Add React.StrictMode to default templates (facebook#8558)

Co-authored-by: Dan Abramov <[email protected]>

Author: connkat

packages/cra-template-typescript/template/src/index.tsx

@@ -4,7 +4,12 @@ import './index.css';
 import App from './App';
 import * as serviceWorker from './serviceWorker';
 
-ReactDOM.render(<App />, document.getElementById('root'));
+ReactDOM.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>,
+  document.getElementById('root')
+);
 
 // If you want your app to work offline and load faster, you can change
 // unregister() to register() below. Note this comes with some pitfalls.

packages/cra-template/template/src/index.js

@@ -4,7 +4,12 @@ import './index.css';
 import App from './App';
 import * as serviceWorker from './serviceWorker';
 
-ReactDOM.render(<App />, document.getElementById('root'));
+ReactDOM.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>,
+  document.getElementById('root')
+);
 
 // If you want your app to work offline and load faster, you can change
 // unregister() to register() below. Note this comes with some pitfalls.