index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';//<app/>js的普通对象
let app = < App / >let root = document.getElementById('root')let h1=<h1>helloworld<span>我是</span></h1>
ReactDOM.render(h1, root
);

运行结果