React的路由功能:将Index.js的页面，指派到Router.js，再由它去指派要渲染的元件。

1.将index.js的页面，指派到Router.js

import React from 'react';import ReactDOM from 'react-dom';import Router from 'Router';ReactDOM.render(<Router />,document.getElementById('root'))

2.再由Router.js去指派要呼叫那个元件
Router.js

import App from 'pages/App.js'import Login from 'pages/login.js'import NotFound from 'pages/NotFound'import {BrowserRouter,Switch,Route} from 'react-router-dom'const Router =()=>(    <BrowserRouter><Switch><Route path='/' exact component={App} />             <Route path='/login' component={Login} />            <Route component={NotFound}/>                //找不到就用这一个        </Switch>    </BrowserRouter>)   export default Router;