비동기... ? 값을 읽지 못하는 오류

undefined ChatRoom.js:273 TypeError: res.data.data.messageList is not iterable at ChatRoom.js:257:1 at basicStateReducer (react-dom.development.js:15196:1) at updateReducer (react-dom.development.js:15319:1) at updateState (react-dom.development.js:15642:1) at Object.useState (react-dom.development.js:16443:1) at useState (react.development.js:1509:1) at ChatRoom (ChatRoom.js:229:1) at renderWithHooks (react-dom.development.js:14985:1) at updateFunctionComponent (react-dom.development.js:17356:1) at beginWork (react-dom.development.js:19064:1) redux-logger.js:1

• 오류상황 : useEffect 안에 setState를 넣어놓았는데 변경되기전에 return 이 실행되는지 값이 undefined로 읽지 못한다고 나옴... 비동기 .... 그건 무엇인가............. 순서대로 왜 안읽음 ?

---- > 비슷한 문제를 겪은듯!
http://daplus.net/javascript-usestate-%EC%84%A4%EC%A0%95-%EB%A9%94%EC%86%8C%EB%93%9C%EA%B0%80-%EC%A6%89%EC%8B%9C-%EB%B3%80%EA%B2%BD-%EC%82%AC%ED%95%AD%EC%9D%84-%EB%B0%98%EC%98%81%ED%95%98%EC%A7%80-%EC%95%8A%EC%9D%8C/

https://ko.reactjs.org/docs/hooks-rules.html#explanation