reactjs - 您可能忘记从定义它的文件中导出组件，或者您可能混淆了默认导入和命名导入

Question

import React from 'react';
import ReactDOM from 'react-dom';
import { createStore } from 'redux';
import  createHistory  from 'history/createBrowserHistory';
import { Provider } from 'react-redux';
import  ConnectedRouter  from 'react-router-redux';
import { Route, Switch } from 'react-router';
import Home from "./pages/Home";
import Register from "./pages/Register";
import CourseManagerDashboard from "./pages/CourseManagerDashboard";
import CourseDetail from "./pages/CourseDetail";
import App from './app/App';
import LoginForm from './components/LoginForm';

const store = createStore(
    state => state
);
const history = createHistory();

ReactDOM.render((
    <Provider store={store}>
        <ConnectedRouter history={history}>
            <Switch>
                <Route name="home" exact path="/" component={Home} />
                <Route name="register" path="/register" component={Register} />
                <Route name="course-manager-dashboard" path="/course-manager-dashboard" component={CourseManagerDashboard} />
                <Route name="course-detail" path="/course-detail" component={CourseDetail} />
                <Route name="login" path="/login" component={LoginForm} />
                <Route path="/" component={App} />
            </Switch>
        </ConnectedRouter>
    </Provider>
),document.getElementById('app'));

出现以下错误:

元素类型无效:需要一个字符串(对于内置组件)或一个类/函数(对于复合组件)，但得到:未定义。您可能忘记从定义它的文件中导出组件，或者您可能混淆了默认导入和命名导入。无法追踪问题到底出在哪里。

Answer 1

我知道这可能听起来很愚蠢，但尝试使用简单的 console.log 检查所有导入的组件:

console.log('Provider', Provider);
console.log('ConnectedRouter', ConnectedRouter);
console.log('Route', Route);
console.log('Switch', Switch);
console.log('Home', Home);
console.log('Register', Register);
console.log('CourseManagerDashboard', CourseManagerDashboard);
console.log('CourseDetail', CourseDetail);
console.log('App', App);
console.log('LoginForm', LoginForm);

将其放在 ReactDOM.render 之前，const history = createHistory();

之后

其中包含 undefined 的行导致了问题。