Why React useState with functional update form is needed?

I stumbled into a need for this recently. For example let's say you have a component that fills up an array with some amount of elements and is able to append to that array depending on some user action (like in my case, I was loading a feed 10 items at a time as the user kept scrolling down the screen. the code looked kind of 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]);     // <---- this breaks the rules of hooks, missing feedItems

...
<button onClick={()=>run()}>get more</button>
...

Obviously, you can't just add feedItems to the dependency list in the useEffect hook because you're invoking setFeedItems in it, so you'd get in a loop.

functional update to the rescue:

useEffect(() => {
    if (data) {
      setFeedItems(prevItems => [...prevItems, ...data.items]);
    }
  }, [data]);     //  <--- all good now

I have answered a similar question like this and it was closed because this was the canonical question - that i did not know of, upon looking the answers i decided to repost my answer here since i think it adds some value.

If your update depends on a previous value found in the state, then you should use the functional form. If you don't use the functional form in this case then your code will break sometime.

Why does it break and when

React functional components are just closures, the state value that you have in the closure might be outdated - what does this mean is that the value inside the closure does not match the value that is in React state for that component, this could happen in the following cases:

1- async operations (In this example click slow add, and then click multiple times on the add button, you will later see that the state was reseted to what was inside the closure when the slow add button was clicked)

const App = () => {
  const [counter, setCounter] = useState(0);

  return (
    <>
      <p>counter {counter} </p>
      <button
        onClick={() => {
          setCounter(counter + 1);
        }}
      >
        immediately add
      </button>
      <button
        onClick={() => {
          setTimeout(() => setCounter(counter + 1), 1000);
        }}
      >
        Add
      </button>
    </>
  );
};

2- When you call the update function multiple times in the same closure

const App = () => {
  const [counter, setCounter] = useState(0);

  return (
    <>
      <p>counter {counter} </p>
      <button
        onClick={() => {
          setCounter(counter + 1);
          setCounter(counter + 1);
        }}
      >
        Add twice
      </button>
   
    </>
  );
}

State update is asynchronous in React. So it is possible that there would be old value in count when you're updating it next time. Compare, for example, result of these two code samples:

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