React.createElement: type is invalid -- expected a string

Question

Trying to get react-router (v4.0.0) and react-hot-loader (3.0.0-beta.6) to play nicely, but getitng the following error in the browser console:

Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.

index.js:

import React from 'react';
import ReactDom from 'react-dom';
import routes from './routes.js';
require('jquery');
import 'bootstrap/dist/css/bootstrap.min.css';
import 'bootstrap/dist/js/bootstrap.min.js';
import './css/main.css';

const renderApp = (appRoutes) => {
    ReactDom.render(appRoutes, document.getElementById('root'));
};

renderApp( routes() );

routes.js:

import React from 'react';
import { AppContainer } from 'react-hot-loader';
import { Router, Route, browserHistory, IndexRoute } from 'react-router';
import store from './store/store.js';
import { Provider } from 'react-redux';
import App from './containers/App.jsx';
import Products from './containers/shop/Products.jsx';
import Basket from './containers/shop/Basket.jsx';

const routes = () => (

    <AppContainer>
        <Provider store={store}>
            <Router history={browserHistory}>
                <Route path="/" component={App}>
                    <IndexRoute component={Products} />
                    <Route path="/basket" component={Basket} />
                </Route>
            </Router>
        </Provider>
    </AppContainer>

);

export default routes;

Answer 1

Most of the time this is due to an incorrect export/import.

Common error:

// File: LeComponent.js
export class LeComponent extends React.Component { ... }

// File: App.js
import LeComponent from './LeComponent';

Possible option:

// File: LeComponent.js 
export default class LeComponent extends React.Component { ... }

// File: App.js
import LeComponent from './LeComponent';

There are a few ways it could be wrong, but that error is because of an import/export mismatch 60% of the time, everytime.

Edit

Typically you should get a stacktrace that indicates an approximate location of where the failure occurs. This generally follows straight after the message you have in your original question.

If it doesn't show, it might be worth investigating why (it might be a build setting that you're missing). Regardless, if it doesn't show, the only course of action is narrowing down where the export/import is failing.

Sadly, the only way to do it, without a stacktrace is to manually remove each module/submodule until you don't get the error anymore, then work your way back up the stack.

Edit 2

Via comments, it was indeed an import issue, specifically importing a module that didn't exist