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useTransition is a React Hook that lets you update the state without blocking the UI.
const [isPending, startTransition] = useTransition()
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();
// ...
}
Parameters
useTransition does not take any parameters.
Returns
useTransition returns an array with exactly two items:
- The
isPendingflag that tells you whether there is a pending Transition. - The
startTransitionfunction 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 moresetfunctions. React immediately callsscopewith no parameters and marks all state updates scheduled synchronously during thescopefunction call as Transitions. They will be non-blocking and will not display unwanted loading indicators.
Returns
startTransition does not return anything.
Caveats
-
useTransitionis 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 standalonestartTransitioninstead. -
You can wrap an update into a Transition only if you have access to the
setfunction of that state. If you want to start a Transition in response to some prop or a custom Hook value, tryuseDeferredValueinstead. -
The function you pass to
startTransitionmust 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:
- The
isPendingflag that tells you whether there is a pending Transition. - The
startTransitionfunction 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.
Example 2 of 2:
Updating the current tab without a TransitionIn this example, the “Posts” tab is also artificially slowed down so that it takes at least a second to render. Unlike in the previous example, this state update is not a Transition.
Click “Posts” and then immediately click “Contact”. Notice that the app freezes while rendering the slowed down tab, and the UI becomes unresponsive. This state update is not a Transition, so a slow re-render freezed the user interface.
[ARTIFICIALLY SLOW] Rendering 500 <SlowPost />
[ARTIFICIALLY SLOW] Rendering 500 <SlowPost />
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.
[ARTIFICIALLY SLOW] Rendering 500 <SlowPost />
[ARTIFICIALLY SLOW] Rendering 500 <SlowPost />
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.
Note
Transitions will only “wait” long enough to avoid hiding already revealed content (like the tab container). If the Posts tab had a nested <Suspense> boundary, the Transition would not “wait” for it.
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.
Note
Suspense-enabled routers are expected to wrap the navigation updates into Transitions by default.
Displaying an error to users with an error boundary
Canary
Error Boundary for useTransition is currently only available in React’s canary and experimental channels. Learn more about React’s release channels here.
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.
import { useTransition } from "react"; import { ErrorBoundary } from "react-error-boundary"; export function AddCommentContainer() { return ( <ErrorBoundary fallback={<p>⚠️Something went wrong</p>}> <AddCommentButton /> </ErrorBoundary> ); } function addComment(comment) { if (comment == null) { throw new Error("Example Error: An error thrown to trigger error boundary"); } } function AddCommentButton() { const [pending, startTransition] = useTransition(); return ( <button disabled={pending} onClick={() => { startTransition(() => { addComment(); }); }} > Add comment </button> ); }
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:
- 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.
- Alternatively, you can have one state variable, and add
useDeferredValuewhich 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 ...
}
}
I’m getting more 
Also I’m posting the final six pages of the latest 
Title thanks to stve3000 from the stream! 
