facebook/react · error · Error

While flushing work, something yielded a value. Use an asser

Error message

While flushing work, something yielded a value. Use an assertion helper to assert on the log of yielded values, e.g. expect(Scheduler).toFlushAndYield([...])

What it means

unstable_flushAll() re-checks the yielded-value log after draining all queued work; if any task called Scheduler.log(value) during the flush it throws 'While flushing work, something yielded a value...'. The mock insists that tests which produce logged values declare them via an assertion helper such as expect(Scheduler).toFlushAndYield([...]) instead of a bare flush that would silently discard them.

Source

Thrown at packages/scheduler/src/forks/SchedulerMock.js:619

function unstable_clearLog(): Array<mixed> {
  if (yieldedValues === null) {
    return [];
  }
  const values = yieldedValues;
  yieldedValues = null;
  return values;
}

function unstable_flushAll(): void {
  if (yieldedValues !== null) {
    throw new Error(
      'Log is not empty. Assert on the log of yielded values before ' +
        'flushing additional work.',
    );
  }
  unstable_flushAllWithoutAsserting();
  if (yieldedValues !== null) {
    throw new Error(
      'While flushing work, something yielded a value. Use an ' +
        'assertion helper to assert on the log of yielded values, e.g. ' +
        'expect(Scheduler).toFlushAndYield([...])',
    );
  }
}

function log(value: mixed): void {
  // eslint-disable-next-line react-internal/no-production-logging
  if (console.log.name === 'disabledLog' || disableYieldValue) {
    // If console.log has been patched, we assume we're in render
    // replaying and we ignore any values yielding in the second pass.
    return;
  }
  if (yieldedValues === null) {
    yieldedValues = [value];
  } else {
    yieldedValues.push(value);

View on GitHub (pinned to eafeac097b)

Solutions

1. Replace the bare flush with expect(Scheduler).toFlushAndYield([...]) listing exactly the values the tasks log.
2. If the values are irrelevant, use unstable_flushAllWithoutAsserting() (optionally followed by unstable_clearLog()) instead of unstable_flushAll().
3. Use toFlushAndYieldThrough([...]) when only a prefix of the logged values is deterministic.
4. Delete stray Scheduler.log() calls left in shared helpers.

Example fix

// before
Scheduler.scheduleCallback(NormalPriority, () => Scheduler.log('hi'));
Scheduler.unstable_flushAll(); // throws: something yielded a value

// after
Scheduler.scheduleCallback(NormalPriority, () => Scheduler.log('hi'));
expect(Scheduler).toFlushAndYield(['hi']);

// If you do not intend to assert values, use the non-asserting variant up front.
Scheduler.unstable_flushAllWithoutAsserting(); // never throws on yielded values
Scheduler.unstable_clearLog(); // discard whatever was logged
// otherwise declare expectations explicitly:
// expect(Scheduler).toFlushAndYield(['a', 'b']);

try {
  Scheduler.unstable_flushAll();
} catch (e) {
  if (e instanceof Error && e.message.includes('something yielded a value')) {
    Scheduler.unstable_clearLog();
    Scheduler.unstable_flushAllWithoutAsserting(); // re-flush without the strict check
  } else {
    throw e;
  }
}

When it happens

Trigger: Calling Scheduler.unstable_flushAll() when the tasks scheduled in the test call Scheduler.log(...) — the do-while loop appends the values to yieldedValues and the post-flush check finds a non-null log.

Common situations: New tests modeled on React fixture tests where tasks log values; conditional logging that only fires for certain inputs; a leftover Scheduler.log() call inside a shared task helper copied from an older fixture.

Related errors

• Log is not empty. Assert on the log of yielded values before
• Cannot reset while already flushing work.
• Already flushing work.
• react-dom/unstable_testing is not supported in React Server
• 361

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/9618c72c67eb82c5. Report an issue: GitHub.