The Impact of React 18 on Server-Side Rendering

Anton Ioffe - November 18th 2023 - 10 minutes read

In the dynamic frontier of modern web development, React 18 emerges as a game-changer, especially in how we approach server-side rendering. With the advent of its concurrency model, developers have at their fingertips a suite of revolutionary features that promise to refine performance, enhance user experience, and alter the very fabric of our development practices. This deep dive will guide you through the transformations and practical applications of these advancements, enabling you to not only grasp the nuances of the new server-side rendering paradigm but to effectively harness its potential in your sophisticated web solutions. Prepare for an exploration of code-laden insights and expert tips that will elevate your understanding and challenge you to rethink server-side rendering in the era of React 18.

React 18 and the Concurrency Model

React 18 introduces a concurrency model that revolutionizes server-side rendering by enabling a more flexible update mechanism known as Concurrent React. The cornerstone of this new model is the ability to start, interrupt, and resume rendering work, which allows for more efficient resource usage and improved user experience. Traditional React rendering paradigms operated in a synchronous and blocking manner, once an update commenced, it had to complete before the system could handle further interactions. However, with Concurrent React, this linear approach is abandoned. Rendering tasks become non-blocking and can be managed according to their priority, ensuring that the UI remains responsive.

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

const container = document.getElementById('app');
// Create a root.
const root = createRoot(container);
// Initial render: Render an element to the root.
root.render(<App tab="home" />);

In the code above, we see a move away from the legacy ReactDOM.render() API to the createRoot() function with Concurrent React. This change signifies the opt-in nature of the concurrent features. The moment you use createRoot(), you open the door to Concurrent React's features, although you still have control over when to utilize concurrent capabilities within your application. This design preserves compatibility with existing code and offers a gradual migration path.

Another aspect of Concurrent React's implementation detail is the internal use of mechanisms like priority queues and multiple buffering. These allow React to work on multiple states of the UI in parallel, leading to a more optimal use of system resources without exposing the complexity to the developer. It's a significant behind-the-scenes enhancement that unlocks new features such as transitions and improved Suspense support, all without altering how developers write components.

function MyComponent() {
  // Start rendering MyComponent
  // ...

  // At this point, rendering can be paused or aborted
  // if a higher priority update comes in
  // ...

  // Resume and finish rendering MyComponent
  // ...

By taking a closer look at how a component like MyComponent might render under this new model, we can better grasp the benefits of interruptible rendering. A high-priority update, such as a user interaction, could suspend the ongoing rendering of MyComponent, which improves the overall responsiveness of the application. The rendering process would then be resumed or even aborted based on current priorities. It is worth noting that while many components will function correctly under Concurrent React with no adjustments, some might require minor revisions to align with the new concurrency features.

React 18's concurrency model offers substantial improvements but also introduces some intricacies with server-side rendering. The interruptible nature of concurrent rendering implies that components may behave differently in this new environment. In the server context, it becomes feasible to generate HTML streams that may be paused and resumed as additional data becomes available or user priorities change. This intrinsic capability of Concurrent React ensures the server can prepare complex user interfaces with minimal delay, effectively improving time-to-first-byte without compromising the interactivity of the final rendered result.

To grasp the full potential of Concurrent React, developers need to understand its implications. Now thorny scenarios, such as rendering large lists or fetching data at a granular component level, are managed more gracefully. Previously, such operations risked blocking the render thread and hindering user interactions. With React 18, developers now have an arsenal that includes concurrency, facilitating the crafting of SSR applications that truly approximate the fluidity and reactivity of their client-side counterparts. Through careful architecting and embracing Concurrent React, server-side rendering ascends to new peaks of performance without compromising on user experience.

Adopting New Concurrent Features in SSR

One critical step in incorporating concurrent features into SSR involves establishing a concise baseline of behavior by ensuring your application runs without modification on the latest environment. Only then should you begin the process of integrating new concurrent enhancements.

Utilizing <StrictMode> during development helps surface unintentional side effects and reinforces best practices for concurrent rendering. Remember to employ <StrictMode> wrapped around your components in the development build to catch potential issues early. However, be aware that <StrictMode> itself does not play a direct role in SSR and should not impact production code.

A major shift in the SSR workflow is moving from renderToString() to renderToPipeableStream(). The latter is a more powerful API offered by React 18, enabling streaming of content and better error handling mechanisms. To transition effectively, you should adapt your server code to utilize this new API. Here is a more complete example, handling possible errors that may arise during streaming:

import { renderToPipeableStream } from 'react-dom/server';

function renderApp(res) {
  let didError = false;

  const {pipeableStream} = renderToPipeableStream(<App />, {
    onShellReady() {
      // Headers can be set now since content stream will begin
      res.statusCode = didError ? 500 : 200;
      res.setHeader('Content-Type', 'text/html');
    onAllReady() {
      // The entire HTML shell is ready and sent to the browser 
    onError(error) {
      didError = true;

  setTimeout(() => {
    pipeableStream.abort(); // Abort rendering if it's not finished in timeout
  }, 5000);  // Set timeout as needed

When integrating concurrent SSR features, it's crucial to prevent any client-side code from being bundled with the server-rendered components to avoid hydration mismatches. Decouple components and employ patterns that keep client and server rendering responsibilities clear, such as lazy loading or dynamic imports, ensuring a seamless hydration process when the client takes over from the server.

Be mindful when shifting data-fetching patterns to align with concurrent features. Remember, Suspense-driven SSR enables independent streaming of different UI segments as their data becomes readable, necessitating changes to how you previously managed data-fetching and state.

Careful planning and methodical application of these concurrent SSR features can smoothly propel your application into the modern architecture that React 18 facilitates. How will your existing SSR setup change to accommodate these advanced patterns, and what steps will you take to mitigate the common hazards that come with server-side Suspense and streaming?

Suspense for Data Fetching with SSR

Utilizing Suspense with Server-Side Rendering (SSR) redefines the data fetching paradigm by offering strategic control over the loading experience. Previously, the entire page would render in a "waterfall" manner, waiting for all data to load before anything could be displayed. With Suspense, developers now have the flexibility to show immediate fallback content, signaling to the user that loading is in progress.

function App() {
    return (
        <Suspense fallback={<Spinner />}>
            <UserDetails />

Here, if <UserDetails /> awaits data at render, the <Spinner /> maintains user engagement. The challenge, however, lies in avoiding multiple loading indicators that detract from the experience. Thoughtfully orchestrating Suspense boundaries preserves a cohesive user journey while data is fetched in the background.

Implementing selective hydration, SSR streams render-ready chunks of the component tree, interweaving client-side interactivity as data arrives. This pattern balances server load with client-side dynamism.

function UserProfile({ userId }) {
    const { data, isLoading, error } = useUserData(userId);
    if (isLoading) return <SkeletonProfile />;
    if (error) return <ErrorFallback error={error} />;
    return <Profile userData={data} />;

function useUserData(userId) {
    // Assume fetchData is a hook managing state and side effects
    return fetchData(`/api/users/${userId}`);

In this real-world design, useUserData acts as a hook integrating Suspense for an incomparable user experience. Yet, developers must discern the proper application of this strategy. SSR should be reserved for critical content, while ancillary data might be best loaded client-side, optimizing server efficiency for a premium initial load experience.

Error management with Suspense in SSR is non-trivial. A robust pattern establishes a parallel path for error states akin to the loading fallbacks:

function ErrorBoundary({ children, fallback }) {
    return (
        <ErrorBoundaryPrimitive fallback={fallback}>

Encapsulating children with ErrorBoundary ensures user experience prevails, despite asynchronous errors. Devising a comprehensive fallback strategy for loading and errors alike safeguards a consistent facade under all conditions.

In sum, the thoughtful incorporation of Suspense alongside SSR unlocks a new realm of data fetching sophistication. It facilitates a progressive loading experience, judiciously employs server resources, and guarantees robust error handling—key components in fostering a seamless user interface. Apply these practices with discretion, considering their sophistication and the responsibility they introduce to your application's architecture.

Performance Enhancements and Memory Efficiency

HTTP streaming significantly enhances server-side performance in modern web applications by surmounting the limitations of traditional SSR. Previously, users experienced delay until the entire page had been served, impacting metrics like time to first byte. With the introduction of new SSR tech, HTML can be streamed in segments, allowing the browser to commence rendering immediately. This strategy significantly improves the largest contentful paint metric, delivering content to the user faster.

Efficiency in interactivity has been refined through selective hydration, which streamlines the manner in which JavaScript is loaded. By focusing on critical components first, this approach allows sections of the page to become interactive even as other parts are still loading. This partial hydration can lead to a marked boost in how responsive an application feels to end-users, especially when interacting with initial page loads.

Memory management has received a noticeable increase in efficiency. More internal fields are now purged when a component unmounts, mitigating the effects of potential memory leaks within application code. This proactive garbage collection is crucial in applications that persist over extended periods, as it prevents the incremental buildup of unused data that can impair performance.

Despite these advances, developers are required to adapt to the nuances of partitioning and optimization. Striking a balance between server-rendered content and client-side interactivity can introduce complexity. Developers must judiciously plan and implement rendering strategies to ensure performance enhancements are fully realized without encumbering the application’s architecture or maintainability.

Here's an example of an improved server-side rendering strategy using Express, a popular Node.js server framework:

import express from 'express';
import { renderToPipeableStream } from 'react-dom/server';
import React from 'react';
import App from './App';

const server = express();

server.get('*', (req, res) => {
  const { pipe } = renderToPipeableStream(<App />, {
    onShellReady() {
      res.statusCode = 200;
      res.setHeader('Content-type', 'text/html');
    onShellError(error) {
      res.statusCode = 500;
      res.end('<!doctype html><p>Error loading the App</p>');
    onAllReady() {
    onError(error) {
      if (!res.headersSent) {
        res.statusCode = 500;
        res.end('<!doctype html><p>Error loading the App</p>');
      } else {
        res.end('<!doctype html><p>Error occurred after streaming had started</p>');

const PORT = 3000;
server.listen(PORT, () => console.log(`Server is listening on port ${PORT}`));

In the improved example, streamed responses provide immediate access to content, setting the stage for a much more performant and user-centric application. This example also includes comprehensive error handling, ensuring that didError accurately reflects the state of the SSR process. By understanding the dependencies and rendering behavior of components, developers can effectively employ these SSR improvements. It becomes a matter of expertly navigating the interplay between server load times and client experiences to deliver the optimal balance.

Real-world Implications of Server Components and Streaming

In the evolution of complex web application development, Server Components significantly complement traditional server-side rendering by enabling certain components to render solely on the server. This mitigates the client-side bundle's size since bulky libraries or components no longer need to be shipped to the client. Consider a dashboard application that includes intricate charting libraries, which are not pertinent to client-side interactivity; Server Components permit these processor-intensive libraries to remain on the server, relieving the client from the burden.

// Dashboard.server.js
import HeavyChartingLibrary from 'heavy-charting-library';

// This function fetches the data and uses the heavy library to create chart data
function generateChartData(data) {
    return HeavyChartingLibrary.process(data);

// Assuming HeavyChartPresentation is a component that takes chartData and renders it
export function Dashboard({ data }) {
    const chartData = generateChartData(data);

    // Render the complex chart as static HTML through HeavyChartPresentation
    return <HeavyChartPresentation chartData={chartData} />;

Such an approach profoundly affects the modularity and reusability of components, necessitating developers to judiciously separate what functions will be performed on the server from those essential on the client. It propels us to enforce stricter separation of concerns, leading us to ponder on how to architect applications that are both efficient and maintainable in the long haul.

Streaming server-rendered content introduces a paradigm shift where the server transmits content fragments that the browser immediately begins to render, thereby augmenting perceived performance. In a typical e-commerce platform, for instance, we can stream the framework of the product listing page while the server is still obtaining the finer details of individual products. With an optimized setup, the server can also intersperse placeholders for client-exclusive interactive elements, fostering a more fluid page loading experience.

// ProductList.server.js
async function fetchProducts(categoryId) {
    try {
        // Simulating an asynchronous call to fetch product data
        const response = await fetch(`/api/products?category=${categoryId}`);
        const products = await response.json();
        return products;
    } catch (error) {
        console.error('Failed to fetch products:', error);
        return [];

export async function ProductList({ categoryId }) {
    const products = await fetchProducts(categoryId); // Data fetching on the server
    // map over products to create list items
    return (
            { => (
                <ProductListItem key={} product={product} />
            {/* Placeholder for client-side interactivity */}
            <ClientComponentPlaceholder />

The deep integration of Server Components raises pivotal questions about the trajectory of full-stack JavaScript development. How will this innovation influence how developers design databases, or the surface area of APIs if direct server-side access to databases becomes more prevalent? Are we observing a shift back toward more robust server layers, and how will this coexist with the demand for highly interactive, rich client experiences? The boundaries between client and server are blurring, challenging developers to reassess the best practices when constructing state-of-the-art web applications.


React 18 brings revolutionary changes to server-side rendering in JavaScript web development. The new concurrency model allows for non-blocking rendering tasks, enhancing performance and user experience. By adopting new concurrent features, developers can optimize their server-rendered applications and improve time-to-first-byte. However, they must be mindful of the nuances and complexities introduced by these changes. A challenging task for developers is to implement selective hydration and streaming techniques to balance server load and client-side interactivity effectively. By navigating this interplay, developers can achieve optimal performance and user-centric applications.

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