react-ssr-resolve-all.js

// Note, doesn't deal with timeouts, infinite recursion, errors etc..
async function resolveAll(render, promiseTracker) {
    const renderResult = render(promiseTracker)
    if (promiseTracker.hasWork) {
        await promiseTracker.waitForCompletion()
        return resolveAll(render, promiseTracker)
    }

    return renderResult
}

// And finally we can use it:
resolveAll(() => ReactDOM.renderToString(<App />), new PromiseTracker())