Isomorphic Support

Isomorphic code layout is a special feature which builds projects in a JavaScript layout that can be potentially processed by any environment, such as NodeJS or the browser. One main benefit is that this code can be evaluated at build-time and prerendered into the HTML document ahead of time. When the isomorphic option is used, prerendering will be attempted.

What is “Prerendering”?

Prerendering, with regards to Enact apps, means that we render out the initial state at build time. The app’s initial state is rendered via React into an HTML string and embedded statically into the index.html file.

Why Prerender?

Having the initial app state as HTML allows the app to be visible as soon as the HTML is rendered. We don’t have to wait for the JavaScript to be fetched, parsed, and executed to see the general app layout. Furthermore, once the JavaScript does load, the underlying React core will recognize the HTML and just add event listeners; no extra rendering needs to be done. The end result is the appearance of significantly faster app load time.

How to Create an Isomorphic Build

Within your package.json file, add an isomorphic property to the enact object:

    "enact": {

If the value is a string filepath instead, it will use that file as the main app entry point instead of the default. Whatever the entry point, ensure it exports the ReactElement in non-browser environments. Additionally, ensure the entry point also conditionally renders to to the DOM if the window is available. An example index.js entry point can be see here and is the default included in the Enact app template.

Then, you can choose to build with isomorphic code layout by adding the --isomorphic flag to the pack command:

enact pack --isomorphic
npm pack -- --isomorphic
npm pack-p -- --isomorphic

If you are using React18, you need to call hydrateRoot instead of createRoot to hydrate prerendered HTML.
Enact CLI provides a global variable ENACT_PACK_ISOMORPHIC to selectively call those two APIs in your app’s index.js. If you build with isomorphic option, ENACT_PACK_ISOMORPHIC will be true, otherwise false.
For more detailed information, please refer to the React 18 migration guide.

Whithin your src/index.js file, add a conditional statement to render or hydrate your app:

import {createRoot, hydrateRoot} from 'react-dom/client';

import App from './App';

const appElement = (<App />);

// In a browser environment, render the app to the document.
if (typeof window !== 'undefined') {
		hydrateRoot(document.getElementById('root'), appElement);
	} else {

export default appElement;

When to Build Isomorphically

By default, the Enact CLI will not use isomorphic code layout, and it should not be considered part of the regular development workflow. It is advisable to only build in isomorphic format when you want to test isomorphic features or in production mode builds.

How to Debug When Prerendering Fails

If prerendering fails, there will be a stack trace printed to the console and the build will continue without prerendering. It’s useful to build in development mode so that you can use the stack trace to determine where in the code the issue lies without any minification getting in the way.

Generally, when a prerender fails, it’s due to window or document being used during initial state creation (prior to mounting). Leave all access of those globals until after mount or wrap in an if-statement to check global variable existence.

Important Note: Prerendering requires an app to be coded such that it does not require access to window or document to create its initial state. The act of prerendering take place in a Node-based environment, so neither window nor document are available. If your app must use document or window in creation of its initial state, be sure to wrap those in if-statements to avoid prerender failure. For example:

   if(typeof window !== 'undefined') {
       // able to access window

Prerendering in Specific Locales

While the app content built with Enact is designed to run for all locales, the prerendered index.html content is based on the en-US locale. While the page should load correctly on other locales, the en-US-based content will be visible to the user until the app is fully loaded/executed. Enact CLI provides command line options to alter and customize the locales that should be prerendered. The -l/--locales option accepts a comma-separated list of locales, path to a file containing a JSON array of desired locales, or a preset name (e.g. tv, signage, none, etc.). For example:

enact pack -i --locales=en-US,ko-KR
enact pack -i --locales=my-locales.json
enact pack -i --locales=tv

When multiple locales are specified for prerendering, each locale is evaluated into its own HTML file (index.en-US.html, index.ko-KR.html, etc.) with a non-prerendered index.html file as a fallback. Deduping is applied to simplify and minify the number of HTML files output, simplifying by root language and across languages themselves. Additionally, with full webOS support, locale-specific appinfo.json files will be generated to correctly target locale-specific app entry points.

How It Works

With an isomorphic build, the app is built in a special pseudo-library layout, in a universal module definition (UMD) format. In a Node environment, the top-level export is the ReactElement export of the ‘isomorphic’ file. In a browser environment, the app executes normally.

During the build process, a custom webpack plugin, PrerenderPlugin, will access the build within its Node environment and use React’s ReactDOMServer to render the initial state of the app into an HTML string and inject that into the index.html file, within the root ID div element. This is the same API used in server-side rendering.

When the webpage loads up in a browser environment, the built JavaScript is loaded normally (and is expected to render itself into the HTML), except React will detect the DOM tree and will simply attach event listeners and go through the React lifecycle methods.