+    return <div ref="div" />;
+  }
+}