There is no global “Concurrent Mode” switch in current React. Instead, use APIs such as useTransition and useDeferredValue to let non-urgent work render without making urgent interactions wait. Add a deliberate <Suspense> boundary when a subtree can suspend, and check your React version and framework integration before adopting examples.
What “Concurrent Mode” means in current React
“Concurrent Mode” is a useful search phrase, but it is not a single setting to enable. React 18 introduced concurrent features through APIs, including startTransition and useDeferredValue; its launch materials described those features as available after upgrading, while noting that frameworks and libraries would often apply them on developers’ behalf. See the React 18 release post and the historical React 18 upgrade guide.
For a current app, first identify the interaction that feels slow, then choose the API that matches it. Current React documentation includes additional APIs, so examples and availability can differ by installed React version and integration. Check the built-in APIs reference and your framework or router’s documentation rather than treating a React 18-era example as a universal setup recipe.
Choose the API that fits the work
| Your situation | Start with | Why |
|---|---|---|
| You own a state update that should be non-urgent | useTransition, or standalone startTransition where appropriate |
Marks the state update as a Transition. The Hook also exposes pending status. Keep controlled text-input updates outside it. |
| A changing value feeds a costly region that can update later | useDeferredValue(value) |
The urgent part can use the latest value while the downstream region catches up. It does not impose a fixed delay or prevent extra network requests. |
| A subtree suspends while content is unavailable | <Suspense fallback={...}> |
Sets where a fallback appears. A Transition can preserve already revealed content during a non-urgent update. |
| Navigation suspends in a framework or router | The integration’s documented navigation behavior | React expects Suspense-enabled routers to wrap navigation updates in Transitions; confirm that your specific library and version support this. |
Use useTransition for state you control
Call useTransition at the top level of a component or custom Hook. It returns an isPending flag and a startTransition function. Wrap the state setter for work that should be non-blocking:
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import { useState, useTransition } from 'react';
function PageTabs({ pages }) {
const [page, setPage] = useState('overview');
const [isPending, startTransition] = useTransition();
function selectPage(nextPage) {
startTransition(() => {
setPage(nextPage);
});
}
return (
<>
<nav>
{pages.map((item) => (
<button key={item.id} onClick={() => selectPage(item.id)}>
{item.label}
</button>
))}
</nav>
{isPending && <span role="status">Updating…</span>}
<PageContent page={page} />
</>
);
}Use pending status for modest feedback, such as a status message or a visual indication that content is updating. A Transition lets React interrupt its non-blocking render when a more urgent update arrives; it does not make expensive rendering free. Keep controlled input state urgent: React’s useTransition reference states, “Transition updates can’t be used to control text inputs.”
For code outside a component or custom Hook, use the standalone startTransition API documented by React instead of calling useTransition outside its valid context.
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Mind the synchronous scope
A Transition covers state updates made synchronously inside its callback. If an update occurs after an await, the current useTransition reference says it needs another startTransition wrapper. Likewise, a state update scheduled in a timer is outside the original synchronous scope. React labels the post-await behavior a limitation it plans to fix, so check the live reference when relying on this detail.
Use useDeferredValue to let a costly region catch up
Use useDeferredValue when a value changes urgently but a downstream part of the UI can render later. For example, keep a search field responsive by updating its controlled value immediately and passing a deferred version to the result list:
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import { useDeferredValue, useState } from 'react';
function Search({ items }) {
const [query, setQuery] = useState('');
const deferredQuery = useDeferredValue(query);
return (
<>
<input
value={query}
onChange={(event) => setQuery(event.target.value)}
aria-label="Search"
/>
<SearchResults items={items} query={deferredQuery} />
</>
);
}The input renders with the latest query; the results subtree receives deferredQuery and may catch up in a later render. This is not a debounce, request throttle, or fixed timeout. React says, “There is no fixed delay caused by useDeferredValue itself,” and “useDeferredValue does not by itself prevent extra network requests.” If each keystroke triggers a request, add an appropriate request strategy separately. See the useDeferredValue reference.
Place Suspense boundaries where fallback content makes sense
When a child suspends, the closest parent <Suspense> boundary displays its fallback. Put boundaries around meaningful regions and choose fallbacks that make sense if that region is temporarily unavailable. Suspense does not automatically fetch arbitrary data: the data source must integrate with Suspense through the framework or library environment. Consult the Suspense reference for the supported behavior.
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If an update is non-urgent, marking it as a Transition can prevent already revealed content from being replaced by a fallback while React works on the update. A newly rendered Suspense boundary can still show its fallback. This distinction helps avoid blanking out content users are already viewing while allowing genuinely new content to show its loading state.
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Verify version and framework support before adopting an example
- Check the installed React version and whether the API you plan to use is available there.
- For routing or data loading, follow the documentation for your specific framework or router version; integrations may apply Transitions or Suspense behavior for you.
- Use the current React references for API details. The React 18 release post and upgrade guide describe the 2022 launch and are historical context, not a guarantee about every present-day integration.
- For newer APIs such as
ViewTransition, consult the current ViewTransition reference rather than assuming they are part of every app’s supported setup.
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