Optimizing Performance with React Suspense and Concurrent Mode

    Tuesday, May 14, 202410 min read106 views
    Optimizing Performance with React Suspense and Concurrent Mode

    Suspense is used to display a fallback until its children have finished loading. This feature in React makes it easy to manage loading times in your components. With Suspense, you can say "wait" for some code to load and clearly say what should happen while waiting. This makes your code simpler and gives users a smoother experience while things are loading.


    		<Suspense fallback={<Loading />}>
      			<SomeComponent />

    In the ever-evolving landscape of web development, React has remained a dominant force due to its efficiency, flexibility, and robust ecosystem. With each iteration, the React team introduces new features and optimizations to enhance user experiences and streamline development workflows. One such advancement is Concurrent Mode, coupled with the powerful Suspense mechanism.


    • Children: Children is the actual UI that is rendered.

    • fallback: If the user interface (UI) hasn't fully loaded yet, we can show something else instead. This could be a simple placeholder like a spinning loading symbol or an outline of where the content will be.

    Leveraging React Suspense for Data Fetching

    One of the most significant use cases of Concurrent Mode is optimizing fetching in React applications. Traditionally, fetching data in React involves asynchronous requests using techniques like callbacks or promises. While these methods work well, managing loading states and error handling can become cumbersome, leading to complex code and potential bugs.

    React Suspense provides a more elegant solution to this problem by allowing components to "suspend" rendering while waiting for data to arrive. Suspense components can specify a fallback UI to display during the loading state, improving the perceived performance of the application.

    Let's explore a simple example of fetching using Suspense and Concurrent Mode

    import React, { Suspense } from 'react';
    // Simulated asynchronous fetching function
    const fetchData = () => new Promise(resolve => setTimeout(resolve, 2000));
    // Component displaying fetched data
    const DataComponent = () => {
      // Simulating fetching with Suspense
      const data = fetchData();
      return <div>{data}</div>;
    // Component using Suspense for fetching
    const App = () => (
      <Suspense fallback={<div>Loading...</div>}>
        <DataComponent />
    export default App;

    In this example, the `DataComponent` suspends rendering while waiting for the data to be fetched. Meanwhile, the `Suspense` component displays a loading message (`Loading...`) as a fallback until the data arrives.

    Handling User Interactions with Suspense

    Suspense isn't limited to data fetching; it can also be used to handle other asynchronous tasks, such as user interactions or server rendering. For instance, Suspense can be employed to lazily load components, defer rendering until a specific event occurs, or handle transitions between different states of the application.


    import React, { Suspense, useState } from 'react';
    // Asynchronous user interaction
    const simulateUserInteraction = () =>
      new Promise(resolve => setTimeout(resolve, 1000));
    // Component for user interaction
    const InteractionComponent = () => {
      const [isClicked, setIsClicked] = useState(false);
      const handleClick = async () => {
        await simulateUserInteraction();
      return (
          <button onClick={handleClick}>Click Me</button>
          {isClicked && <div>Interaction Completed!</div>}
    // App component using Suspense for handling interactions
    const App = () => (
      <Suspense fallback={<div>Waiting for Interaction...</div>}>
        <InteractionComponent />
    export default App;

    In this example, the `InteractionComponent` suspends rendering while waiting for the user interaction to complete. The `Suspense` component displays a waiting message until the interaction is finished.

    Streaming server rendering

    It means sending HTML to the client as soon as it is ready, rather than waiting for the entire server-rendered page to be generated.Example of how Suspense can be used in streaming server rendering:

    1. Server-side rendering with suspense boundary

    In our server-side code, we have components that use Suspense to wait for data before rendering. These components will define a boundary where suspense can be triggered.

    	import React, { Suspense } from 'react';
    	import { renderToString } from 'react-dom/server';
    	import App from './App';
    	const html = renderToString(
     	 <Suspense fallback={<div>Loading...</div>}>
      	  <App />
    	// Stream the HTML to the client as it's generated

    2. Client-side Hydration

    Client-side Hydration allows client-side React to recognize the ReactDOM components which then rendered on the server. React will re-render the page which is initially occurs from the server, and update them if any changes occur.

    	import React from 'react';
    	import { hydrate } from 'react-dom';
    	import App from './App';
    	hydrate(<App />, document.getElementById('root'));

    Advantages of using Suspense fallback

    1. It is used to display fallback while content is in a loading state.

    2. It reveals the content together at once.

    3. It reveals the nested content as it loads.

    4. It shows stale content while fresh content is loading.

    5. It prevents already revealed content from hiding.

    6. Indicating that the transition is happening.

    7. Resetting Suspense boundaries on navigation.

    8. Providing a fallback for server errors and client-only content.

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    Optimizing Performance with React Concurrent Mode

    React Concurrent Mode is a groundbreaking addition to the React library that enables more responsive and fluid user interfaces by allowing components to render asynchronously.

    Traditionally, React updates its UI in a synchronous manner, which means that when a component re-renders, it blocks the main thread until the update is complete. This synchronous rendering model can lead to janky user experiences, especially when dealing with complex UIs or slow fetching processes.

    Concurrent Mode, on the other hand, introduces the concept of "time-slicing," where React can pause and resume rendering work to prioritize high-priority updates while keeping the UI responsive. This enables smoother transitions, faster loading times, and better overall performance, particularly on low-powered devices or networks.

    Concurrent Mode is a feature in React that allows for more responsive user interfaces by breaking down the rendering work into smaller chunks and prioritizing which parts of the UI to render first. It was introduced in React version 16. It used in React to pause, abort, or resume rendering updates based on their priority, thus ensuring that high priority updates, e.g. user interactions or animations are processed without delay, but in case of lower priority updates, it can be deferred or interrupted to prevent blocking the main thread.

    Concurrent features

    1. Prioritizing Updates: Concurrent Mode allows react to prioritize updates based on their importance. It ensures that higher priority updates are processed first, lower priority updates processed only after the higher priority updates. It improves the perceived performance of the application.

    2. Time slicing: Time slicing is a technique used in Concurrent Mode to break down the rendering work into smaller units called "time slices" or "tasks". These tasks are executed increamentally, allowing React to pause and resume rendering as needed without blocking the main thread.

    3. Suspense: Suspense is another feature of Concurrent Mode. It allows component to suspend rendering while waiting for asynchronous data to resolve. This helps in creating smoother user experiences by avoiding empty loader states or placeholder content.

    4. Error Boundaries: Concurrent Mode enhances error handling in React by introducing improvements to error boundaries. Error boundaries in Concurrent Mode can gracefully handle errors that occur during rendering, suspending, or resuming components.

    5. Improved User Experience: By enabling smoother rendering and prioritizing important updates, Concurrent Mode significantly improves the perceived performance and responsiveness of React applications, especially in scenarios involving complex UIs or heavy rendering workloads.

    In React, concurrent rendering revolutionizes how apps handle heavy loads and manage updates. Unlike traditional synchronous rendering, concurrent rendering allows React to multitask without blocking the main thread.

    Concurrent rendering prioritizes a fluid user experience, enabling React to pause ongoing rendering to address urgent tasks, like responding to user input. This ensures immediate responses to user interactions, even during extensive rendering tasks.

    Consider this straightforward example

    import { unstable_createRoot } from 'react-dom';
    const root = unstable_createRoot(document.getElementById('root'));
    root.render(<App />);

    Here, unstable_createRoot facilitates concurrent rendering for the entire app. This approach lets React prepare multiple UI versions in memory and swiftly switch between them based on user actions and network status.

    While concurrent rendering enhances responsiveness, it introduces complexity. Adapting to its nuances requires adjusting how you approach app updates and rendering. However, mastering it can elevate your React apps to new heights.

    How Concurrent Mode Differs from React 16

    In React 18, a significant update introduces a concurrent renderer, diverging from the synchronous renderer used in React 16. This new approach enables React to manage tasks more flexibly, including responding to user input while rendering.

    Previously, in React 16, rendering was strictly synchronous, continuing until the entire tree was rendered. This could lead to UI freezes if a large task blocks the main thread.

    In React 18, concurrent rendering offers multitasking capabilities. For instance, it can pause rendering to handle urgent tasks, ensuring a smoother user experience.

    Here's a simple comparison

    // React 16
    ReactDOM.render(<App />, document.getElementById('root'));
    // React 18
    const root = ReactDOM.createRoot(document.getElementById('root'));
    root.render(<App />);

    With React 18's concurrent mode, you gain new features like automatic batching of state updates, streaming server rendering, and Suspense for smoother loading states.

    Overall, React 18's concurrent mode marks a significant shift, making rendering faster, more responsive, and user-friendly.

    Concurrent React: Real-World Examples

    Now that we've talked about how concurrent mode works, let's see how it can improve user experiences in real-life scenarios.

    Example 1: Prioritizing User Input

    Concurrent mode lets React prioritize certain tasks, like handling user input. For instance, imagine a search bar filtering a large list of items as the user types. In a traditional setup, filtering might lag behind typing. But with concurrent mode, React can pause filtering to keep the input responsive.

    Example 2: Smooth Loading States with Suspense

    Another cool feature of concurrent mode is Suspense. It's great for creating smooth loading states in your components.

    Let's say you're fetching data from an API. Normally, managing the loading state manually can be clunky. But with Suspense, you can just tell React what to show while waiting for data.

    <Suspense fallback={<div>Loading...</div>}>
      <DataFetchingComponent />

    Here, if `DataFetchingComponent` is still fetching data, React shows the fallback loading message.

    These examples showcase how concurrent React improves user experiences. There's so much you can do with these new features, and I can't wait to see what you create!

    Handling Concurrency with Ease

    React introduces a concurrent renderer that works on multiple tasks at the same time, without blocking the main thread. In concurrent mode react interrupts an ongoing task to handle a new task that is more urgent than the current task.

    import { startTransition } from "react";
    function MyComponent() {
      const [state, setState] = useState(initialState);
      function handleClick() {
        startTransition(() => {
      // ...

    In this example, we are using the startaTransition function to mark a state update as a transition. This is useful to tell React that this update is non-urgent and this can be interrupted if there is another urgent task.

    Concurrent Rendering and Concurrent Features in React 18

    You can improve your app's performance using concurrent rendering by declaring some states as non-important or we can say that non-urgent. The new concurrent features in your app will work automatically in the specific areas that use them since these features were built on top of concurrent rendering.

    1. StartTransition API

    It is used to keep the app responsive without blocking user interaction by allowing us you to mark specific updates as transitions.


        import { startTransition } from 'react';
        // Urgent
        // Mark any state updates inside as transitions
        startTransition(() => {
          // Transition

    2. useTransition API

    By using useTransition API React can track and update pending state transitions with the usePending flag. It helps to display loading feedback to users so that users will know that something is happening in the background.


     import { useTransition } from 'react';
        const [isPending, startTransition] = useTransition();
        {isPending && <Spinner />}

    3. useDefferedValue API

    This API helps keep the UI responsive by letting React delay updates to slow parts of the screen. For instance, if one part of the UI updates quickly and another part takes longer, React can hold off updating the slower part, showing its old value while other components update.


    import { useState, useDeferredValue } from "react";
        function App() {
          const [input, setInput] = useState("");
          const deferredValue = useDeferredValue(text, { timeoutMs: 3000 }); 
          return (
              <input value={input} onChange={handleChange} />
              <MyList text={deferredValue} />

    Final Thoughts

    Suspense and Concurrent Mode represent significant advancements in React's capabilities, empowering developers to create more responsive and performant applications. By leveraging these features, developers can streamline data fetching, handle user interactions seamlessly, and optimize rendering processes. Whether it's improving loading states, managing asynchronous tasks, or enhancing the overall user experience, Suspense and Concurrent Mode offer a robust toolkit for tackling performance optimization challenges in modern web development.

    As you continue your journey with React, consider integrating Suspense and Concurrent Mode into your projects to unlock their full potential. With their ability to simplify complex asynchronous workflows and enhance perceived performance, these features will undoubtedly lead to better user experiences and more efficient development workflows. Keep exploring, experimenting, and embracing the latest advancements in React to stay at the forefront of web development innovation.

    Remember, the key to mastering Suspense and Concurrent Mode lies in understanding their concepts, experimenting with different use cases, and staying informed about the latest updates from the React team. With dedication, practice, and a willingness to embrace new technologies, you'll be well-equipped to harness the full power of React and build exceptional web applications that delight users and developers alike.


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