I encountered this need recently. For example, let's say you have a component that populates an array and is able to load a dynamic source of 10 items each time based on some action by the user (like in my case when the user keeps scrolling down the screen .The code looks a bit like this:

function Stream() {
  const [feedItems, setFeedItems] = useState([]);
  const { fetching, error, data, run } = useQuery(SOME_QUERY, vars);

  useEffect(() => {
    if (data) {
      setFeedItems([...feedItems, ...data.items]);
    }
  }, [data]);     // <---- 这违反了hooks的规则，缺少了feedItems

...
<button onClick={()=>run()}>获取更多</button>
...

Obviously, you can't simply add feedItems to the dependency list of the useEffect hook, because you call setFeedItems in it, so you'll get stuck in a loop.

Functional updates to the rescue:

useEffect(() => {
    if (data) {
      setFeedItems(prevItems => [...prevItems, ...data.items]);
    }
  }, [data]);     //  <--- 现在一切都好了

In React, status updates are asynchronous. Therefore, on the next update, there may be old values ​​in count. For example, compare the results of the following two code examples:

function Counter({initialCount}) {
  const [count, setCount] = useState(initialCount);
  return (
    <>
      Count: {count}
      <button onClick={() => setCount(initialCount)}>重置</button>
      <button onClick={() => {
        setCount(prevCount => prevCount + 1); 
        setCount(prevCount => prevCount + 1)}
      }>+</button>
    </>
  );
}

and

function Counter({initialCount}) {
  const [count, setCount] = useState(initialCount);
  return (
    <>
      Count: {count}
      <button onClick={() => setCount(initialCount)}>重置</button>
      <button onClick={() => {
        setCount(count + 1); 
        setCount(count + 1)}
      }>+</button>
    </>
  );
}