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loadable-components

loadable-components

React code splitting made easy. Reduce your bundle size without stress ✂️✨.

Read the intro blogpost

npm install loadable-components

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Webpack allows modern code splitting via the dynamic import syntax. loadable-components makes it possible to use that awesome feature with React components. It is compatible with react-router and server side rendering. The API is designed to be as simple as possible to avoid useless complexity and boilerplate.

We use it in production on our website, it's open source https://github.com/smooth-code/website.

Motivation

Splitting your React application and rendering it server-side is complicated. Several have tried, react-router gave up, today only next.js is doing it right. First I decided to not do it (afraid by react-router 😱) on my website. But then I think "Fuck code splitting shouldn't be a problem today, let's do it.".

I tried several solutions, react-async-components, react-loadable and for each of them server-side rendering is very complicated. I decided to create loadable-components with for main goal: reducing API in order to make it as easier as possible for the developer. I inspired from styled-components and Apollo for the API and loadable-components was born.

Getting started

// Routes.js
import loadable from 'loadable-components'
 
export const Home = loadable(() => import('./Home'))
export const About = loadable(() => import('./About'))
export const Contact = loadable(() => import('./Contact'))
// App.js
import React from 'react'
import { Route } from 'react-router'
import * as Routes from './Routes'
 
export default () =>
  <div>
    <Route exact path="/" component={Routes.Home} />
    <Route path="/about" component={Routes.About} />
    <Route path="/contact" component={Routes.Contact} />
  </div>

Custom loading

It is possible to add a custom loading component, by default it will render nothing:

export const Home = loadable(() => import('./Home'), {
  LoadingComponent: (props) => <div>Loading...</div>,
})

Error handling

You can configure the component rendered when an error occurs during loading, by default it will render nothing:

export const Home = loadable(() => import('./Home'), {
  ErrorComponent: ({ error, props }) => <div>Oups an error occurs.</div>,
})

Delay

To avoid flashing a loader if the loading is very fast, you could implement a minimum delay. There is no built-in API in loadable-components but you could do it using p-min-delay.

import loadable from 'loadable-components'
import pMinDelay from 'p-min-delay'
 
// Wait a minimum of 200ms before loading home.
export const Home = loadable(pMinDelay(() => import('./Home'), 200))

If you want to avoid these delay server-side:

import loadable from 'loadable-components'
import pMinDelay from 'p-min-delay'
 
const delay = (promise) => {
  if (typeof window === 'undefined') return promise
  return pMinDelay(promise, 200)
}
 
export const Home = loadable(delay(() => import('./Home')))

Timeout

Infinite loading is not good for user experience, to avoid it implementing a timeout is a good workaround. You can do it using a third party module like promise-timeout:

import loadable from 'loadable-components'
import { timeout } from 'promise-timeout'
 
// Wait a maximum of 2s before sending an error.
export const Home = loadable(timeout(() => import('./Home'), 2000))

Prefetching

To enhance user experience you can fetch routes before they are requested by the user.

Prefetch on route loading

import React from 'react'
import { Contact } from './Routes'
 
Contact.load()
 
export default () => <div>Hello</div>

Prefetch on hover

import React from 'react'
import { Contact } from './Routes'
 
export default () =>
  <div>
    <Link
    <Link to="/contact" onHover={Contact.load}>Contact</Link>
  </div>

Server-side rendering

First create a Routes.js containing all your loadable routes:

// Routes.js
import loadable from 'loadable-components'
 
export const Home = loadable(() => import('client/Home'))

You can use them in your application:

// App.js
import React from 'react'
import { Home } from './Routes'
 
const App = () =>
  <div>
    <Route exact path="/" component={Home} />
  </div>

Then bootstrap your application client-side using loadComponents:

// main.js
import React from 'react'
import ReactDOM from 'react-dom'
import { BrowserRouter } from 'react-router-dom'
import { loadComponents } from 'loadable-components'
import App from './App'
 
// Load all components needed before starting rendering
loadComponents().then(() => {
  ReactDOM.render(
    <BrowserRouter>
      <App />
    </BrowserRouter>,
    document.getElementById('main'),
  )
})

The only thing you have to do on the server is calling getLoadableState() and inserting the loadable state in your html:

// server.js
import React from 'react'
import { renderToString } from 'react-dom/server'
import { StaticRouter } from 'react-router'
import { getLoadableState } from 'loadable-components/server'
import App from './App'
 
let context = {}
 
const app = (
  <StaticRouter location={...} context={context}>
    <App />
  </StaticRouter>
)
 
// Extract loadable state from application tree
getLoadableState(app).then(loadableState => {
  const html = renderToString(<YourApp />)
  // Insert style tag into page
  const page = `
    <!doctype html>
    <html>
    <head></head>
    <body>
      <div id="main">${html}</div>
      ${loadableState.getScriptTag()}
    </body>
    </html>
  `
})

Configuring Babel

Dynamic import syntax is natively supported by Webpack but not by node. That's why you have to configure Babel differently for server and client:

To have a different configuration for client and server, you can use Babel env option.

Snapshoting

An alternative to server-side rendering is snapshoting. Basically, you crawl your React website locally and you generate HTML pages.

You need to instruct your snapshot solution to save state of loadable-components to the window in the end.

getState() will return {__LOADABLE_COMPONENT_IDS__: [...]}, and this should be converted to <script>window.__LOADABLE_COMPONENT_IDS__ = [...]</script> in the resulting html.

For example, to do this with react-snap you can use following code:

import { getState } from 'loadable-components/snap'
 
// Set up for react-snap.
window.snapSaveState = () => getState()

API Reference

loadable

This is the default export. It's a factory used to create a loadable component. Props are passed to the loaded component.

Arguments

  1. getComponent (Function): Function to load component asynchronously.
  2. options (Object): Facultative options to configure component behavior.

options

  1. ErrorComponent (ReactComponent): Component rendered when an error occurs, take two props: error and props.
  2. LoadingComponent (ReactComponent): Component rendered during loading, take the same props from loadable component.
import loadable from 'loadable-components'
 
const MyLoadableComponent = loadable(() => import('./MyComponent'), {
  ErrorComponent: ({ error }) => <div>{error.message}</div>,
  LoadingComponent: () => <div>Loading...</div>,
})

loadComponents

This method is only required if you use server-side rendering. It loads components used in the page that has been rendered server-side.

import React from 'react'
import ReactDOM from 'react-dom'
import { BrowserRouter } from 'react-router-dom'
import { loadComponents } from 'loadable-components'
import App from './App'
 
// Load all components needed before starting rendering
loadComponents().then(() => {
  ReactDOM.render(
    <BrowserRouter>
      <App />
    </BrowserRouter>,
    document.getElementById('main'),
  )
})

getLoadableState

This method is only required if you use server-side rendering. It loads components recursively and extract a loadable state from a React tree.

import React from 'react'
import { renderToString } from 'react-dom/server'
import { StaticRouter } from 'react-router'
import { getLoadableState } from 'loadable-components/server'
import App from './App'
 
const app = (
  <StaticRouter>
    <App />
  </StaticRouter>
)
 
// Extract loadable state from application tree
getLoadableState(app).then(loadableState => {
  const html = renderToString(<YourApp />)
  // Insert style tag into page
  const page = `
    <!doctype html>
    <html>
    <head></head>
    <body>
      <div id="main">${html}</div>
      ${loadableState.getScriptTag()}
    </body>
    </html>
  `
})

A loadable state has two methods to extract state:

  • loadableState.getScriptTag(): Returns a string representing a script tag.
  • loadableState.getScriptElement(): Returns a React element.

Interoperability

You can implement a loadable component by your own. To do it you have to add LOADABLE Symbol to your component:

import React from 'react'
import { LOADABLE } from 'loadable-components'
 
class ComponentWithTranslations extends React.Component {
  // Required
  static componentId = 'custom-loadable'
  static async load = () => {
    const response = await fetch('/translations.json')
    const translations = await response.json()
    ComponentWithTranslations.translations = translations
    return translations
  }
 
  state = { translations: ComponentWithTranslations.translations }
 
  componentWillMount() {
    ComponentWithTranslations[LOADABLE].load()
    .then(translations => this.setState({ translations }))
  }
 
  render() {
    const { translations = { hello = 'hello' } } = this.props;
 
    return <div>{hello}</div>
  }
}
 
ComponentWithTranslations[LOADABLE] = () => ({
  componentId: 'custom-loadable',
  load: async () => {
    const response = await fetch('/translations.json')
    const translations = await response.json()
    ComponentWithTranslations.translations = translations
  }
})

Other solutions

react-loadable offers an elegant API to load a component and enhance it. It supports a lot of features like delay and timeout. I chose to not implement it because it delay can be done in LoadingComponent and timeout can be done in getComponent function.

react-async-component offers a simple API, very similar to loadable-components API.

react-code-splitting is the basic approach of an async component, it doesn't support LoadingComponent, ErrorComponent and server-side rendering.

The main difference between these two libraries is the server-side rendering approach:

  • react-loadable requires a babel plugin. I think it's too complicated and we should not rely on it.
  • react-async-component has a better approach, analyzing tree + context, it also rely on another library. I like the idea but not the API.

loadable-components has a simpler approach, it relies on dynamic-import-specification and assumes that it is working for node and Webpack. Then it analyzes the tree server-side and waiting for every modules to be loaded. Client-side it loads modules before rendering the application. The API is as simple as possible, no context, no babel plugin, no magic variable.

Inspirations

MIT