What does flushSync do, and why is it considered a last-resort React API?

Direct answer

flushSync forces React to synchronously flush qualifying updates and update the DOM before returning, which is useful for rare browser or third-party integration requirements but can hurt performance and interfere with normal scheduling.

print.js

function handleBeforePrint() {
  flushSync(() => {
    setIsPrinting(true);
  });

  // By here, the DOM reflects isPrinting.
  window.print();
}

Useful when: an external browser or third-party API requires the DOM to reflect a React update synchronously before the next statement.
Performance cost: it bypasses normal scheduling advantages and may force other pending work to flush.
Suspense impact: a synchronous flush can cause pending Suspense fallbacks to appear.