@magento/venia-concept/src/index.js

import React from 'react';
import { render } from 'react-dom';

import store from './store';
import app from '@magento/peregrine/lib/store/actions/app';
import Adapter from '@magento/venia-ui/lib/components/Adapter';
import { registerSW } from './registerSW';
import './index.css';

// server rendering differs from browser rendering
const isServer = !globalThis.document;

// TODO: on the server, the http request should provide the origin
const origin = isServer
    ? process.env.MAGENTO_BACKEND_URL
    : globalThis.location.origin;

// on the server, components add styles to this set and we render them in bulk
const styles = new Set();

const tree = <Adapter origin={origin} store={store} styles={styles} />;

if (isServer) {
    // TODO: ensure this actually renders correctly
    import('react-dom/server').then(({ default: ReactDOMServer }) => {
        console.log(ReactDOMServer.renderToString(tree));
    });
} else {
    render(tree, document.getElementById('root'));
    registerSW();

    globalThis.addEventListener('online', () => {
        store.dispatch(app.setOnline());
    });
    globalThis.addEventListener('offline', () => {
        store.dispatch(app.setOffline());
    });
}