useAsyncState.js

function useAsyncState(initialValue) {
  const [value, setValue] = useState(initialValue);
  const setter = x =>
    new Promise(resolve => {
      setValue(x);
      resolve(x);
    });
  return [value, setter];
}

function App() {
  // const [count, setCount] = useState(0);
  // const [message, setMessage] = useState("");
  const [count, setCount] = useAsyncState(0);
  const [message, setMessage] = useAsyncState("");

  function increment() {
    setCount(count + 1).then(count => setMessage(`count is ${count}`));
  }

  function decrement() {
    setCount(count - 1).then(count => setMessage(`count is ${count}`));
  }

// OR use async/await...

  async function increment() {
    const newCount = await setCount(count + 1)
    setMessage(`count is ${newCount}`);
  }

  async function decrement() {
    const newCount = await setCount(count - 1)
    setMessage(`count is ${newCount}`);
  }
  
  ...
}