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React flushSync called inside a lifecycle method

flushSync was called from inside a lifecycle method. It cannot be called when React is already rendering.

React reported that flushSync was called during render or lifecycle work, where React is already in the middle of rendering and cannot synchronously flush again.

React error #187runtime

Observed message

flushSync was called from inside a lifecycle method. It cannot be called when React is already rendering.

Why it happens

  • flushSync was called during component render.
  • flushSync was used inside useEffect, useLayoutEffect, or a class lifecycle method.
  • Synchronous DOM flushing was attempted while React was already rendering.

How to fix it

  1. Move flushSync into an event handler when possible.
  2. If it cannot happen in an event, defer it with queueMicrotask after the current render completes.
  3. Remove unnecessary flushSync usage and rely on normal React scheduling when possible.

Official reference

React Docs: Minified React error #187

Related errors

React error #426React component suspended while responding to synchronous inputReact error #307React Hooks called outside a function componentReact error #404React invalid hook callReact error #321React invalid hook call with mismatched versions or duplicate React

Explore more

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React error #187 flushSync lifecycle method: causes and fixes | Error Atlas