useEffect Clean-Up Function Executes After Component is Removed

This behavior has changes in React version 17.0.x.

Clean-up function should execute when the component unmounts, and I've noticed in React v.17 by the time cleanup function executes, React referenced elements are no longer available.

Working example with React v.16
Example with React v.17

Notice unmountSuccessful logs before divRef in React v.17 example.

This is the only relevant change I noticed from the changelog.

If the behavior of useEffect clean-up has changed it should be reflected on the docs.

Code Snippet:

const App = () => {
  const divRef = React.useRef(null);
  
  React.useEffect(() => {
    console.log('divRef ::', divRef.current)
    return function cleanUp() {
      console.log('[cleanUp] divRef ::', divRef.current)
    }
  }, [])
  return (
    <div ref={divRef}>
      Open Browser's Console For Output
    </div>
  )
}

ReactDOM.render(
  <App />,
  document.getElementById('root')
);

const unmountApp = () => {
  console.log('unmounting root...')
  const node = document.getElementById('root');
  const unmountSuccessful = ReactDOM.unmountComponentAtNode(node);
  console.log('unmountSuccessful ::', unmountSuccessful)
};

window.setTimeout(unmountApp, 1500);

從 issue 中連結的 React useEffect 文件開始，將其中關於清理的措辭與兩個 CodePen 範例中展示的 React 17 行為進行比較。閱讀與相關變更對應的已連結 changelog pull request。完成的標準是文件準確說明清理何時執行，以及被參照的元素是否仍然可用。