React Concurrent Mode allows your application to stay responsive even during heavy rendering or data fetching by interleaving, pausing, and resuming rendering tasks.
Let’s see it in action. Imagine a simple app with a list of items and a search input.
import React, { useState, useDeferredValue } from 'react';
function App() {
const [query, setQuery] = useState('');
const deferredQuery = useDeferredValue(query); // This is the key!
const items = ['Apple', 'Banana', 'Cherry', 'Date', 'Elderberry', 'Fig', 'Grape'];
const filteredItems = items.filter(item =>
item.toLowerCase().includes(deferredQuery.toLowerCase())
);
return (
<div>
<input
type="text"
value={query}
onChange={e => setQuery(e.target.value)}
placeholder="Search items..."
/>
<ul>
{filteredItems.map((item, index) => (
<li key={index}>{item}</li>
))}
</ul>
</div>
);
}
export default App;
When you type into the input, setQuery updates the query state. Normally, this would trigger an immediate re-render of the entire App component, including filtering the items. If items were a very large array, or the filtering logic was complex, this could make the input feel sluggish because the UI is blocked until the filtering and rendering are complete.
This is where useDeferredValue comes in. deferredQuery is a "deferred" version of query. When query updates, React doesn’t immediately re-render App with the new query. Instead, it schedules a lower-priority update for deferredQuery. This means React can continue to render the UI with the previous value of deferredQuery (so the input remains responsive) while it works on calculating the new deferredQuery and its associated filteredItems in the background.
The core problem Concurrent Mode solves is the "blocking" nature of traditional React rendering. In older React, a render was a single, indivisible unit of work. If it took 100ms to render, your browser was busy for 100ms, unable to handle user input or other tasks. Concurrent Mode breaks rendering into smaller chunks. It can start a render, pause it if a higher-priority task (like user input) comes along, and then resume the paused render later.
Internally, React uses a scheduler to manage these rendering tasks. When you call setState or useReducer’s dispatch, it doesn’t directly trigger a re-render. Instead, it creates a "task" and gives it a priority. The scheduler then decides which task to work on based on these priorities. User interactions (like typing in an input) are high-priority, while rendering a large list might be a lower-priority task. useDeferredValue explicitly tells React to treat the updates stemming from its value as lower priority.
The mental model to build is one of a cooperative multitasking operating system, but for your UI. React is the OS, and different rendering updates are processes. High-priority processes (user input, animations) preempt lower-priority ones (background data fetching, complex list rendering). useDeferredValue is like telling a process, "Hey, your work isn’t critical right now, do it when you have spare CPU cycles."
The mechanism behind useDeferredValue is that React will actually render the component twice in quick succession when the deferred value changes. First, it renders with the old deferred value, allowing the UI to remain interactive. Then, as soon as possible, it renders again with the new deferred value. This second render is what updates the filteredItems list. If the user types again before the second render completes, React might discard the in-progress second render and start over with the latest input.
This ability to interrupt and resume rendering is what unlocks features like useTransition and useDeferredValue, allowing you to create more fluid and responsive user experiences even when dealing with computationally intensive UI updates.
The next concept you’ll likely encounter is how to manage loading states explicitly during these deferred updates, which is precisely what useTransition is designed for.