代码：
import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import App from "./App";
import * as serviceWorker from "./serviceWorker";
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
function Hello() {
return <div>hello</div>;
}
ReactDOM.render(
<Router>
<Switch>
<Route exact path="/" component={Hello} />
<Route component={App} />
</Switch>
</Router>,
document.getElementById("root")
);

本地调试，在浏览器中访问首页是可以显示hello的，如下图

但是npm run build后，访问就不行了，只能访问App,<Hello>访问不到，如下图