Understanding useDeferredValue in React

What is useDeferredValue?

useDeferredValue allows you to delay updating a state or prop value, improving UI responsiveness during heavy rendering tasks. It helps prevent laggy UI updates by letting React handle expensive rendering operations when the browser is free.

Example Code

import { Suspense, useState, useDeferredValue } from "react";
import SearchResults from "./SearchResults.js";

export default function App() {
  const [query, setQuery] = useState("");
  const deferredQuery = useDeferredValue(query);

  return (
    <>
      <label>
        Search albums:
        <input value={query} onChange={(e) => setQuery(e.target.value)} />
      </label>
      <Suspense fallback={<h2>Loading...</h2>}>
        <SearchResults query={deferredQuery} />
      </Suspense>
    </>
  );
}

In this example:

• query updates immediately as you type.
• deferredQuery lags behind and updates only when the browser has time.

How useDeferredValue Works

1. Initial Update:

   • When you type "ab", React first updates the query to "ab" but keeps the deferredQuery as "a" for now.
   • The deferredQuery value is delayed, so it "lags behind" the actual query.

2. Background Update:

   • React then tries to update both query and deferredQuery to "ab".
   • If the data for "ab" is ready, React will show the updated results.
   • If the data isn’t ready, React will keep showing the old value ("a") until the data is loaded.

3. Interruptible Rendering:

   • If you type again, React will stop the current update and start fresh with the latest value.
   • React always works with the newest value you provide.

4. Network Requests:

   • React still makes a network request for every keystroke.
   • What’s delayed is showing the results, not the network request itself.
   • Responses are cached, so pressing Backspace won’t trigger a new request.

---

Showing Outdated Content

To show that the latest query is still loading, you can dim the list when query and deferredQuery don’t match:

<div style={{ opacity: query !== deferredQuery ? 0.5 : 1 }}>
  <SearchResults query={deferredQuery} />
</div>

• This visually indicates that the displayed results are not yet updated.
• You can also add a CSS transition to make the dimming feel smoother.

---

Problem Without useDeferredValue

Without useDeferredValue, React updates the state immediately with every keystroke. This can lead to the following issues:

1. Laggy UI:
   If rendering the result list is expensive (e.g., large datasets or complex components), the UI may become unresponsive while React processes each update.

2. Incomplete Display:
   Since the state updates instantly, React will try to show the results for every keystroke, even if the data is not yet ready. This can cause flickering or showing empty/partial results.

3. Unclear Feedback:
   The user might not know if the results are still loading or if the search is complete, leading to a confusing experience.

By using useDeferredValue, you allow React to prioritize immediate UI updates (like typing) over rendering expensive components, keeping the app responsive.

Check out this example to see how useDeferredValue affects rendering.

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Deferring vs Debouncing vs Throttling

Deferring (useDeferredValue) only delays updating the UI — it doesn’t limit the number of state updates or function calls. React will still process every state update and function call, but it shows the old value until the browser has time to update the UI with the latest value.

In contrast:

• Debouncing limits how often a function is called by waiting until the user stops triggering events.
• Throttling allows a function to run at most once in a fixed time interval.

So, deferring improves UI responsiveness, while debouncing and throttling reduce the number of function calls. 😎