Understanding React Transitions

27. May, 2023 5 min read Teach

Using the startTransition Function

Using startTransition is a powerful function that can significantly enhance your web application's performance and user experience.

This technique enables developers to optimize the rendering process and deliver a smoother, more responsive interface. Regardless of your level of expertise, grasping the concept of startTransition will empower you to build high-performing web applications that leave a lasting impression on users.

startTransition is a new function introduced in React 18. It serves as a performance optimization tool, allowing React to batch updates and prioritize rendering more efficiently. It is valuable when dealing with asynchronous updates or rendering multiple components simultaneously.

What does it do?

Traditionally, a state update in React triggers a re-rendering of affected components, leading to a potential performance overhead, especially when dealing with complex UIs or frequent updates. However, startTransition solves this problem by intelligently batching and prioritizing updates.

When you wrap a code section with the function, React treats it as a low-priority update. Instead of rendering immediately, React defers the rendering until the end of the transition, allowing higher-priority updates to take precedence. This approach minimizes unnecessary re-renders and enhances the perceived performance of the application.

In its simplest form, startTransition is used to wrap a section of code where you expect updates to occur. By wrapping the code, React will delay rendering until the end of the transition, reducing unnecessary re-renders.

Let’s have a look at an example:

import { startTransition, useEffect, useState } from 'react';

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

  useEffect(() => {
    startTransition(() => {
      // perform asynchronous data fetching or state updates
      fetchData().then(result => {
  }, []);

  return <div>{data ? <App data={data} /> : <AppLoading />}</div>;

In the above example, the code inside startTransition will be treated as a low priority and rendered after higher priority updates. It prevents intermediate renders and ensures a smoother user experience.

The function becomes even more powerful in complex scenarios involving multiple components and state updates. Let’s consider a chat application where new messages arrive frequently. Without startTransition, each message update could cause unnecessary re-renders of all affected components, potentially causing a sluggish UI. However, with it, we can optimize the rendering:

import { startTransition, useEffect, useState } from 'react';

const ChatApplication = () => {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const messageSubscription = subscribeToNewMessages(message => {
      startTransition(() => {
        setMessages(prevMessages => [...prevMessages, message]);

    return () => {
  }, []);

  return (
      {messages.map(message => (
        <Message key={message.id} content={message.content} />

In this example, the arrival of a new message triggers an update using setMessages wrapped in startTransition. By doing so, React delays rendering until all high-priority updates are completed, resulting in a smoother experience for the user.

Why should I use it?

Under the hood, the function leverages React’s new concurrent rendering architecture, which enables React to work on multiple updates concurrently and prioritize them based on their importance. This concurrent mode is the foundation for smoother UI transitions and responsiveness in React applications.

Using startTransition strategically, you can optimize your applications by minimizing UI disruptions, reducing visual glitches, and ensuring a more fluid user experience, even in scenarios involving frequent state updates or asynchronous operations.

It’s important to note that startTransition is not a silver bullet for all performance issues. It is most effective when used selectively and in situations where timely updates will maintain the responsiveness of the user interface. It’s crucial to strike a balance between performance optimization and providing timely feedback to user interactions.

What are the caveats?

While startTransition offers significant benefits, it’s essential to understand its limitations and considerations:

  • Responsiveness: Delaying rendering can improve perceived performance but may also delay visual feedback for specific user interactions. Carefully consider the trade-off between performance and immediate response when deciding where to use it.
  • Usage Scenarios: It is most effective when dealing with non-essential updates or interactions that don’t require a quick response. Use it selectively in areas where its benefits outweigh potential drawbacks.
  • Error Boundaries: When using the function, error boundaries may not catch errors within the transitioned code. Ensure you have appropriate error-handling mechanisms in place to prevent unhandled errors.
  • Asynchronous Updates: If you have concurrent state updates within a transition block, React will batch them together and render them in the order they were called. Be mindful of the order of updates to maintain the expected behaviour.


React’s startTransition function is a powerful tool for optimizing rendering and improving user experience in React applications. Delaying non-essential updates ensures that high-priority updates are rendered first, resulting in smoother UI transitions. When used thoughtfully, it can significantly enhance the performance of your React components and contribute to a more responsive application. However, be aware of its caveats and make informed decisions on where to apply this optimization.

‘Till next time!