Error: Cannot flush updates when React is not rendering in Next.js: Causes and How to Fix

Error: Cannot Flush Updates When React is Not Rendering in Next.js

• This error typically arises from the asynchronous nature of React combined with complexities in frameworks like Next.js. React's concurrent mode allows for non-blocking UI updates, but when certain components are not ready or rendering has not been triggered, attempts to flush updates can lead to this error.

 

• Flushing updates in React involves committing changes to the DOM, but when React is in a non-rendering phase, either due to paused or suspended components, this flushing process encounters issues.

Understanding the Rendering Phases in React and Next.js

• React's rendering process involves two main phases: the "render phase" and the "commit phase." The render phase is when React calculates what changes to make to the UI in memory, while the commit phase is when it actually updates the DOM.

 

• In Next.js, which is a hybrid framework allowing both server-side and client-side rendering, understanding when the rendering phase occurs is crucial, especially when dealing with components that depend on server-side data fetching.

function MyComponent() {
  // State and effect hooks that may lead to errors when flushing updates
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch('/api/data').then(response => response.json()).then(receivedData => {
      setData(receivedData);
      // Attempt to update state based on fetched data
    });
  }, []);

  return (
    <div>
      {data ? (
        // Render component once data is available
        <p>{data.content}</p>
      ) : (
        // During initial rendering or when render phase is interrupted
        <p>Loading...</p>
      )}
    </div>
  );
}

Implications and Considerations

• The "Cannot flush updates" error acts as a safeguard in React's rendering system. It prevents unwanted or premature updates that can lead to inconsistencies in the user interface or unexpected behaviors.

 

• While the error might not crash an application, it can indicate potential issues in the render logic, such as inappropriate handling of asynchronous data or improper component lifecycles.

Best Practices to Keep in Mind

• Be cautious with asynchronous operations in hooks like `useEffect`. Ensure your fetch calls, subscriptions, or any other side-effects are set up and cleaned correctly to align with React's rendering phases.

 

• Use placeholder content or fallback UIs to handle unmounting or suspension of components gracefully, ensuring that flushing updates happens when components are guaranteed to be in a rendering state.

useEffect(() => {
  const controller = new AbortController();
  const signal = controller.signal;

  fetch('/api/data', { signal }).then(response => response.json()).then(receivedData => {
    if (!signal.aborted) {
      setData(receivedData);
    }
  });

  return () => controller.abort();
}, []);