useTransition

const [isPending, startTransition] = useTransition()

• Reference
  • useTransition()
  • startTransition function
• Usage
  • Marking a state update as a non-blocking Transition
  • Updating the parent component in a Transition
  • Displaying a pending visual state during the Transition
  • Preventing unwanted loading indicators
  • Building a Suspense-enabled router
  • Displaying an error to users with an error boundary
• Troubleshooting
  • Updating an input in a Transition doesn’t work
  • React doesn’t treat my state update as a Transition
  • I want to call useTransition from outside a component
  • The function I pass to startTransition executes immediately

Reference

useTransition()

Call useTransition at the top level of your component to mark some state updates as Transitions.

import { useTransition } from 'react';

function TabContainer() {
  const [isPending, startTransition] = useTransition();
  // ...
}

See more examples below.

Parameters

useTransition does not take any parameters.

Returns

useTransition returns an array with exactly two items:

1. The isPending flag that tells you whether there is a pending Transition.
2. The startTransition function that lets you mark a state update as a Transition.

startTransition function

The startTransition function returned by useTransition lets you mark a state update as a Transition.

function TabContainer() {
  const [isPending, startTransition] = useTransition();
  const [tab, setTab] = useState('about');

  function selectTab(nextTab) {
    startTransition(() => {
      setTab(nextTab);
    });
  }
  // ...
}

Parameters

• scope: A function that updates some state by calling one or more set functions. React immediately calls scope with no parameters and marks all state updates scheduled synchronously during the scope function call as Transitions. They will be non-blocking and will not display unwanted loading indicators.

Returns

startTransition does not return anything.

Caveats

• useTransition is a Hook, so it can only be called inside components or custom Hooks. If you need to start a Transition somewhere else (for example, from a data library), call the standalone startTransition instead.

• You can wrap an update into a Transition only if you have access to the set function of that state. If you want to start a Transition in response to some prop or a custom Hook value, try useDeferredValue instead.

• The function you pass to startTransition must be synchronous. React immediately executes this function, marking all state updates that happen while it executes as Transitions. If you try to perform more state updates later (for example, in a timeout), they won’t be marked as Transitions.

• A state update marked as a Transition will be interrupted by other state updates. For example, if you update a chart component inside a Transition, but then start typing into an input while the chart is in the middle of a re-render, React will restart the rendering work on the chart component after handling the input update.

• Transition updates can’t be used to control text inputs.

• If there are multiple ongoing Transitions, React currently batches them together. This is a limitation that will likely be removed in a future release.

Usage

Marking a state update as a non-blocking Transition

Call useTransition at the top level of your component to mark state updates as non-blocking Transitions.

import { useState, useTransition } from 'react';

function TabContainer() {
  const [isPending, startTransition] = useTransition();
  // ...
}

useTransition returns an array with exactly two items:

1. The isPending flag that tells you whether there is a pending Transition.
2. The startTransition function that lets you mark a state update as a Transition.

You can then mark a state update as a Transition like this:

function TabContainer() {
  const [isPending, startTransition] = useTransition();
  const [tab, setTab] = useState('about');

  function selectTab(nextTab) {
    startTransition(() => {
      setTab(nextTab);
    });
  }
  // ...
}

Transitions let you keep the user interface updates responsive even on slow devices.

With a Transition, your UI stays responsive in the middle of a re-render. For example, if the user clicks a tab but then change their mind and click another tab, they can do that without waiting for the first re-render to finish.

Updating the parent component in a Transition

You can update a parent component’s state from the useTransition call, too. For example, this TabButton component wraps its onClick logic in a Transition:

export default function TabButton({ children, isActive, onClick }) {
  const [isPending, startTransition] = useTransition();
  if (isActive) {
    return <b>{children}</b>
  }
  return (
    <button onClick={() => {
      startTransition(() => {
        onClick();
      });
    }}>
      {children}
    </button>
  );
}

Because the parent component updates its state inside the onClick event handler, that state update gets marked as a Transition. This is why, like in the earlier example, you can click on “Posts” and then immediately click “Contact”. Updating the selected tab is marked as a Transition, so it does not block user interactions.

Displaying a pending visual state during the Transition

You can use the isPending boolean value returned by useTransition to indicate to the user that a Transition is in progress. For example, the tab button can have a special “pending” visual state:

function TabButton({ children, isActive, onClick }) {
  const [isPending, startTransition] = useTransition();
  // ...
  if (isPending) {
    return <b className="pending">{children}</b>;
  }
  // ...

Notice how clicking “Posts” now feels more responsive because the tab button itself updates right away:

Preventing unwanted loading indicators

In this example, the PostsTab component fetches some data using a Suspense-enabled data source. When you click the “Posts” tab, the PostsTab component suspends, causing the closest loading fallback to appear:

Hiding the entire tab container to show a loading indicator leads to a jarring user experience. If you add useTransition to TabButton, you can instead indicate display the pending state in the tab button instead.

Notice that clicking “Posts” no longer replaces the entire tab container with a spinner:

Read more about using Transitions with Suspense.

Building a Suspense-enabled router

If you’re building a React framework or a router, we recommend marking page navigations as Transitions.

function Router() {
  const [page, setPage] = useState('/');
  const [isPending, startTransition] = useTransition();

  function navigate(url) {
    startTransition(() => {
      setPage(url);
    });
  }
  // ...

This is recommended for two reasons:

• Transitions are interruptible, which lets the user click away without waiting for the re-render to complete.
• Transitions prevent unwanted loading indicators, which lets the user avoid jarring jumps on navigation.

Here is a tiny simplified router example using Transitions for navigations.

Displaying an error to users with an error boundary

If a function passed to startTransition throws an error, you can display an error to your user with an error boundary. To use an error boundary, wrap the component where you are calling the useTransition in an error boundary. Once the function passed to startTransition errors, the fallback for the error boundary will be displayed.

Troubleshooting

Updating an input in a Transition doesn’t work

You can’t use a Transition for a state variable that controls an input:

const [text, setText] = useState('');
// ...
function handleChange(e) {
  // ❌ Can't use Transitions for controlled input state
  startTransition(() => {
    setText(e.target.value);
  });
}
// ...
return <input value={text} onChange={handleChange} />;

This is because Transitions are non-blocking, but updating an input in response to the change event should happen synchronously. If you want to run a Transition in response to typing, you have two options:

1. You can declare two separate state variables: one for the input state (which always updates synchronously), and one that you will update in a Transition. This lets you control the input using the synchronous state, and pass the Transition state variable (which will “lag behind” the input) to the rest of your rendering logic.
2. Alternatively, you can have one state variable, and add useDeferredValue which will “lag behind” the real value. It will trigger non-blocking re-renders to “catch up” with the new value automatically.

React doesn’t treat my state update as a Transition

When you wrap a state update in a Transition, make sure that it happens during the startTransition call:

startTransition(() => {
  // ✅ Setting state *during* startTransition call
  setPage('/about');
});

The function you pass to startTransition must be synchronous.

You can’t mark an update as a Transition like this:

startTransition(() => {
  // ❌ Setting state *after* startTransition call
  setTimeout(() => {
    setPage('/about');
  }, 1000);
});

Instead, you could do this:

setTimeout(() => {
  startTransition(() => {
    // ✅ Setting state *during* startTransition call
    setPage('/about');
  });
}, 1000);

Similarly, you can’t mark an update as a Transition like this:

startTransition(async () => {
  await someAsyncFunction();
  // ❌ Setting state *after* startTransition call
  setPage('/about');
});

However, this works instead:

await someAsyncFunction();
startTransition(() => {
  // ✅ Setting state *during* startTransition call
  setPage('/about');
});

I want to call useTransition from outside a component

You can’t call useTransition outside a component because it’s a Hook. In this case, use the standalone startTransition method instead. It works the same way, but it doesn’t provide the isPending indicator.

The function I pass to startTransition executes immediately

If you run this code, it will print 1, 2, 3:

console.log(1);
startTransition(() => {
  console.log(2);
  setPage('/about');
});
console.log(3);

It is expected to print 1, 2, 3. The function you pass to startTransition does not get delayed. Unlike with the browser setTimeout, it does not run the callback later. React executes your function immediately, but any state updates scheduled while it is running are marked as Transitions. You can imagine that it works like this:

// A simplified version of how React works

let isInsideTransition = false;

function startTransition(scope) {
  isInsideTransition = true;
  scope();
  isInsideTransition = false;
}

function setState() {
  if (isInsideTransition) {
    // ... schedule a Transition state update ...
  } else {
    // ... schedule an urgent state update ...
  }
}