Concurrent Features in React 18: A Deep Dive

Anton Ioffe - November 21st 2023 - 10 minutes read

Dive into the beating heart of React 18 with our in-depth exploration of its groundbreaking concurrent features. In this comprehensive guide tailored for seasoned developers, we'll peel back the layers of React's concurrent rendering engine to reveal the potent blend of performance and user experience enhancements it can inject into your web applications. From the streamlined magic of automatic batching and the nuanced control of startTransition, to leveraging the powerful <Suspense> for data-heavy scenarios, and the subtle artistry of new hooks such as useId and useDeferredValue, this article will arm you with the knowledge to transform your applications into models of efficiency and responsiveness. If you're plotting your migration path to React 18, our strategic insights on upgrading will illuminate your trail, all the while sidestepping the common pitfalls that ensnare the unwary. Prepare to embark on a journey through React's concurrency, where expert insights and practical code examples lie in wait to elevate your development prowess to new heights.

Exploring Concurrent Rendering in React 18

React 18's concurrent rendering engine marks a transformative step in frontend development by enabling updates and rendering cycles that no longer lock users out during intensive tasks. This groundbreaking feature allows React to pause ongoing rendering to take care of more pressing matters, such as user interactions, thereby keeping the user interface smooth and responsive even during complex operations.

The concurrent renderer operates under an innovative strategy of splitting rendering tasks into smaller chunks and handling them based on their urgency. When a user interaction occurs that requires an immediate response, React swiftly interrupts the current render, addresses the interaction, and then resumes the render at the point of interruption. This approach of deferring less critical work behind user-triggered updates is pivotal to delivering an uninterrupted user experience.

Consider a scenario where we're managing an operation that could potentially be a long-running calculation or data fetch. With concurrent rendering, we'd need to ensure that the user interface remains interactive. The code to handle this asynchronously may look something like this:

// Fragment of a component that may cause a long calculation
function MyComponent() {
    const [data, setData] = useState(null);

    useEffect(() => {
        // Simulate a fetch that takes time to complete
        fetchData().then(response => setData(response));
    }, []);

    // The UI will remain responsive, rendering placeholders if necessary
    // ...render UI based on `data` and show loading state if data is null

In the snippet above, React will handle the asynchronous fetchData() operation in a way that does not block the main rendering process, ensuring that the UI remains interactive for other user inputs or events.

Using concurrency in rendering, React 18 ensures that the user interface is regularly updated and interactions remain seamless. The renderer's new ability to multitask based on priority rather than simply in a sequential order allows large tasks and API communications to happen in the background without rendering the UI unresponsive.

By prioritizing tasks aimed at enhancing the user experience, React's concurrent rendering feature paves the way for web applications that not only perform well but also respond to user inputs instantaneously. As a result, developers can craft complex, stateful applications that maintain elegance and performance, setting new standards for web app responsiveness. This represents a significant milestone where the fluidity of the interface and instant feedback become the norm, forming the foundation for building upon the next generation of responsive web applications.

Advancements in Automatic Batching and StartTransition

In the realm of web development, React 18's enhancements to automatic batching have been a boon for performance. This feature ensures that multiple state updates, regardless of their origin within promises, timeouts, or native event handlers, culminate in a single re-render cycle. This consolidated process reduces the number of render phases and consequently minimizes the workload on the browser's rendering engine, promoting smoother user experiences. To elucidate:

const [counter, setCounter] = useState(0);

// Previously, these would each trigger a re-render
setTimeout(() => {
  setCounter(counter + 1);
  setCounter(prevCounter => prevCounter + 1);
}, 1000);

// Now, in React 18, they result in a single re-render

The automatic batching system discerns when state changes are correlated, orchestrating a single re-render that encapsulates all updates. This is particularly advantageous in scenarios involving complex state logic or interactions that span across various components.

The startTransition API introduces another layer of distinction by segregating updates into two categories: urgent and non-urgent. Urgent updates are tied to direct user interactions, like clicks, which demand immediate feedback. By contrast, non-urgent updates, such as data fetching or significant view changes, can often wait without degrading the user experience. This differentiation allows React to prioritize updates in a manner that aligns with the user's immediate expectations. For example:

const [inputValue, setInputValue] = useState('');
const [searchResults, setSearchResults] = useState([]);

function handleInputChange(event) {
  startTransition(() => {
    // This update can be interrupted if more urgent updates come in

In the above snippet, typing in an input field triggers an immediate state update, while the potentially time-consuming performSearch is wrapped in startTransition, indicating it can be deferred if more critical operations demand attention.

Notably, while automatic batching and transitions offer performance boosts, they may also require developers to rethink their approach to state management. Developers should be cautious to avoid triggering excessive updates within a single batch, which could lead to unintentional side effects or performance regressions. It's essential to consider the granularity of state updates and employ batching judiciously.

Moreover, startTransition demands an acute awareness of user interaction patterns. Erroneously marking urgent updates as non-urgent could foil the user's expectation for immediate feedback, leading to a perceived sluggishness in the user interface. Conversely, not utilizing startTransition for deferrable updates squanders the opportunity to keep the application responsive during heavier background tasks.

In conclusion, React 18's advancements in state update efficiency significantly improve the fluidity of user interfaces. Developers have at their disposal a nuanced toolkit to create highly interactive and performant applications. The interplay between automatic batching and startTransition forms an intricate dance of updates—a dance that, when choreographed well, results in a seamless and gratifying user experience. How might you refactor current React applications to leverage these enhancements optimally? Consider this question as you explore the potential for performance gains in your projects.

Unlocking for Data Fetching and Progressive Rendering

React 18's enhancement of the <Suspense> component has been a game-changer for data fetching and progressive rendering. Traditionally, developers had to manage loading states and asynchronous data fetching imperatively, often resulting in complex lifecycle management and suboptimal user experiences. <Suspense> now simplifies this by providing a declarative approach to handle asynchronous operations and render a fallback UI during the wait.

import { Suspense } from 'react';

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <MyDataFetchingComponent />

In this snippet, <MyDataFetchingComponent /> integrates data fetching and rendering logic. Should the data fetching take time, <Suspense> displays the specified fallback UI, offering a much smoother experience to the end-user. However, it's crucial to note that <Suspense> is not a data fetching library itself; its role is to suspend rendering until the components within are ready to be displayed.

The adoption of <Suspense> for server-side rendering is equally transformative. Server components now have the ability to defer heavier rendering to the client-side. Initially, the server sends a partial render—often including a site header, sidebar, and static content—while data-intensive sections are temporarily represented by fallback content, such as loaders or spinners. These placeholders keep the user engaged while the client-side fetches and renders the remaining content.

<Suspense fallback={<div>Loading complex list...</div>}>
  <HeavyDataComponent />

For developers, transitioning to this pattern means embracing the consistent usage of <Suspense> across client and server contexts. It's vitally important to resist rewriting Suspense-like mechanisms, as this leads to unnecessary complication and code duplication. Instead, leverage pre-integrated frameworks or libraries, such as Next.js or Relay, which adhere to best practices and reduce the likelihood of common coding mistakes like mixing client-side only logic in server-rendered components.

In practice, one must be wary of over-reliance on Suspense for every element of data fetching. While it's tempting to wrap each asynchronous component in <Suspense>, this can lead to a scattered user experience with multiple loaders. Consider the user's perspective and consolidate data fetching where possible, utilizing error boundaries to handle failed loads, and ensure progressive rendering prioritizes content according to relevance and urgency, thus providing a cohesive and fluid interaction.

Through <Suspense>, React 18 offers the means to create a more responsive and interactive web by optimizing data-rich pages with server-side rendering in conjunction with strategic client-side hydration. When implemented mindfully, it leads to an immediate and engaging user interface. Are there sections in your application that could benefit from this granular control over rendering? How might such optimizations improve your users' perception of application speed and reliability?

Utilizing New Hooks: useId and useDeferredValue

React 18's useId hook is an elegant solution for generating unique identifiers that remain stable across both server and client rendering, thereby addressing the common problem of mismatches during hydration in server-side rendered applications. Primarily, this hook is beneficial in scenarios involving accessible components that require stable IDs, such as when associating form labels with their corresponding inputs or creating complex ARIA relationships. A typical use case might look like this:

function MyInputComponent() {
    const id = useId();
    return (
            <label htmlFor={id}>Name:</label>
            <input id={id} name="name" type="text" />

With useDeferredValue, developers can significantly improve user experience when dealing with state updates that may lead to expensive re-renders. This hook allows developers to defer updates to the less critical parts of the UI while maintaining responsiveness. For example, in a search functionality where the rendering of filtered results is compute-intensive, useDeferredValue can be used to keep the input responsive while the list of results is being updated asynchronously:

function SearchComponent({ searchText }) {
    const deferredSearchText = useDeferredValue(searchText);
    const results = useSearchResults(deferredSearchText);

    return (
            <SearchInput searchText={searchText} />
            <SearchResults results={results} />

Here, the SearchResults component receives a deferred version of the search text and renders based on that, ensuring user typing isn't hindered by the potentially slow rendering of search results. This approach enhances the perceived performance even on lower-powered devices or situations with complex UI, such as rendering large data sets or sophisticated visualizations.

However, it's important to understand that useDeferredValue is not a silver bullet for all performance issues. Its use should be carefully considered in the context of the user experience. Overusing it in situations where real-time updates are crucial would be detrimental and could lead to confusing UI states. It’s best suited for situations where updates do not require immediate user feedback or where displaying stale data for a short period of time is acceptable.

One common mistake when working with useDeferredValue is neglecting the timeout configuration option, which can cause the deferred state to update too slowly or too quickly:

const deferredValue = useDeferredValue(value, { timeoutMs: 2000 });

Setting an appropriate timeout ensures that React understands the urgency of your updates and strikes a balance between performance and user experience. In the example above, specifying a 2000 millisecond timeout informs React to not defer updates for more than two seconds.

While these hooks can provide better performance and responsiveness, developers should consider their impact on code complexity and maintainability. Overuse or misuse can introduce new challenges, such as managing multiple states of truth within your components. Thus, it is crucial to weigh these factors carefully against the benefits in each use case.

Are you leveraging these new hooks to their full potential while maintaining clean and understandable code? How can useId and useDeferredValue enhance the components you are currently working on? Reflecting on these questions can help you integrate these hooks thoughtfully into your React applications for maximum benefit.

Strategies for Upgrading to React 18 and Embracing Concurrency

Upgrading to React 18 is a critical move to unlock the responsive power of the concurrent features. Begin this process by updating your application to the latest versions with npm install react@18 react-dom@18. It is imperative to ensure that all your third-party libraries are compatible to prevent runtime issues.

Enabling Strict Mode is a key step in preparing for concurrent features. It helps highlight potential problems that could arise with concurrency. Encapsulate the entire application within <StrictMode> for a robust start to working with concurrent functionalities:

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

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

Transition to Concurrent Mode by substituting ReactDOM.render with ReactDOM.createRoot. Take this opportunity to refactor class components into function components with hooks to align with the React 18 philosophy. Replacing lifecycle methods with hooks like useEffect will set you up for adopting concurrent patterns effectively:

import { useEffect } from 'react';
import { createRoot } from 'react-dom';
import App from './App';

const container = document.getElementById('app');
const root = createRoot(container);

function MyComponent() {
    useEffect(() => {
        // Side effects to run after render goes here

    // Rest of the functional component logic

root.render(<App />);

When designing your application to thrive in a concurrent environment, it's essential to distinguish between urgent and non-urgent updates. React 18 introduces startTransition for this purpose, allowing your application to maintain responsiveness during state transitions:

import { useState, useTransition } from 'react';

function SearchComponent() {
    const [inputValue, setInputValue] = useState('');
    const [isPending, startTransition] = useTransition();

    function handleSearchInput(input) {

        startTransition(() => {

    // You can use isPending to show a loader if the transition is delayed
    return (
            <input type="text" value={inputValue} onChange={(e) => handleSearchInput(} />
            {isPending ? <Loader /> : <SearchResults />}

As you move forward with adopting concurrent features in React 18, ensure thorough testing, particularly for side effects, state handling, and effect synchronization. Embrace the asynchrony brought by these features, and rearchitect your data fetching, state management, and side-effect logic. Adhering to these principles will dramatically improve interactivity and overall application performance, offering users a seamless and engaging experience.


In this article, we dive into the concurrent features of React 18 and explore how they enhance performance and user experience in web applications. We cover topics such as automatic batching, startTransition, , and new hooks like useId and useDeferredValue. Key takeaways include the benefits of concurrent rendering for maintaining a responsive UI, the use of automatic batching and startTransition for efficient state updates, the power of for data fetching and progressive rendering, and the potential of hooks like useId and useDeferredValue. As a challenge, consider refactoring your React applications to leverage these enhancements and optimize their performance and responsiveness.

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