useState 후크가 게으른 초기 상태 함수가 동기식이어야 하는지 확인해야 합니다.

const [state] = useState(async () => 'a');
console.log(state); // "state" is a promise.

const syncCallback = () => 'a';
const asyncCallback = async () => 'b';

const AsyncFunction = (async () => { }).constructor;

function isAsync(fn: Function) {
  return fn instanceof AsyncFunction;
}

function useState<S>(initialState: S | (() => S)) {
  if (typeof initialState === 'function' && isAsync(initialState)) throw new Error('initial state function must be synchronous.')
  if (initialState instanceof Function) {
    return initialState();
  }
  return initialState
}

const state1 = useState(syncCallback); // correct usage
console.log('state1: ', state1); 
const state2 = useState(asyncCallback); // Only allow synchronous function, so it will throw error to developer.