把组件写入单独的文件中

src/components/Welcome.js

@@ -0,0 +1,8 @@
+import React, { Component } from 'react';
+
+class Welcome extends Component{
+    render(){
+        return <h1>Hello, Component</h1>;
+    }
+}
+export default Welcome

src/index.js

@@ -2,12 +2,13 @@ import React from 'react';
 import ReactDOM from 'react-dom';
 import './index.css';
 import App from './App';
+import Welcome from './components/Welcome'
 import * as serviceWorker from './serviceWorker';
 
 ReactDOM.render(
-    <h1>Hello, world!</h1>,
-    document.getElementById('root')
-  );
+  <Welcome/>,
+  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.