Exploring Concurrent Mode in React 18

Anton Ioffe - November 18th 2023 - 9 minutes read

As the digital tapestry becomes increasingly complex, React 18 heralds a new era with the introduction of Concurrent Mode—a silent revolution reaching into the heart of modern web development. This article will journey through the transformation Concurrent Mode prompts, from reimagining the rendering pipeline to fostering fluid user experiences that sing to the beat of concurrent operations. We'll navigate the practicalities of implementing this paradigm shift, wrangle the expanded capabilities of Suspense and transitions in data fetching, debunk myths ensnaring unwary developers, and look to the horizon to envision how React's evolution will sculpt the web of tomorrow. Gear up, senior developers, as we guide you through the concurrent frontier, where performance converges with possibility in the dynamic realm of React 18.

Concurrent Mode Unveiled: The Paradigm Shift in React Rendering

Concurrent Mode in React marks a significant architectural divergence from traditional synchronous rendering. In essence, it introduces a non-blocking rendering strategy that allows React to prepare new updates in the background without interrupting the user's experience. This asynchrony means that React can work on rendering a component, pause that work if a more urgent update like a user's input arrives, and resume it later. This is a fundamental departure from the synchronous approach, in which React would perform render operations in a single, unyielding pass, inevitably blocking user interactions for the duration of rendering.

The shift towards Concurrent Mode necessitates a reconsideration of the mental model underpinning React development. Developers must now account for the possibility of multiple render phases before committing to the DOM. This alters how state changes are handled and demands vigilance for side effects that could be triggered more than once. Debugging becomes more nuanced, as component states may not immediately lead to DOM updates. This demand for a shift in mindset is substantial, yet it enables a more resilient approach to creating interactive and performance-sensitive applications.

Architecturally, Concurrent Mode's underpinnings lie in a complete reconsideration of the reconciliation process. Through incremental rendering, React can process changes in discrete chunks, prioritizing updates based on user interactions and perceived performance impacts. This ability to interleave rendering work with browser events paves the way for smoother animations and uninterrupted user experiences, even in applications with complex state graphs and deep component trees.

By leveraging this non-blocking UI paradigm, intensive tasks no longer compromise responsiveness. The implications are profound: applications can start large data-fetching operations or complex UI updates without creating a sluggish user experience. With this mode, React essentially provides a better frame budget management, allowing developers to maintain high frame rates necessary for modern web applications while deferring less critical updates without sacrificing overall app performance.

One of the most significant impacts of Concurrent Mode is on rendering large lists and computing expensive operations within components. By breaking down updates into smaller, manageable pieces and prioritizing user-driven updates, React ensures the main thread remains unblocked and responsive. This is particularly crucial for maintaining interactivity on low-powered devices and in performance-critical applications where every millisecond counts. It is a leap forward in creating web applications that feel as seamless and fluid as native applications, and as developers adapt to this new mode of operation, the gap between web and native interactivity continues to narrow.

Implementing Concurrent Mode: From Theory to Practice

Transitioning an existing React application to Concurrent Mode begins with upgrading to React 18. For this, you can update your React and ReactDOM versions using the command:

npm install react@18 react-dom@18

Once you've upgraded, you'll need to replace the traditional ReactDOM.render with ReactDOM.createRoot. This is a critical shift as createRoot lays the groundwork for concurrent capabilities.

import { createRoot } from 'react-dom';

// Legacy way
// ReactDOM.render(<App />, document.getElementById('root'));

// New way with createRoot
const root = createRoot(document.getElementById('root'));
root.render(<App />);

Embracing Strict Mode is advisable when moving to Concurrent Mode. Wrap your application's root component in <StrictMode>. This doesn't enable Concurrent Mode, but it helps identify potential issues in your application that could conflict with Concurrent Mode's expectations, such as detecting side-effects during the render phase, deprecated methods, or legacy patterns.

import { StrictMode } from 'react';
import { createRoot } from 'react-dom';

const root = createRoot(document.getElementById('root'));
    <App />

As you implement Concurrent Mode, watch out for potential migration pitfalls. One common mistake is to overlook the invasive impact of legacy lifecycle methods (UNSAFE_componentWillMount, UNSAFE_componentWillReceiveProps, and UNSAFE_componentWillUpdate) which may not operate reliably in a concurrent environment. Refactoring to use new lifecycle methods or Hooks is a more robust approach.

Reflect on the architecture of your state management. Using external libraries not designed for Concurrent Mode could lead to inconsistent states due to the preemptive capabilities of Concurrent Mode causing rerenders at unexpected times. Therefore, assess the compatibility of state management libraries and consider migrating to React's built-in state management tools or alternatives that support Concurrent Mode.

Through these steps, you transition your application to leverage the new concurrent features in React 18. It's a structured process, but one that requires careful consideration of the more nuanced aspects such as life cycles, state management, and adherence to the latest patterns prescribed by the React team. As you refactor, question the dependencies and patterns that might impede the full benefits of Concurrent Mode. Are there any specific areas in your app, such as complex state transitions or third-party UI libraries, that may require close attention during the switch? How can you ensure your codebase is resilient against possible race conditions or inconsistencies that might emerge from concurrent rendering? Addressing these questions early in the migration will pave the way for a smoother transition and a more robust application posture in the concurrent era of React.

Concurrent Data Fetching: The Power of Suspense and Transitions

React 18 solidifies and enhances the capabilities of Suspense, expanding its utility in the realm of data fetching. Utilizing the <Suspense> component, developers can orchestrate complex rendering sequences, particularly beneficial when handling intensive operations. Suspense adopts a declarative strategy: you define a fallback UI to exhibit while waiting for the component's necessary data, avoiding the pitfalls of rendering an incomplete state and fostering a smoother user experience.

Pairing Suspense with the startTransition API enhances its concurrency benefits, aiding in the prioritization of user-centric interactions. Utilizing startTransition renders certain updates as low-priority, granting precedence to user actions for a seamless experience. This proves invaluable during tasks like state transitions in sluggish network environments, ensuring responsive input fields while postponing less critical updates.

Examine this practical code example, which showcases startTransition in a data fetching scenario:

import { useState, [useTransition]( } from 'react';
import { fetchData } from './api';

function SearchComponent() {
  const [inputValue, setInputValue] = useState('');
  const [isPending, startTransition] = useTransition();
  const [data, setData] = useState(null);
  const [error, setError] = useState(null);

  function handleSearchChange(e) {
    startTransition(() => {
        .then(response => {
        .catch(err => {

  return (
      {isPending ? <div>Loading...</div> : <SearchResults data={data} />}
      {error && <div>Error fetching data</div>}

In this snippet, the search input is rapidly processed, deferring the update of search results, thus preventing interface stalls. The isPending state clues us into ongoing transitions, enabling the rendering of a loading indicator during data retrieval without compromising input responsiveness. Notably, this code adheres to best practices by embracing error management, ensuring the UI gracefully handles problematic data fetching events.

For robust exception handling with asynchronous data fetches, leverage Suspense in conjunction with error boundaries. Enclosing Suspense-wrapped components within an error boundary ensures that any issues are managed discretely, yielding a durable UI capable of withstanding unexpected data retrieval failures or network lags.

To further refine your control over rendering, take advantage of useDeferredValue which interfaces smoothly with concurrent rendering functionalities. This hook lets you postpone heavy state updates, safeguarding crucial renderings from being obstructed by intensive operations. Here's an implementation of useDeferredValue:

import { useState, useDeferredValue } from 'react';

function ListComponent({ items }) {
  const [filter, setFilter] = useState('');
  const deferredFilter = useDeferredValue(filter, { timeoutMs: 2000 });

  const filteredItems = items.filter(item => 

  function handleFilterChange(e) {

  return (
      <input type="text" value={filter} onChange={handleFilterChange} />
      <LongListDisplay items={filteredItems} />

Utilizing this pattern allows for instantaneous updates in the filter field, while the more resource-intensive filtering process occurs post-interaction, maintaining a brisk and interactive user experience.

Common Misconceptions and Coding Traps in Concurrent Mode

When developers first explore the capabilities of Concurrent Mode in React 18, many fall prey to the idea that "double render" is a sign of inefficiency or a bug in their code. In truth, this is a deliberate feature during development, designed to expose side-effects in the pre-commit phase, which can then be eliminated or controlled for a more predictable UI. By recognizing its purpose to enhance reliability, developers can avoid misguided attempts at optimization that stem from a misinterpretation of its intent.

Developers accustomed to synchronous rendering patterns sometimes incorrectly assume that state updates with useState and useEffect will immediately reflect in the DOM in Concurrent Mode. This mismatch between expectation and reality could introduce race conditions. With renders in Concurrent Mode being interruptible, developers must leverage useTransition for state updates that can afford a delay, thus guaranteeing a responsive interface across various load conditions.

Here's a common pitfall related to side effects, illustrating both the mistake and the remedy:

// Incorrect:
useEffect(() => {
    const subscription = props.source.subscribe();
    return () => {
        // Improperly handled cleanup, leading to potential memory leaks
}, [props.source]);

// Corrected:
useEffect(() => {
    const subscription = props.source.subscribe();
    return () => {
        subscription.unsubscribe(); // Ensures side effects are cleaned up
}, [props.source]);

In the context of error handling, Concurrent Mode requires a strategic placement of error boundaries due to the nature of asynchronous rendering. Errors can arise unpredictably, and it's paramount to encapsulate at-risk code paths with error boundaries to maintain a robust and user-friendly UI in adverse conditions.

Developers sometimes fail to fully capitalize on the startTransition API, resulting in a less-than-ideal user experience. By encapsulating state updates that can be interrupted or deferred within startTransition, crucial interactions such as input responsiveness can be safeguarded. The essential inquiry here is, "Is the smoothness of this interaction critical to the current user experience?" If it's not fundamental, startTransition can be leveraged to optimize the interactivity of your app effectively.

Approaching development with Concurrent Mode's paradigms can catalyze tremendous enhancements in user experience, but it is crucial for developers to maintain awareness of the mode's nuances and to continually evaluate the impact of their decisions regarding state management and component lifespan.

The Future of React: Concurrent Mode and Beyond

Concurrent Mode's trajectory in React development suggests increasing emphasis on constructing interfaces that are not just visually appealing, but also exceptionally performant and responsive. As we ponder the rapid development of these concurrent features, an intriguing reflection emerges regarding the integration of server components into the React ecosystem. Server components promise more efficient data-fetching and streamlined client-side processes, yet their production readiness is a topic of spirited debate among developers. While leveraging these components can significantly slash the JavaScript bundle size and speed up application delivery, the complexity of maintaining hydration consistency and ensuring seamless interactivity pose tangible challenges. Developers must remain cognizant of the added architectural considerations and diligent with their testing strategies to fully harness the potential of server components without compromising user experience.

Progressing further, the ramifications of Concurrent Mode for substantial applications cannot be overstated. Large-scale applications stand to benefit immensely from Concurrent Mode's capability to manage intricate rendering tasks and prioritize updates effectively. However, this sophistication necessitates a fresh perspective on state management and a departure from conventional lifecycle practices. Recognizing the potential for unintended side effects and adopting a mindset that embraces mutable source management will be crucial for developers weaving concurrent patterns into their application architectures.

The evolution of cross-platform development tools like React Native under the influence of Concurrent Mode also holds considerable potential. One can envisage a future where the distinctions between developing for web and native platforms further blur, with performance optimizations and responsive patterns seamlessly transitioning across boundaries. This pursuit of uniformity and efficiency across platforms will likely demand continual refinements in the React codebase and drive innovations that cater to developer ergonomics.

Developer ergonomics, a pivotal aspect of web development, is poised for enhancement as Concurrent Mode evolves. The promise of reduced boilerplate, along with more intuitive patterns of side effect management and state transitions, may reshape the development experience. Yet, it is imperative to acknowledge that this evolution will introduce new paradigms that require adaptation—an investment in understanding that developers will need to make to capitalize on these advancements.

Engaging in discourse about the implications of Concurrent Mode, developers would be wise to juxtapose the excitement of these new capabilities with thorough deliberation on their practicality and long-term impact. Will Concurrent Mode's progressive rendering strategies become the new standard for web applications, and how will it shape the future responsibilities of web developers? As the thresholds of performance and user engagement are continuously redefined by such innovations, the community's collective experience and ongoing experimentation will serve as the foundation for mastering Concurrent Mode's profound capabilities.


In the article "Exploring Concurrent Mode in React 18," the writer discusses the introduction of Concurrent Mode in React 18 and its impact on web development. The article explains how Concurrent Mode allows React to prepare updates in the background without interrupting the user experience, leading to smoother animations and uninterrupted user interactions. The article also provides practical tips for implementing Concurrent Mode and utilizing features like Suspense and transitions in data fetching. The article challenges the reader to consider the implications of Concurrent Mode on their own codebase and to refactor their application to take advantage of the new features introduced in React 18.