Understanding React 18: A Comprehensive Introduction

Anton Ioffe - November 18th 2023 - 10 minutes read

Embark on an exploratory journey through the transformative landscape of React 18 where traditional boundaries of web development are redrawn with innovative features and performance enhancements. This article offers seasoned developers a comprehensive introduction to the profound capabilities introduced in the latest iteration of React, promising to elevate user experiences and streamline development workflows like never before. From the multitasking prowess of Concurrent Mode to the server-side alchemy of React Server Components, prepare to navigate the nuanced world of automatic batching, and master the enhanced Suspense for a seamless migration to the cutting edge of front-end engineering. Join us as we delve deep into the heart of React 18, unlocking insights and strategies that will reshape the way you conceive, build, and optimize React applications in this ever-evolving web development era.

Harnessing the Power of Concurrent Mode

Concurrent Mode stands as a transformative addition to React's rendering process, orchestrating multiple UI versions simultaneously while ensuring that critical user interactions remain fluid and responsive. This advanced mode effectively prevents the blocking of the UI thread, a limitation that historically posed challenges during compute-intensive operations within large-scale applications. To leverage Concurrent Mode, developers must transition their applications to React 18 by updating React and ReactDOM dependencies and embracing the createRoot API which serves as the nexus for Concurrent Mode's functionality.

import { createRoot } from 'react-dom';

const rootElement = document.getElementById('root');
const root = createRoot(rootElement);
root.render(<App />); // Enables Concurrent Mode

In the snippet above, the traditional ReactDOM.render method is supplanted by createRoot, signaling React to engage in a non-blocking mode where render cycles can be interrupted and prioritized. This allows updates triggered by user interactions to leapfrog over tasks deemed less critical, thus reducing latency and enhancing the user experience with smoother transitions.

Concurrent Mode introduces a 'start-and-stop' mechanism of rendering that accords top priority to interactive updates. While it elevates the user experience by minimizing wait times, developers must navigate the nuances of this asynchronicity with care, particularly ensuring state consistency across the application to prevent 'tearing'. Tearing, the anomaly where different parts of an application render from divergent states, could lead to a disjointed UI if updates are asynchronous and disrupted.

const [data, setData] = useState(null);

// Hook accommodates concurrent rendering for data fetching
useEffect(() => {
    let didCancel = false;

    async function fetchData() {
        const result = await myApi.getData();
        // Prevent state update if component unmounted
        if (!didCancel) {


    // Cleanup function marks as canceled
    return () => {
        didCancel = true;
}, []);

In the code above, setData is only called if didCancel is false, mitigating the risk of updating an unmounted component. This pattern is indicative of concurrent-aware coding practices, which developers need to adopt for effective state synchronization.

Concurrent Mode has progressed beyond experimental into mainstream as of React's 18 release. Although transitioned to a fully-fledged feature, it continues to evolve, compelling developers to rethink prevalent coding patterns. The move to Concurrent Mode necessitates an assessment of existing applications, urging a migration towards concurrency-friendly patterns. As you reflect on your project's transition to this concurrent landscape, what parts of your code are most impacted? What strategies will you implement to harness the full potential of concurrency, ensuring that your application reaps the benefits of this high-performance rendering model?

Unveiling React Server Components

React Server Components herald a paradigm shift wherein the traditional boundaries between server-side and client-side rendering become more permeable, offering developers an optimal blend of both worlds. By adopting Server Components, it’s possible to decrease the size of JavaScript bundles shipped to the browser, thus expediting the time to interactive. Since these components are rendered on the server and shipped as HTML, the client only needs to parse and render a subset of the entire application, translating to quicker load times and potentially superior user experiences on resource-constrained devices.

Despite these attractive perks, there are trade-offs to consider. Server Components elevate the server's role in rendering, which necessitates a robust server infrastructure, especially under high traffic conditions. They also introduce additional complexity in application architecture, as developers must now judiciously decide which components to render on the server versus the client, taking into account factors like dynamic interactivity and real-time data requirements.

Consider a scenario where you have a complex dashboard with data-intensive components; converting some components to Server Components can reduce the client-side load.

// DashboardStats.server.js
import fetchStats from './statsFetcher';

function DashboardStats() {
    const stats = fetchStats(); // Assume this function fetches data server-side
    return (
            <h1>Performance Metrics</h1>
            {stats.map(stat => (
                <p key={stat.id}>{stat.label}: {stat.value}</p>

export default DashboardStats;

In the above example, DashboardStats is a server component designated by the .server.js extension. It fetches and renders performance statistics server-side, reducing the initial JavaScript load for the client.

One common mistake is to ignore the distinct lifecycle and capabilities of Server Components compared to Client Components. For instance, Server Components do not support client-side interactivity through event handlers like onClick. Attempting to do so will lead to errors. A correct approach is to utilize Client Components for interactive elements.

// A mistake - using an event handler in a Server Component
function IncorrectServerComponent() {
    return (
        <button onClick={() => console.log('Clicked!')}>Click me</button> // This will not work

// A correct approach - delegating interactivity to a Client Component
function InteractiveClientComponent() {
    return (
        <button onClick={() => console.log('Clicked!')}>Click me</button> // Event handlers work here

The interplay between server and client brings thought-provoking questions: How might we balance the server load without diminishing user experience? What are the best strategies for splitting an application's components between server and client? Reflecting on these can guide developers towards crafting applications that leverage React Server Components effectively without tipping the scales unfavorably in either direction.

Refining Performance with Automatic Batching

Automatic batching in React 18 revolutionizes how updates are processed, significantly refining application performance. By queuing multiple state updates within a single event loop tick, React efficiently groups them into a batch to minimize rendering cycles. For instance, consider an event handler that triggers multiple state changes. Traditionally, each state update might have led to its own rendering process, consuming valuable computation time and potentially leading to visible performance issues on the user's end. However, React 18's new mechanism consolidates these updates, thus reducing redundant render cycles and boosting overall responsiveness.

To appreciate the micro-level impact, let's illustrate with code. In React 17, using setTimeout to trigger back-to-back state updates, each call prompted an individual render, straining performance:

// React 17 behavior with setTimeout
setTimeout(() => {
  setCount(prevCount => prevCount + 1); // This would trigger the first render
  setCount(prevCount => prevCount + 2); // And this would trigger the second render
}, 1000);

Contrarily, in React 18, these updates are automatically batched, leading to a single re-render:

// React 18 behavior with setTimeout
setTimeout(() => {
  setCount(prevCount => prevCount + 1); // Both updates batched for a single render
  setCount(prevCount => prevCount + 2);
}, 1000);

Beyond individual event handlers, automatic batching excels in complex scenarios involving promises, asynchronous operations, or other events outside of React's ecosystem. It offers a macro impact by transforming the handling of entire sequences of updates—previously staggered and rendering-intensive—into seamless, behind-the-scenes affairs. This macro optimization means that intensive processes, like fetching and updating data upon user interaction, can occur without interrupting the user experience. The result is a smoother interface and quicker perceived load times, imparting a sense of immediate response to user actions.

Importantly, while automatic batching is now the default, developers have the option, using flushSync, to opt-out of this behavior for scenarios where immediate state updates and renders are critical. An appropriate use case for opting out might be when maintaining synchronization between React and non-React code is necessary, ensuring that an update within the React component lifecycle is applied immediately. Here's an example of flushSync in action:

import { flushSync } from 'react-dom';

// Using flushSync to force an immediate render for specific state update
flushSync(() => {
  setCount(prevCount => prevCount + 1); // State update inside flushSync causes an immediate render 
// Subsequent updates follow React 18's automatic batching rules
setCount(prevCount => prevCount + 2);

Lastly, while automatic batching is a robust optimization technique, its benefits are most observable when it reduces the render cycles of multiple, rapid state updates. It's less impactful when state updates are naturally spread out over time or are few in number. Developers must assess whether the changes in their application could truly leverage the potential of automatic batching. Profiling application performance before and after the migration to React 18 is essential to quantify improvements. Such an analysis will reveal the real-world impact of automatic batching and help maintain the best practices for state management, ensuring a performant application that delivers an excellent user experience.

Elevating UI with Enhanced Suspense

React 18 extends Suspense's utility well beyond its initial use case with React.lazy, enabling components anywhere in the component tree to be suspended during data fetching. This enhancement opens up new opportunities for creating responsive user interfaces but simultaneously introduces more complex patterns that developers must navigate, especially regarding error handling.

Suspense's integration with React's concurrent features significantly benefits user experience by preventing disruptive state changes during transitions such as data loading or animations. Utilizing the useTransition hook alongside Suspense allows existing content to remain visible while new data is being fetched, providing a smoother transition to new content without a jarring fallback experience. This requires developers to have a profound understanding of React's concurrent capabilities.

Consider this illustrative example demonstrating Suspense with useTransition:

import React, { Suspense, useState, useTransition, useEffect } from 'react';
import { fetchData } from './api';

const DataComponent = ({ data }) => <div>{data}</div>;

function App() {
    const [startTransition, isPending] = useTransition();
    const [data, setData] = useState(null);

    useEffect(() => {
        if (!data) {
            startTransition(() => {
                .catch((error) => {}); // Error handling is essential here
    }, [data]);

    return (
            <button onClick={() => startTransition(() => setData(null))} disabled={isPending}>
                {isPending ? 'Loading...' : 'Load Data'}
            <Suspense fallback={<div>Loading...</div>}>
                {data && <DataComponent data={data} />}

In the case of data fetching errors, it's critical to handle exceptions with a functional component error boundary. Given React's shift towards functional components and hooks, here's how you could handle errors using the ErrorBoundary hook:

import React, { Suspense, useState, useCallback } from 'react';

const useErrorBoundary = () => {
    const [hasError, setHasError] = useState(false);

    const handleError = useCallback(() => {
    }, []);

    const ErrorFallback = () => <div>Error occurred while loading data.</div>;

    return [hasError, handleError, ErrorFallback];

function AppWithBoundary() {
    const [hasError, handleError, ErrorFallback] = useErrorBoundary();

    return (
        <Suspense fallback={<div>Loading...</div>}>
            {hasError ? <ErrorFallback /> :
                // Components likely to throw errors are rendered here

Using useErrorBoundary allows you to catch and handle errors seamlessly within functional components. It's crucial to interrogate and possibly revise your error handling strategies ensuring a more resilient UI.

With Suspense's enhancements in React 18, developers gain more declarative control over UI rendering, yet they must conscientiously handle the added complexity. As you explore these advancements, how can you evolve your application to leverage the full potential of Suspense while maintaining, or even improving, the robustness of user interactions?

Migrating to React 18: Strategies and Best Practices

Upgrading your application to React 18 comes with an array of strategic considerations designed to ensure minimal disruptions while taking advantage of feature enhancements. Your first step should align with compatibility checks for your existing dependencies. It's imperative to ensure they support, or are updated to be compatible with React 18. This usually involves perusing the release notes or documentation pertinent to these dependencies and performing the necessary upgrades. The insightful way forward involves a careful and informed update of your react and react-dom dependencies to the 18th version, typically involving npm or yarn commands.

With React 18, a significant new paradigm is introduced that may shift how your app handles rendering logic and data fetching. However, it would be prudent to stabilize your application on React 18 before diving into the adoption of new features. Once your application is running smoothly post-upgrade, consider taking up novel features one at a time. This strategic incremental adoption allows for focused testing and a better understanding of how each enhancement can be leveraged to optimize your application.

A common pitfall during migration is not addressing newly-introduced warnings and potential deprecations that might surface when you switch to React 18. These warnings are crucial indicators of practices or features that are no longer recommended or might be removed in the future. It's essential to allocate time to resolve these warnings diligently, as they often guide you toward using the updated and more preferred APIs or strategies which are integral to the seamless functioning of your app in the new React ecosystem.

A vital part of migration is being mindful of your application’s performance. Your development toolkit should include React DevTools and performance profiling to monitor the app's behavior as you implement React 18's new features. This vigilance enables you to spot performance regressions or bottlenecks early on, allowing for timely optimizations that maintain, if not improve, the responsiveness and speed of your application.

Lastly, a best practice that should not be overlooked is educating your team on the changes and improvements introduced in React 18. A smooth migration is not only about the technical switch but also involves ensuring that your team is familiar with and capable of utilizing the full spectrum of the new features. This involves internal knowledge sharing, code reviews focusing on new practices, and possibly practice projects to solidify the concepts. Properly equipped with these new advancements, your team will be in an optimal position to enhance your applications and embrace the future of web development with React 18.


In this comprehensive introduction to React 18, developers are introduced to the transformative capabilities of this latest version. The article covers key features such as Concurrent Mode, React Server Components, automatic batching, and enhanced Suspense. The key takeaways include understanding how to harness the power of Concurrent Mode, how React Server Components can optimize performance, the benefits of automatic batching for refining performance, and how to elevate the user interface with enhanced Suspense. The challenging technical task for readers is to analyze their own applications and consider how to migrate to React 18, selecting appropriate features to adopt and ensuring compatibility with existing dependencies.

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