IMPORTED COMPONENT ✂

Code splitting which always works^*

SSR-friendly code splitting compatible with any platform.
Deliver a better experience within a single import.

^* It's really will never let you down. All credits to your bundler.

Library	Suspense	SSR	Hooks	Library	Non-modules	import(`./${value}`)	babel-macro	webpack only
React.lazy	✅	❌	❌	❌	❌	❌	😹	no-ssr
react-loadable	✅	✅	❌	❌	✅	❌	❌	😿
@loadable/component	✅	✅	❌	✅	❌	✅	❌	😿
imported-component	✅	✅	✅	✅	✅	❌	✅	😸

Read more about what this table displays

Key features:

1️⃣ Single source of truth - your bundler drives everything
📖 library level code splitting
🧙️ Hybrid and Prerendering compatible
💡 TypeScript bindings
⚛️ React.Lazy underneath (if hot module updates are disabled)
🌟 Async on client, sync on server. Supports Suspense (even on server side)
📦 could work with any bundler - webpack, rollup, parcel or puppeteer - it does not matter
🤹‍♂️ working as well with any import you may provide

Other features:

🔥 Hot-Module-Replacement/React-Hot-Loader friendly
⛓️ support forwardRef
⚛️ React 16/Async/Hooks ready
🛠 HOC, Component, Hooks API
🐳 stream rendering support
👥 partial hydration out of the box
📦 and yes - this is the only parcel-bundler compatible SSR-friendly React code splitting library, as well as a perfect solution for Create React App

👍 Better than React.Lazy:

It IS Lazy, just with some stuff around^*
SSR, Prerendering and Preloading support
With or without Suspense, and easier Error cases support

👍 Better than others:

Not bound to webpack
Easy way to use per-browser(modern/legacy) bundles - you down have to mess with actual browser support
Strong typing
Working with any imports, even native, external or derived ones.

👌 Client-side module resolution

Loads chunks only after the main one, as long as loader code is bundled inside the main chunk, so it should be loaded first.
Not an issue with the progressive hydration, and might provide a better UX via feature detection.
Provides 👨‍🔬 technological workaround - see here

📦 Optional bundler integration for the best experience

prefetching backed by webpack stat.json and asset.json
parcel-manifest.json support

👯‍♀️Works better in pair

react-prerendered-component for prerendering, partial hydration and react fragment caching
used-style for CSS and critical CSS extraction
devolution for shipping legacy/modern bundles
webpack-imported for deep webpack integration, the only bundler dependent tool in this list.

Usage

Server side

Just a proper setup and a bit of magic

Client side

Component

imported provides 2 common ways to define a component, which are more different inside than outside

using pre-lazy API.

import importedComponent from 'react-imported-component';
const Component = importedComponent( () => import('./Component'));

const Component = importedComponent( () => import('./Component'), {
  LoadingComponent: Spinner, // what to display during the loading
  ErrorComponent: FatalError // what to display in case of error
});

Component.preload(); // force preload

// render it
<Component... />

using lazy API. It's almost the same React.lazy outside, and exactly the same inside.

import { lazy, LazyBoundary } from 'react-imported-component';
const Component = lazy(() => import('./Component'));

const ClientSideOnly = () => (
  <Suspense>
    <Component />
  </Suspense>
);

// or let's make it SSR friendly
const ServerSideFriendly = () => (
  <LazyBoundary>
    {' '}
    // LazyBoundary is Suspense* on the client, and "nothing" on the server
    <Component />
  </LazyBoundary>
);

LazyBoundary is a Suspense on Client Side, and React.Fragment on Server Side. Don't forget - "dynamic" imports are sync on a server.

Example: React.lazy vs Imported-component

Hook

However, you may not load only components - you may load anything

import {useImported} from 'react-imported-component'

const MyCalendarComponent = () => {
  const {
      imported: moment,
      loading
    } = useImported(() => import("moment"));

  return loading ? "..." : <span>today is {moment(Date.now).format()}</span>
}

// or we could make it a bit more interesting...

const MyCalendarComponent = () => {
  const {
      imported: format  = x => "---", // default value is used while importing library
    } = useImported(
      () => import("moment"),
      moment => x => moment(x).format // masking everything behind
    );

  return <span>today is {format(Date.now())</span>
}

What you could load using useImported? Everything - imported itself is using it to import components.

useImported is an excellent example for loading translations, which are usually a simple json, in a trackable way.

💡 did you know that there is another hook based solution to load "something might might need"? The use-sidecar pattern.

🤔 Keep in mind - everything here is using useImported, and you can build whatever you need using just it.

Module

A slim helper to help handle modules, you might require using useImported in a component way

import { importedModule, ImportedModule } from 'react-imported-component';

const Moment = importedModule(() => import('moment'));

<Moment fallback="long time ago">
  {(momentjs /* default imports are auto-imported*/) => momentjs(date).fromNow()}
</Moment>;

Yes, this example was taken from loadable.lib

Can I also use a ref, populated when the library is loaded? No, you cant. Use useImported for any special case like this.

Plus, there is a Component helper:

<ImportedModule
 // import is just a _trackable_ promise - do what ever you want
 import={() => import('moment').then(({momentDefault})=> momentDefault(date).fromNow()}
 fallback="long time ago"
>
 {(fromNow) => fromNow()}
</ImportedModule>

ImportedModule will throw a promise to the nearest Suspense boundary if no fallback provided.

Babel macro

If you could not use babel plugin, but do have babel-plugin-macro (like CRA) - consider using macro API:

import { imported, lazy, useImported } from 'react-imported-component/macro';
// notice - there is no default import here

Indirect usage

Just importing react-imported-component/macro would enable babel transformation for the current file. If you have imported definition in one file, and use it from another - just import "react-imported-component/macro" in that another file. See #142

API

Don't forget - there are TS typings provided.

Code splitting components

import {*} from 'react-imported-component';

importedComponent

importedComponent(importFunction, [options]): ComponentLoader - main API, default export, HOC to create imported component.

importFunction - function which resolves with Component to be imported.

options - optional settings

options.async - activates react suspense support. Will throw a Promise in a Loading State - use it with Suspense in a same way you use React.lazy. See working with Suspense

options.LoadingComponent - component to be shown in Loading state

options.ErrorComponent - component to be shown in Error state. Will re-throw error if ErrorComponent is not set. Use ErrorBoundary to catch it.

options.onError - function to consume the error, if one will thrown. Will rethrow a real error if not set.

options.exportPicker - function to pick not default export from a importFunction

options.render(Component, state, props)- function to render the result. Could be used to tune the rendering.

[static] .preload - static method to preload components.

lazy

lazy(importFunction) - helper to mimic React.lazy behavior

useImported

useImported(importFunction, [exportPicker], [options]) - code splitting hook
- importFunction - a function which resolves to default or wildcard import(T | {default:T})
- [exportPicker] - function to pick "T" from the import
- [options] - options to the hook
  - [options.import] - controls import. Hooks would be executed only if this is not false
  - [options.track] - ability to disable server-side usage tracking.

useImported returns complex object(ImportedShape):

imported - the imported resource
error - error (if present)
loading - is it loading right now?
loadable - the underlying Loadable object
retry - retry action (in case of error)

Hints:

use options.import=false to perform conditional import - importFunction would not be used if this option set to `false.
use options.track=true to perform SSR only import - to usage would be tracked if this option set to `false.

ImportedController

<ImportedController> - a controller for Suspense Hydration. Compulsory for async/lazy usecases

Misc

There is also API method, unique for imported-component, which could be useful on the client side

addPreloader(fn):fn - adds a function, result of which would be awaited when any component is loaded. Returns cancel method.

Server side API

import {*} from 'react-imported-component/server';

whenComponentsReady():Promise - will be resolved, when all components are loaded. Usually on the next "Promise" tick.
drainHydrateMarks([stream]) - returns the currently used marks, and clears the list.
printDrainHydrateMarks([stream]) - print our the drainHydrateMarks.

Stream API

createLoadableStream - creates a steam
ImportedStream - wraps another component with import usage tracker.
createLoadableTransformer - creates nodejs StreamTransformer
getLoadableTrackerCallback - helper factory for the stream transformer

Client side API

import {*} from 'react-imported-component/boot';

whenComponentsReady():Promise, will be resolved, when all (loading right now) marks are loaded.
rehydrateMarks([marks]):Promise, loads marked async chunks.
injectLoadableTracker - helper factory for the stream transformer

Types

Loadable

All imports inside library are converted into Loadable object, and it's often accessible from outside via useImported().loadable, useLoadable(not documented), getLoadable(not documented). Even if it's documented from TS point of view - let's keep all fields in a secret, except one:

resolution - promise reflecting resolution of this loadable object

Setup

In short

Add babel plugin

Run yarn imported-components src src/imported.js to extract all your imports into a run time chunk (aka async-requires).

Replace React.lazy with our lazy, and React.Suspense with our LazyBoundary. Literraly monkey-patch React to do so
Add printDrainHydrateMarks to the server code.
Add rehydrateMarks to the client code
Done. Just read the rest of readme for details.

There are examples for webpack, parcel, and react-snap. Just follow them.

1. Configure babel plugin

On the server:

{
  "plugins": ["react-imported-component/babel", "babel-plugin-dynamic-import-node" /* might be optional for babel 7*/]
}

On the client:

{
  "plugins": ["react-imported-component/babel"]
}

Imported-Component will hook into dynamic imports, providing extra information about files you want to load.

2. Add one more command into package.json

CLI command imported-components [sources ROOT] [targetFile.js] (use .ts for TypeScript)

 "generate-imported-component": "imported-components src src/imported.js"

When you will execute this command - all imports among your codebase would be found and extracted to a file provided. This will gave ability to orchestrate code-splitting later.

If you need to search inside more that one top-level directory - just define more command, saving information into more than one target file.

The current implementation will discover and use all imports, even // commented ones

💡 Feel free to .gitignore these autogenerated files

3. Start using `imported`, `lazy` or `useImported`

Without you using API provided nothing would work.

4. Add server side tracking

There are two ways to do it - in a single threaded way, and async

Single threaded

import { printDrainHydrateMarks, drainHydrateMarks } from 'react-imported-component';
// this action will "drain" all currently used(by any reason) marks
// AND print a script tag
const html = renderToString(<YourApp />) + printDrainHydrateMarks();

// OR return list of usedmarks, and yet again CLEAR the marks list.
const html = renderToString(<YourApp />) + '<script>const marks=' + JSON.stringify(drainHydrateMarks()) + '</script>';

renderToStream or async render

import {createLoadableStream} from 'react-imported-component/server';

let importedStream = createLoadableStream();
// ImportedStream is a async rendering "provider"
const stream = renderToStream(
  <ImportedStream stream={importedStream}>
    <YourApp />
  </ImportedStream>
);

// you'd then pipe the stream into the response object until it's done
stream.pipe(res, { end: false });

// and finalize the response with closing HTML
stream.on('end', () =>
    // print marks used in the file
    res.end(`${printDrainHydrateMarks(importedStream)}</body></html>`),
)

However, the idea is just to use streams to separate renders

const html =
  renderToString(
    <ImportedStream stream={importedStream}>
      <YourApp />
    </ImportedStream>
  ) + printDrainHydrateMarks(importedStream);

5. Add `rehydrateMarks` to the client code

Before rendering your application you have to ensure - all parts are loaded. rehydrateMarks will load everything you need, and provide a promise to await.

import { rehydrateMarks, ImportedController } from 'react-imported-component';

// this will trigger all marked imports, and await for competition.
rehydrateMarks().then(() => {
  // better (note ImportedController usage)
  ReactDOM.hydrate(
    <ImportedController>
      <App />
    </ImportedController>,
    document.getElementById('main')
  );
  // or
  ReactDOM.render(<App />, document.getElementById('main'));
});

rehydrateMarks accepts a list of marks from a server side(drainHydrateMarks), loads all necessary chunks and then resolves.

A VERY IMPORTANT MOMENT - Concurrent Loading

All other code splitting libraries are working a bit differently - they amend webpack building process, gathering information about how the final chunks are assembled, and injects the real scripts and styles to the server response, thus all scripts, used to render something on the Server would be loaded in a parallel in on Client. Literally - they are defined in the HTML. React-imported-component is different, it starts "working" when the bundle is loaded, thus the loading of chunks is deferred.

💡 In the normals conditions react-imported-component would be "slower" than a "webpack" library. Refer to bundle integration section.

However, it is not a problem, as long as (for now), script execution is single threaded, and even you if can load multiple scripts simultaneously - you can't run them in parallel*.

And there a way to utilize this limitation - just change your entry point, .

And let's call it - a Scheduler optimization. See loading prediction section for more details.

Scheduler optimization + simply static render

Split your app into boot and main parts
rehydrate at the boot

// index.js (boot)
import './src/imported'; // the file generated by "generate-imported-component" (.2)
import { rehydrateMarks } from 'react-imported-component/boot';

rehydrateMarks(); // just start loading what's needed

// load/execute the rest after letting the browser kick off chunk loading
// for example wrapping it in two Promises (1ms timeout or setImmediate)
Promise.resolve().then(() =>
  Promise.resolve().then(() => {
    // load the rest
    require('./main'); // <--- your main scripts
    // ! and don't forget to `await rehydrateMarks()` before render
  })
);

// main.js
rehydrateMarks().then(() => {
  ReactDOM.hydrate(<App />, document.getElementById('root'));
});

This will just start loading extra chunks before the main bundle got completely parsed and executed.

Defer till DOMReady

💡 Another perfect option would be to wait till DomReady event.

// index.js (boot)
import './src/imported'; // the file generated by "generate-imported-component" (.2)
import { rehydrateMarks } from 'react-imported-component/boot';

rehydrateMarks(); // just start loading what's needed

const startApp = () => require('./main'); // <--- your main scripts

// it's "not safe" to start you application before DOM is "ready"
if (document.readyState === 'loading') {
  document.addEventListener('DOMContentLoaded', startApp);
} else {
  startApp();
}

Scheduler optimization + stream render

See examples/SSR/parcel-react-ssr/server-stream for details

Add your main bundle to the head, using async script tag. Not defer! We have to do it async
Add loadableTracker at server side

import {createLoadableTransformer, getLoadableTrackerCallback} from 'react-imported-component/server';
const importedTracker = createLoadableTransformer(
  loadableStream, // stream to observe
  getLoadableTrackerCallback() // helper factory to create global tracker.
);

// pipe result of `renderToStream` throught it
const reactRenderStream = ReactDOM.renderToNodeStream(...).pipe(importedTracker);

Add loadableTracker at client side

// index.js
import './src/imported'; // the file generated by "generate-imported-component" (.2)
import { injectLoadableTracker } from 'react-imported-component/boot';

injectLoadableTracker();

// load the rest after letting the browser kick off chunk loading
// for example wrapping it in two Promises (1ms timeout or setImmediate)
Promise.resolve().then(() =>
  Promise.resolve().then(() => {
    require('./main');
  })
);

This "hack", will first introduce all possible imports to the imported-component, then gave it a "tick" to start loading required once, and only then execute the rest of the bundle. While the rest(99%) of the bundle would make CPU busy - chunks would be loaded over the network.

💡This is utilizing the differences between parse(unenviable) phase of script, and execute(more expensive) one.

Cooking receipts

Replace React.lazy by lazy

import React from 'react';
import { lazy, LazyBoundary } from 'react-imported-component';
React.lazy = lazy;
React.Suspense = LazyBoundary;

That's all the work required to get it working.

Partial hydration

Just wrap "partial hydrated" component with another ImportedStream so everything needed for it would be not automatically loaded with the main stream.

Hybrid render (CSR with prerendering)

This library could support hybrid rendering (aka pre-rendering) compatible in two cases:

pre-render supports state hydration, like getState in react-snap. See our example.
for rendertron or https://prerender.io follow react-snap example, just dump state using setTimeout.
You may use react-prerendered-component to maintain a component state until async chunk is not loaded. See example below.

Works better in pair (boiled-place-less code splitting)

You might not need to wait for all the chunks to be loaded before you can render you app - just use react-prerendered-component.

import imported from 'react-imported-component';
import { PrerenderedComponent } from 'react-prerendered-component';

const AsyncComponent = imported(() => import('./myComponent.js'));

<PrerenderedComponent
  // component will "go live" when chunk loading would be done
  live={AsyncComponent.preload()}
>
  // until component is not "live" prerendered HTML code would be used // that's why you need to `preload`
  <AsyncComponent />
</PrerenderedComponent>;

React-prerendered-component is another way to work with code splitting, which makes everything far better.

Timeout to display "spinners"

There is no build in timeouts to display Error or Loading states. You could control everything by yourself

use react-delay, p-delay, p-timeout, or Suspense :P.

Component loader

You may use component api if you need it by any reason.

import { ComponentLoader } from 'react-imported-component';

const MyPage = () => (
  <ComponentLoader
    loadable={() => import('./Page.js')}
    // all fields are optional, and matches the same field of importedComponent.
    LoadingComponent={Loading}
    ErrorComponent={Error}
    onError
    exportPicker
    render
    async
  />
);

CSS Support

CSS-in-JS Support

Out-of-the-box. Literally. CSS-in-JS library, like styled-component will do it by themselves, and there is nothing to be managed by this library.

Static CSS Files Support

imported could knew only about JS you've used, not the CSS that js've used...

This library does not support CSS as CSS, as long it's bundler independent and such deep integration is not possible. Even if deep bundler integration is next following this section - the recomended solution is to skip css part of it, and use a more bundler independent way to support CSS:

Configure you bundler, and server side rendering to emit the right classNames (just remove style-loader from webpack configuration)
Use used-styles to inject used css files to the resulting HTML.

In short (streamed example is NOT short)

const lookup = discoverProjectStyles('./dist');
// ....
const markup = ReactDOM.renderToString(<App />);
const usedStylesAsCSSFiles = getUsedStyles(markup, lookup);
// generate `link` (better rel='preload') to load CSS files

const usedStylesAsStyles = getCriticalStyles(markup, lookup);
// extract critical CSS and inline in to the server response

If you need stream render example with reduced TTFB - please refer to used-styles documentation, or our parcel-bundler stream server example.

Critical style extraction and Preloader

With the critical style extracting you might not need the "real" and "full" styles on the page load - only js is really required for hydration. Thus - you might skip loading styles for the initial render, however still need them for the next render. However - your bundler might think that you have loaded them, this is not true. To handle this case you need addPreloader method

// load initial bundle
// await dom ready
// -> HYDRATE APP <-
// add preloader

import { addPreloader } from 'react-imported-component/boot';

addPreloader(() => {
  // check that styles are loaded
  // or return Promise if they are not
});

// any not yet loaded ImportedComponent
// will await for it's own `import` as well as for
// promise returned from preloaders

See critical css example for details

Create React App

Use react-imported-component/macro for CRA compatible SSR code splitting without ejecting.

It is safe to always use react-imported-component/macro without babel-plugin-macros, BUT with react-hot-loader/babel enabled, as long as it will remove all macros.

Bundler integration

Keep in mind - you dont "need" this. It will just make integration slightly better in terms of prefetching (which affects network recourse priority) and thus startup time.

Webpack integration

You might preload/prefetch used styles and scripts, which were defined with webpackChunkName

// get mark somehow (drainHydrateMarks(importedStream))
import { getMarkedChunks } from 'react-imported-component/server';

const chunkNames = getMarkedChunks(marks);

Via webpack-imported

webpack-imported - provides webpack plugin to get data from the build, as well as to use it. Supports prefetching and critical style guidance.

import { WebpackImport } from 'webpack-imported/react';
import importedStat from 'build/imported.json';

<WebpackImport stats={importedStat} chunks={getMarkedChunks(drainHydrateMarks())} />;

Via stat.json

If you do have only stat.json - it could be converted into "importedStat" by webpack-imported without adding plugin.

Alternatively - you can discover all resources you have to preload using flush-webpack-chunks

const { js, styles } = flushChunks(webpackStats, {
  chunkNames,
});

const prefetch = (targets, as) => targets.map(url => `<link as="${as}" rel="preload" href="${url}" />`).join('');

res.send(prefetch(scripts, 'script'));
res.send(prefetch(stylesheets, 'style'));

DO NOT actually load reported resources - only preload them, and let webpack do rest.

Via assets.json

You you are using assets-webpack-plugin then you have only list of assets, without dependencies between. That's enought.

const prefetchChunks = (chunks, assets) =>
  chunks
    .map(chunk =>
      [
        assets[chunk].js && `<link rel="preload" as="script" href="${assets[chunk].js}" />`,
        assets[chunk].css && `<link rel="preload" as="style" href="${assets[chunk].css}" />`,
      ].join('')
    )
    .join('');

res.send(prefetchChunks(chunkNames, assets));

Preloading

Proper preloading is available only with bundler integration. Calling component.preload will not only preload it, but will execute as well.

.preload is equal do loading script as usual.

If you seek the best experience you might want to prefetch or preload only (network only), not load (network and CPU).

In order to prefetch(or preload) scripts - use webpack-imported, as seen in examples above.

You need SSR step in order to use preloading!

Step 1 - expose known chunks

In order to prefetch or preload you should know scripts locations. This information is stored inside webpack, and you cannot access it from user code. As a result we have to duplicate this information:

import { importAssets } from 'webpack-imported';
// react-imported-component data file
import applicationImports from '../async-requires';
// webpack-imported data file
import importedChunks from '../build/imported.json';
// 👆 this file is generated during the build, you cannot use it "IN" the build
// that's why this is Server Side only component

export const ScriptLocations = () => (
  <script
    dangerouslySetInnerHTML={{
      __html: `
        window.KNOWN_SCRIPT_MARKS = ${JSON.stringify(
          applicationImports.reduce((acc, [, chunkName]) => {
            if (chunkName) {
              acc[chunkName] = importAssets(importedChunks, chunkName).raw.load;
            }
            return acc;
          }, {})
        )};        
    `,
    }}
  />
);

Then add <ScriptLocations/> somewhere in your SSR-ed head.

Step 2 - preload

Then you will be able to prefetch(() => import('./home').

import { getMarkedChunks, loadableResource } from 'react-imported-component';

// this function is partially duplicating webpack internals
export const prefetch = importCallback => {
  // get a "lodable" attached for a given import
  const loadable = loadableResource(importCallback);
  if (typeof document !== 'undefined') {
    const prefix = __webpack_public_path__;
    if (loadable) {
      // get chunks used in loadable
      const chunks = getMarkedChunks(loadable.mark);
      chunks.forEach(chunk => {
        // note the usage of KNOWN_SCRIPT_MARKS
        const { js = [], css = [] } = window.KNOWN_SCRIPT_MARKS[chunk];
        js.forEach(script => {
          const link = document.createElement('link');

          link.rel = 'prefetch';
          link.as = 'script';

          link.href = prefix + script;
          document.head.appendChild(link);
        });
        css.forEach(style => {
          const link = document.createElement('link');

          link.rel = 'prefetch';
          link.as = 'style';

          link.href = prefix + style;
          document.head.appendChild(link);
        });
      });
    }
  }
};

Parcel integration

Use parcel-manifest and getMarkedFileNames(instead of getMarkedChunks) to find which files were required and has to be imported. Keep in mind - the base path would be different and you have to resolve the right files in to the right files (/app/index to index.js).

CRA integration

Consider using concurrent-loading technique only.

React-snap

react-imported-component is compatible with react-snap out of the box. Works even better with react-prerendered-component.

Webpack-external-import (or native import)

react-imported-component is compatible with webpack-external-import, as long as it executes imports, thus executes custom logic

`useImported` and `Suspense`

useImported is not supposed to be used with Suspense (by design), while it could

const MyComponent = () => {
  const { loading, error, loadable, imported } = useImported(() => import('...'));

  if (loading) throw loadable.resolution; // throw to the nearest Suspense boundary
  if (error) throw error; // throw to the nearest Error boundary

  // do something with `imported` value
};

SSR (Server side rendering)

It was usually a headache - async components and SSR, which is currently sync. React-imported-component break this cycle, making ServerSide rendering sync, and providing comprehensive ways to rehydrate rendered tree on client. It will detect server-side environment and precache all used components.

Server Side Auto Import

On the server side imported would auto-import any found import. As long as not all imports could be executed at Server Side you might pop out this feature using a "magic comment".

import(/* client-side */ './file');

Or file filtering

import { setConfiguration } from 'react-imported-component';

setConfiguration({
  fileFilter: fileName => file.indexOf('/client') !== 0,
});

.imported.js

There is possibility to finely control both with files are scanned, which imports are added and which chunks would be generated(webpackonly) via .imported.js at the root directory

💩 .js, not .ts

// 👉 use provided helper for documentation and type safety
const { configure } = require('react-imported-component');

modules.export = configure({
  testFile: fileName => true | false,
  testImport: (targetFile, sourceFile) => true | false,
  clientSideOnly: (targetFile, sourceFile, sourceComments) => true | false,
  shouldPrefetch: (targetFile, sourceFile, sourceComments) => true | false,
  shouldPreload: (targetFile, sourceFile, sourceComments) => true | false,
  chunkName: (targetFile, sourceFile, sourceComments) => string | null | undefined,
});

None of those methods are required.

.imported could be used to:

remove some files or imports
autogenerate chunk names or prefetch/preload

Bundler independent SSR

It does not matter how do you bundle your application - it could be even browser. The secrect sause is a cli command, to extract all your imports into imports map, and use it later to load chunks by request.

You might even dont have any separated chunk on the server side - it would still works.
You might even ship module/nomodule scripts, using, for example, devolution - no additional configuration would be required.

Why you need SSR

In case of imported component SSR is a "dry run" of your application - an easy way to discover required pieces powered by zero latency(you are already on the server) and super fast speed connection (all scripts are in memory).

However - you dont need SSR to get the same benefits on pure ClientSideRendered solutions - prediction would be enought. The common approach is to

load first part of components, including Providers, which would load something they need. Like translations.
then hit, and load the current route
then do the same with the sub route

This causes effect known as loading waves, the effect SSR could mitigate almost in full.

However, nothing stops you from loading translation data in the parallel to the routes, and loading route and sub route in the same time, not sequentially. You can have backend-for-frontend, why not to have frontend-for-frontend? Just handle route, cookies, and whatever you could handle, outside of React. Redux-first-router and principles behind it are the great example of this idealogy.

Not using React.Lazy with React-Hot-Loader

There is design limitation with React.lazy support from RHL size, so they could not be reloaded without state loss if lazy is created not in the user space. At it would be created inside imported.

If React-Hot-Loader is detected lazy switches to imported async mode, this behaves absolutely the same.

Comparison table legend

Library - the library name
Suspense - does it support Suspense feature
SSR - does it support SSR
Hooks - does it have hooks API
Library - does it support library, not Component level splitting
Non-modules - could it "import" generic promise, not a real dynamic module import
import('./${value}') - does it support a full dynamic import - ability to import any file.
babel macro - ability to work with babel-plugin-macros
webpack only - is this solution hard bould to webpack

Featured in

Other loaders

Another loaders exist, and the only difference is in API, and how they manage (or not manage) SSR.

(no SSR) React.Lazy
(webpack only) With react-loadable
(not compatible with hooks) react-async-component
(webpack only) loadable-components
(webpack only) react-universal-component
(not a component loader) use-sidecar