react usestate functional update

1// declare state using useState hook.
2// someState is set to someInitialState
3const [someState, setSomeState] = useState(someInitialState);
4
5// setSomeState updates the current state 
6setSomeState(someOtherState);

1function DelayedCount() {
2  const [count, setCount] = useState(0);
3
4  const handleClickAsync = () => {
5    setTimeout(function delay() {
6      setCount(count => count + 1);    }, 3000);
7  }
8
9  return (
10    <div>
11      {count}
12      <button onClick={handleClickAsync}>Increase async</button>
13    </div>
14  );
15}

1() => setCount(prevCount => prevCount - 1)