API Reference
ReactDOM APIs

flushSync

Pitfall

Using flushSync is uncommon and can hurt the performance of your app.

flushSync lets you force React to flush any updates inside the provided callback synchronously. This ensures that the DOM is updated immediately.

flushSync(callback)

Usage

Flushing updates for third-party integrations

When integrating with third-party code such as browser APIs or UI libraries, it may be necessary to force React to flush updates. Use flushSync to force React to flush any state updates inside the callback synchronously:

flushSync(() => {
  setState(true);
});
// By this line, the DOM is updated.

This ensures that, by the time the next line of code runs, React has already updated the DOM.

Using flushSync is uncommon, and using it often can significantly hurt the performance of your app. If your app only uses React APIs, and does not integrate with third-party libraries, flushSync should be unnecessary.

However, it can be helpful for integrating with third-party code like browser APIs.

Some browser APIs expect results inside of callbacks to be written to the DOM synchronously, by the end of the callback, so the browser can do something with the rendered DOM. In most cases, React handles this for you automatically. But in some cases it may be necessary to break outside of React and force a synchronous update.

For example, the browser onbeforeprint API allows you to change the page immediately before the print dialog opens. This is useful for applying custom print styles that allow the document to display better for printing.

In the example below, you use flushSync inside of the onbeforeprint callback to immediately “flush” the React state to the DOM. By doing this, when the print dialog opens, the state has been updated in isPrinting is “yes”: 

import { useState, useEffect } from 'react';
import {flushSync} from 'react-dom';

export default function PrintApp() {
  const [isPrinting, setIsPrinting] = useState(false);
  
  useEffect(() => {
    function handleBeforePrint() {
      flushSync(() => {
        setIsPrinting(true);
      })
    }
    
    function handleAfterPrint() {
      setIsPrinting(false);
    }

    window.addEventListener('beforeprint', handleBeforePrint);
    window.addEventListener('afterprint', handleAfterPrint);
    return () => {
      window.removeEventListener('beforeprint', handleBeforePrint);
      window.removeEventListener('afterprint', handleAfterPrint);
    }
  }, []);
  
  return (
    <>
      <h1>isPrinting: {isPrinting ? 'yes' : 'no'}</h1>
      <button onClick={() => window.print()}>
        Print
      </button>
    </>
  );
}

If you remove the call to flushSync, then when the print dialog will display isPrinting as “no”. This is because React batches the updates asynchronously and the print dialog is displayed before the state is updated.

Pitfall

flushSync can significantly hurt performance, and may unexpectedly force pending Suspense boundaries to show their fallback state.

Most of the time, flushSync can be avoided, so use flushSync as last resort.

Reference

flushSync(callback)

Call flushSync to force React to flush any pending work and update the DOM synchronously.

flushSync(() => {
  setState(true);
});

Most of the time, flushSync can be avoided. Use flushSync as last resort.

See examples above.

Parameters

  • callback: A function. React will immediately call this callback and flush any updates it contains synchronously. It may also flush any pending updates, or Effects, or updates inside of Effects. If an update suspends as a result of this flushSync call, the fallbacks may be re-shown.

Returns

flushSync returns undefined.

Caveats

  • flushSync can significantly hurt performance. Use sparingly.
  • flushSync may force pending Suspense boundaries to show their fallback state.
  • flushSync may run pending effects and synchronously apply any updates they contain before returning.
  • flushSync may flush updates outside the callback when necessary to flush the updates inside the callback. For example, if there are pending updates from a click, React may flush those before flushing the updates inside the callback.
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