react-loadable

What is react-loadable?

react-loadable is a higher-order component for loading components with dynamic imports. It helps in code-splitting and lazy loading of components, which can improve the performance of React applications by reducing the initial load time.

What are react-loadable's main functionalities?

Basic Usage

This feature allows you to dynamically import a component and display a loading indicator while the component is being loaded.

const LoadableComponent = Loadable({
  loader: () => import('./MyComponent'),
  loading: () => <div>Loading...</div>,
});

function App() {
  return (
    <div>
      <LoadableComponent />
    </div>
  );
}

Custom Loading Component

This feature allows you to use a custom loading component that can handle different states like loading and error.

const LoadableComponent = Loadable({
  loader: () => import('./MyComponent'),
  loading: ({ isLoading, error }) => {
    if (isLoading) {
      return <div>Loading...</div>;
    } else if (error) {
      return <div>Error loading component!</div>;
    } else {
      return null;
    }
  },
});

function App() {
  return (
    <div>
      <LoadableComponent />
    </div>
  );
}

Preloading Components

This feature allows you to preload a component before it is actually needed, which can help in reducing the load time when the component is eventually rendered.

const LoadableComponent = Loadable({
  loader: () => import('./MyComponent'),
  loading: () => <div>Loading...</div>,
});

LoadableComponent.preload();

function App() {
  return (
    <div>
      <LoadableComponent />
    </div>
  );
}

Other packages similar to react-loadable

react-lazyload

react-lazyload is a package that provides lazy loading functionality for React components. It is similar to react-loadable but focuses more on lazy loading components based on their visibility in the viewport.

react-suspense

react-suspense is a built-in feature of React that allows you to suspend rendering while waiting for some asynchronous operation to complete, such as loading a component. It is more integrated into the React ecosystem compared to react-loadable.

loadable-components

loadable-components is a package that provides a similar functionality to react-loadable but with a more modern API and better support for server-side rendering. It also offers features like preloading and code-splitting.

README

A higher order component for loading components with dynamic imports.

Install

yarn add react-loadable

Example

import Loadable from 'react-loadable';
import Loading from './my-loading-component';

const LoadableComponent = Loadable({
  loader: () => import('./my-component'),
  loading: Loading,
});

export default class App extends React.Component {
  render() {
    return <LoadableComponent/>;
  }
}

Happy Customers:

• "I'm obsessed with this right now: CRA with React Router v4 and react-loadable. Free code splitting, this is so easy."
• "Webpack 2 upgrade & react-loadable; initial load from 1.1mb to 529kb in under 2 hours. Immense."
• "Oh hey - using loadable component I knocked 13K off my initial load. Easy win!"
• "Had a look and its awesome. shaved like 50kb off our main bundle."
• "I've got that server-side rendering + code splitting + PWA ServiceWorker caching setup done 😎 (thanks to react-loadable). Now our frontend is super fast."
• "Using react-loadable went from 221.28 KB → 115.76 KB @ main bundle. Fucking awesome and very simple API."

Users

• Analog.Cafe
• Appbase.io
• Atlassian
• Cloudflare
• Curio
• Dresez
• Flyhomes
• Gogo
• Gofore
• MediaTek MCS-Lite
• Officepulse
• Render
• Snipit
• Spectrum.chat
• Talentpair
• Tinder
• Unsplash
• Wave

If your company or project is using React Loadable, please open a PR and add yourself to this list (in alphabetical order please)

Also See:

• react-loadable-visibility - Building on top of and keeping the same API as react-loadable, this library enables you to load content that is visible on the screen.

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Guide

So you've got your React app, you're bundling it with Webpack, and things are going smooth. But then one day you notice your app's bundle is getting so big that it's slowing things down.

It's time to start code-splitting your app!

Code-splitting is the process of taking one large bundle containing your entire app, and splitting them up into multiple smaller bundles which contain separate parts of your app.

This might seem difficult to do, but tools like Webpack have this built in, and React Loadable is designed to make it super simple.

Route-based splitting vs. Component-based splitting

A common piece of advice you will see is to break your app into separate routes and load each one asynchronously. This seems to work well enough for many apps– as a user, clicking a link and waiting for a page to load is a familiar experience on the web.

But we can do better than that.

Using most routing tools for React, a route is simply a component. There's nothing particularly special about them (Sorry Ryan and Michael– you're what's special). So what if we optimized for splitting around components instead of routes? What would that get us?

As it turns out: Quite a lot. There are many more places than just routes where you can pretty easily split apart your app. Modals, tabs, and many more UI components hide content until the user has done something to reveal it.

Example: Maybe your app has a map buried inside of a tab component. Why would you load a massive mapping library for the parent route every time when the user may never go to that tab?

Not to mention all the places where you can defer loading content until higher priority content is finished loading. That component at the bottom of your page which loads a bunch of libraries: Why should that be loaded at the same time as the content at the top?

And because routes are just components, we can still easily code-split at the route level.

Introducing new code-splitting points in your app should be so easy that you don't think twice about it. It should be a matter of changing a few lines of code and everything else should be automated.

Introducing React Loadable

React Loadable is a small library that makes component-centric code splitting incredibly easy in React.

Loadable is a higher-order component (a function that creates a component) which lets you dynamically load any module before rendering it into your app.

Let's imagine two components, one that imports and renders another.

import Bar from './components/Bar';

class Foo extends React.Component {
  render() {
    return <Bar/>;
  }
}

Right now we're depending on Bar being imported synchronously via import, but we don't need it until we go to render it. So why don't we just defer that?

Using a dynamic import (a tc39 proposal currently at Stage 3) we can modify our component to load Bar asynchronously.

class MyComponent extends React.Component {
  state = {
    Bar: null
  };

  componentWillMount() {
    import('./components/Bar').then(Bar => {
      this.setState({ Bar });
    });
  }

  render() {
    let {Bar} = this.state;
    if (!Bar) {
      return <div>Loading...</div>;
    } else {
      return <Bar/>;
    };
  }
}

But that's a whole bunch of work, and it doesn't even handle a bunch of cases. What about when import() fails? What about server-side rendering?

Instead you can use Loadable to abstract away the problem.

import Loadable from 'react-loadable';

const LoadableBar = Loadable({
  loader: () => import('./components/Bar'),
  loading() {
    return <div>Loading...</div>
  }
});

class MyComponent extends React.Component {
  render() {
    return <LoadableBar/>;
  }
}

Automatic code-splitting on import()

When you use import() with Webpack 2+, it will automatically code-split for you with no additional configuration.

This means that you can easily experiment with new code splitting points just by switching to import() and using React Loadable. Figure out what performs best for your app.

Creating a great "Loading..." Component

Rendering a static "Loading..." doesn't communicate enough to the user. You also need to think about error states, timeouts, and making it a nice experience.

function Loading() {
  return <div>Loading...</div>;
}

Loadable({
  loader: () => import('./WillFailToLoad'), // oh no!
  loading: Loading,
});

To make this all nice, your loading component receives a couple different props.

Loading error states

When your loader fails, your loading component will receive an error prop which will be an Error object (otherwise it will be null).

function Loading(props) {
  if (props.error) {
    return <div>Error! <button onClick={ props.retry }>Retry</button></div>;
  } else {
    return <div>Loading...</div>;
  }
}

Avoiding Flash Of Loading Component

Sometimes components load really quickly (<200ms) and the loading screen only quickly flashes on the screen.

A number of user studies have proven that this causes users to perceive things taking longer than they really have. If you don't show anything, users perceive it as being faster.

So your loading component will also get a pastDelay prop which will only be true once the component has taken longer to load than a set delay.

function Loading(props) {
  if (props.error) {
    return <div>Error! <button onClick={ props.retry }>Retry</button></div>;
  } else if (props.pastDelay) {
    return <div>Loading...</div>;
  } else {
    return null;
  }
}

This delay defaults to 200ms but you can also customize the delay in Loadable.

Loadable({
  loader: () => import('./components/Bar'),
  loading: Loading,
  delay: 300, // 0.3 seconds
});

Timing out when the loader is taking too long

Sometimes network connections suck and never resolve or fail, they just hang there forever. This sucks for the user because they won't know if it should always take this long, or if they should try refreshing.

The loading component will receive a timedOut prop which will be set to true when the loader has timed out.

function Loading(props) {
  if (props.error) {
    return <div>Error! <button onClick={ props.retry }>Retry</button></div>;
  } else if (props.timedOut) {
    return <div>Taking a long time... <button onClick={ props.retry }>Retry</button></div>;
  } else if (props.pastDelay) {
    return <div>Loading...</div>;
  } else {
    return null;
  }
}

However, this feature is disabled by default. To turn it on, you can pass a timeout option to Loadable.

Loadable({
  loader: () => import('./components/Bar'),
  loading: Loading,
  timeout: 10000, // 10 seconds
});

Customizing rendering

By default Loadable will render the default export of the returned module. If you want to customize this behavior you can use the render option.

Loadable({
  loader: () => import('./my-component'),
  render(loaded, props) {
    let Component = loaded.namedExport;
    return <Component {...props}/>;
  }
});

Loading multiple resources

Technically you can do whatever you want within loader() as long as it returns a promise and you're able to render something. But writing it out can be a bit annoying.

To make it easier to load multiple resources in parallel, you can use Loadable.Map.

Loadable.Map({
  loader: {
    Bar: () => import('./Bar'),
    i18n: () => fetch('./i18n/bar.json').then(res => res.json()),
  },
  render(loaded, props) {
    let Bar = loaded.Bar.default;
    let i18n = loaded.i18n;
    return <Bar {...props} i18n={i18n}/>;
  },
});

When using Loadable.Map the render() method is required. It will be passed a loaded param which will be an object matching the shape of your loader.

Preloading

As an optimization, you can also decide to preload a component before it gets rendered.

For example, if you need to load a new component when a button gets pressed, you could start preloading the component when the user hovers over the button.

The component created by Loadable exposes a static preload method which does exactly this.

const LoadableBar = Loadable({
  loader: () => import('./Bar'),
  loading: Loading,
});

class MyComponent extends React.Component {
  state = { showBar: false };

  onClick = () => {
    this.setState({ showBar: true });
  };

  onMouseOver = () => {
    LoadableBar.preload();
  };

  render() {
    return (
      <div>
        <button
          onClick={this.onClick}
          onMouseOver={this.onMouseOver}>
          Show Bar
        </button>
        {this.state.showBar && <LoadableBar/>}
      </div>
    )
  }
}

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Server-Side Rendering

When you go to render all these dynamically loaded components, what you'll get is a whole bunch of loading screens.

This really sucks, but the good news is that React Loadable is designed to make server-side rendering work as if nothing is being loaded dynamically.

Here's our starting server using Express.

import express from 'express';
import React from 'react';
import ReactDOMServer from 'react-dom/server';
import App from './components/App';

const app = express();

app.get('/', (req, res) => {
  res.send(`
    <!doctype html>
    <html lang="en">
      <head>...</head>
      <body>
        <div id="app">${ReactDOMServer.renderToString(<App/>)}</div>
        <script src="/dist/main.js"></script>
      </body>
    </html>
  `);
});

app.listen(3000, () => {
  console.log('Running on http://localhost:3000/');
});

Preloading all your loadable components on the server

The first step to rendering the correct content from the server is to make sure that all of your loadable components are already loaded when you go to render them.

To do this, you can use the Loadable.preloadAll method. It returns a promise that will resolve when all your loadable components are ready.

Loadable.preloadAll().then(() => {
  app.listen(3000, () => {
    console.log('Running on http://localhost:3000/');
  });
});

Picking up a server-side rendered app on the client

This is where things get a little bit tricky. So let's prepare ourselves little bit.

In order for us to pick up what was rendered from the server we need to have all the same code that was used to render on the server.

To do this, we first need our loadable components telling us which modules they are rendering.

Declaring which modules are being loaded

There are two options in Loadable and Loadable.Map which are used to tell us which modules our component is trying to load: opts.modules and opts.webpack.

Loadable({
  loader: () => import('./Bar'),
  modules: ['./Bar'],
  webpack: () => [require.resolveWeak('./Bar')],
});

But don't worry too much about these options. React Loadable includes a Babel plugin to add them for you.

Just add the react-loadable/babel plugin to your Babel config:

{
  "plugins": [
    "react-loadable/babel"
  ]
}

Now these options will automatically be provided.

Finding out which dynamic modules were rendered

Next we need to find out which modules were actually rendered when a request comes in.

For this, there is Loadable.Capture component which can be used to collect all the modules that were rendered.

import Loadable from 'react-loadable';

app.get('/', (req, res) => {
  let modules = [];

  let html = ReactDOMServer.renderToString(
    <Loadable.Capture report={moduleName => modules.push(moduleName)}>
      <App/>
    </Loadable.Capture>
  );

  console.log(modules);

  res.send(`...${html}...`);
});

Mapping loaded modules to bundles

In order to make sure that the client loads all the modules that were rendered server-side, we'll need to map them to the bundles that Webpack created.

This comes in two parts.

First we need Webpack to tell us which bundles each module lives inside. For this there is the React Loadable Webpack plugin.

Import the ReactLoadablePlugin from react-loadable/webpack and include it in your webpack config. Pass it a filename for where to store the JSON data about our bundles.

// webpack.config.js
import { ReactLoadablePlugin } from 'react-loadable/webpack';

export default {
  plugins: [
    new ReactLoadablePlugin({
      filename: './dist/react-loadable.json',
    }),
  ],
};

Then we'll go back to our server and use this data to convert our modules to bundles.

To convert from modules to bundles, import the getBundles method from react-loadable/webpack and the data from Webpack.

import Loadable from 'react-loadable';
import { getBundles } from 'react-loadable/webpack'
import stats from './dist/react-loadable.json';

app.get('/', (req, res) => {
  let modules = [];

  let html = ReactDOMServer.renderToString(
    <Loadable.Capture report={moduleName => modules.push(moduleName)}>
      <App/>
    </Loadable.Capture>
  );

  let bundles = getBundles(stats, modules);

  // ...
});

We can then render these bundles into <script> tags in our HTML.

It is important that the bundles are included before the main bundle, so that they can be loaded by the browser prior to the app rendering.

However, as the Webpack manifest (including the logic for parsing bundles) lives in the main bundle, it will need to be extracted into its own chunk.

This is easy to do with the CommonsChunkPlugin

// webpack.config.js
export default {
  plugins: [
    new webpack.optimize.CommonsChunkPlugin({
      name: 'manifest',
      minChunks: Infinity
    })
  ]
}

Notice: As of Webpack 4 the CommonsChunkPlugin has been removed and the manifest doesn't need to be extracted anymore.

let bundles = getBundles(stats, modules);

res.send(`
  <!doctype html>
  <html lang="en">
    <head>...</head>
    <body>
      <div id="app">${html}</div>
      <script src="/dist/manifest.js"></script>
      ${bundles.map(bundle => {
        return `<script src="/dist/${bundle.file}"></script>`
        // alternatively if you are using publicPath option in webpack config
        // you can use the publicPath value from bundle, e.g:
        // return `<script src="${bundle.publicPath}"></script>`
      }).join('\n')}
      <script src="/dist/main.js"></script>
    </body>
  </html>
`);

Preloading ready loadable components on the client

We can use the Loadable.preloadReady() method on the client to preload the loadable components that were included on the page.

Like Loadable.preloadAll(), it returns a promise, which on resolution means that we can hydrate our app.

// src/entry.js
import React from 'react';
import ReactDOM from 'react-dom';
import Loadable from 'react-loadable';
import App from './components/App';

Loadable.preloadReady().then(() => {
  ReactDOM.hydrate(<App/>, document.getElementById('app'));
});

Now server-side rendering should work perfectly!

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API Docs

Loadable

A higher-order component for dynamically loading a module before rendering it, a loading component is rendered while the module is unavailable.

const LoadableComponent = Loadable({
  loader: () => import('./Bar'),
  loading: Loading,
  delay: 200,
  timeout: 10000,
});

This returns a LoadableComponent.

Loadable.Map

A higher-order component that allows you to load multiple resources in parallel.

Loadable.Map's opts.loader accepts an object of functions, and needs a opts.render method.

Loadable.Map({
  loader: {
    Bar: () => import('./Bar'),
    i18n: () => fetch('./i18n/bar.json').then(res => res.json()),
  },
  render(loaded, props) {
    let Bar = loaded.Bar.default;
    let i18n = loaded.i18n;
    return <Bar {...props} i18n={i18n}/>;
  }
});

When using Loadable.Map the render() method's loaded param will be an object with the same shape as your loader.

Loadable and Loadable.Map Options

opts.loader

A function returning a promise that loads your module.

Loadable({
  loader: () => import('./Bar'),
});

When using with Loadable.Map this accepts an object of these types of functions.

Loadable.Map({
  loader: {
    Bar: () => import('./Bar'),
    i18n: () => fetch('./i18n/bar.json').then(res => res.json()),
  },
});

When using with Loadable.Map you'll also need to pass a opts.render function.

opts.loading

A LoadingComponent that renders while a module is loading or when it errors.

Loadable({
  loading: LoadingComponent,
});

This option is required, if you don't want to render anything, return null.

Loadable({
  loading: () => null,
});

opts.delay

Time to wait (in milliseconds) before passing props.pastDelay to your loading component. This defaults to 200.

Loadable({
  delay: 200
});

Read more about delays.

opts.timeout

Time to wait (in milliseconds) before passing props.timedOut to your loading component. This is turned off by default.

Loadable({
  timeout: 10000
});

Read more about timeouts.

opts.render

A function to customize the rendering of loaded modules.

Receives loaded which is the resolved value of opts.loader and props which are the props passed to the LoadableComponent.

Loadable({
  render(loaded, props) {
    let Component = loaded.default;
    return <Component {...props}/>;
  }
});

opts.webpack

An optional function which returns an array of Webpack module ids which you can get with require.resolveWeak.

Loadable({
  loader: () => import('./Foo'),
  webpack: () => [require.resolveWeak('./Foo')],
});

This option can be automated with the Babel Plugin.

opts.modules

An optional array with module paths for your imports.

Loadable({
  loader: () => import('./my-component'),
  modules: ['./my-component'],
});

This option can be automated with the Babel Plugin.

LoadableComponent

This is the component returned by Loadable and Loadable.Map.

const LoadableComponent = Loadable({
  // ...
});

Props passed to this component will be passed straight through to the dynamically loaded component via opts.render.

LoadableComponent.preload()

This is a static method on LoadableComponent which can be used to load the component ahead of time.

const LoadableComponent = Loadable({...});

LoadableComponent.preload();

This returns a promise, but you should avoid waiting for that promise to resolve to update your UI. In most cases it creates a bad user experience.

Read more about preloading.

LoadingComponent

This is the component you pass to opts.loading.

function LoadingComponent(props) {
  if (props.error) {
    // When the loader has errored
    return <div>Error! <button onClick={ props.retry }>Retry</button></div>;
  } else if (props.timedOut) {
    // When the loader has taken longer than the timeout
    return <div>Taking a long time... <button onClick={ props.retry }>Retry</button></div>;
  } else if (props.pastDelay) {
    // When the loader has taken longer than the delay
    return <div>Loading...</div>;
  } else {
    // When the loader has just started
    return null;
  }
}

Loading({
  loading: LoadingComponent,
});

Read more about loading components

props.error

An Error object passed to LoadingComponent when the loader has failed. When there is no error, null is passed.

function LoadingComponent(props) {
  if (props.error) {
    return <div>Error!</div>;
  } else {
    return <div>Loading...</div>;
  }
}

Read more about errors.

props.retry

A function prop passed to LoadingComponent when the loader has failed, used to retry loading the component.

function LoadingComponent(props) {
  if (props.error) {
    return <div>Error! <button onClick={ props.retry }>Retry</button></div>;
  } else {
    return <div>Loading...</div>;
  }
}

Read more about errors.

props.timedOut

A boolean prop passed to LoadingComponent after a set timeout.

function LoadingComponent(props) {
  if (props.timedOut) {
    return <div>Taking a long time...</div>;
  } else {
    return <div>Loading...</div>;
  }
}

Read more about timeouts.

props.pastDelay

A boolean prop passed to LoadingComponent after a set delay.

function LoadingComponent(props) {
  if (props.pastDelay) {
    return <div>Loading...</div>;
  } else {
    return null;
  }
}

Read more about delays.

Loadable.preloadAll()

This will call all of the LoadableComponent.preload methods recursively until they are all resolved. Allowing you to preload all of your dynamic modules in environments like the server.

Loadable.preloadAll().then(() => {
  app.listen(3000, () => {
    console.log('Running on http://localhost:3000/');
  });
});

It's important to note that this requires that you declare all of your loadable components when modules are initialized rather than when your app is being rendered.

Good:

// During module initialization...
const LoadableComponent = Loadable({...});

class MyComponent extends React.Component {
  componentDidMount() {
    // ...
  }
}

Bad:

// ...

class MyComponent extends React.Component {
  componentDidMount() {
    // During app render...
    const LoadableComponent = Loadable({...});
  }
}

Note: Loadable.preloadAll() will not work if you have more than one copy of react-loadable in your app.

Read more about preloading on the server.

Loadable.preloadReady()

Check for modules that are already loaded in the browser and call the matching LoadableComponent.preload methods.

Loadable.preloadReady().then(() => {
  ReactDOM.hydrate(<App/>, document.getElementById('app'));
});

Read more about preloading on the client.

Loadable.Capture

A component for reporting which modules were rendered.

Accepts a report prop which is called for every moduleName that is rendered via React Loadable.

let modules = [];

let html = ReactDOMServer.renderToString(
  <Loadable.Capture report={moduleName => modules.push(moduleName)}>
    <App/>
  </Loadable.Capture>
);

console.log(modules);

Read more about capturing rendered modules.

Babel Plugin

Providing opts.webpack and opts.modules for every loadable component is a lot of manual work to remember to do.

Instead you can add the Babel plugin to your config and it will automate it for you:

{
  "plugins": ["react-loadable/babel"]
}

Input

import Loadable from 'react-loadable';

const LoadableMyComponent = Loadable({
  loader: () => import('./MyComponent'),
});

const LoadableComponents = Loadable.Map({
  loader: {
    One: () => import('./One'),
    Two: () => import('./Two'),
  },
});

Output

import Loadable from 'react-loadable';
import path from 'path';

const LoadableMyComponent = Loadable({
  loader: () => import('./MyComponent'),
  webpack: () => [require.resolveWeak('./MyComponent')],
  modules: [path.join(__dirname, './MyComponent')],
});

const LoadableComponents = Loadable.Map({
  loader: {
    One: () => import('./One'),
    Two: () => import('./Two'),
  },
  webpack: () => [require.resolveWeak('./One'), require.resolveWeak('./Two')],
  modules: [path.join(__dirname, './One'), path.join(__dirname, './Two')],
});

Read more about declaring modules.

Webpack Plugin

In order to send the right bundles down when rendering server-side, you'll need the React Loadable Webpack plugin  to provide you with a mapping of modules to bundles.

// webpack.config.js
import { ReactLoadablePlugin } from 'react-loadable/webpack';

export default {
  plugins: [
    new ReactLoadablePlugin({
      filename: './dist/react-loadable.json',
    }),
  ],
};

This will create a file (opts.filename) which you can import to map modules to bundles.

Read more about mapping modules to bundles.

getBundles

A method exported by react-loadable/webpack for converting modules to bundles.

import { getBundles } from 'react-loadable/webpack';

let bundles = getBundles(stats, modules);

Read more about mapping modules to bundles.

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FAQ

How do I avoid repetition?

Specifying the same loading component or delay every time you use Loadable() gets repetitive fast. Instead you can wrap Loadable with your own Higher-Order Component (HOC) to set default options.

import Loadable from 'react-loadable';
import Loading from './my-loading-component';

export default function MyLoadable(opts) {
  return Loadable(Object.assign({
    loading: Loading,
    delay: 200,
    timeout: 10000,
  }, opts));
};

Then you can just specify a loader when you go to use it.

import MyLoadable from './MyLoadable';

const LoadableMyComponent = MyLoadable({
  loader: () => import('./MyComponent'),
});

export default class App extends React.Component {
  render() {
    return <LoadableMyComponent/>;
  }
}

Unfortunately at the moment using wrapped Loadable breaks react-loadable/babel so in such case you have to add required properties (modules, webpack) manually.

import MyLoadable from './MyLoadable';

const LoadableMyComponent = MyLoadable({
  loader: () => import('./MyComponent'),
  modules: ['./MyComponent'],
  webpack: () => [require.resolveWeak('./MyComponent')],
});

export default class App extends React.Component {
  render() {
    return <LoadableMyComponent/>;
  }
}

How do I handle other styles .css or sourcemaps .map with server-side rendering?

When you call getBundles, it may return file types other than JavaScript depending on your Webpack configuration.

To handle this, you should manually filter down to the file extensions that you care about:

let bundles = getBundles(stats, modules);

let styles = bundles.filter(bundle => bundle.file.endsWith('.css'));
let scripts = bundles.filter(bundle => bundle.file.endsWith('.js'));

res.send(`
  <!doctype html>
  <html lang="en">
    <head>
      ...
      ${styles.map(style => {
        return `<link href="/dist/${style.file}" rel="stylesheet"/>`
      }).join('\n')}
    </head>
    <body>
      <div id="app">${html}</div>
      <script src="/dist/main.js"></script>
      ${scripts.map(script => {
        return `<script src="/dist/${script.file}"></script>`
      }).join('\n')}
    </body>
  </html>
`);