facebook/react · error · Error

521

521

Error message

flushSyncWork should not be called from builds that support legacy mode. This is a bug in React.

What it means

Error "flushSyncWork should not be called from builds that support legacy mode. This is a bug in React." thrown in facebook/react.

Source

Thrown at packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js:5094

  r: requestFormReset,
  D /* prefetchDNS */: prefetchDNS,
  C /* preconnect */: preconnect,
  L /* preload */: preload,
  m /* preloadModule */: preloadModule,
  X /* preinitScript */: preinitScript,
  S /* preinitStyle */: preinitStyle,
  M /* preinitModuleScript */: preinitModuleScript,
};

function flushSyncWork() {
  if (disableLegacyMode) {
    const previousWasRendering = previousDispatcher.f(); /* flushSyncWork */
    const wasRendering = flushSyncWorkOnAllRoots();
    // Since multiple dispatchers can flush sync work during a single flushSync call
    // we need to return true if any of them were rendering.
    return previousWasRendering || wasRendering;
  } else {
    throw new Error(
      'flushSyncWork should not be called from builds that support legacy mode. This is a bug in React.',
    );
  }
}

function requestFormReset(form: HTMLFormElement) {
  const formInst = getInstanceFromNodeDOMTree(form);
  if (
    formInst !== null &&
    formInst.tag === HostComponent &&
    formInst.type === 'form'
  ) {
    requestFormResetOnFiber(formInst);
  } else {
    // This form was either not rendered by this React renderer (or it's an
    // invalid type). Try the next one.
    //
    // The last implementation in the sequence will throw an error.

View on GitHub (pinned to eafeac097b)

When it happens

Trigger: Thrown at packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js:5094 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/e847bc256534e458. Report an issue: GitHub.