有人可以解釋為什么key箭頭函數內的值未定義：// in parent componentconst Parent = () => {        const [key, setKey] = useState<string>();        // this contains an expensive function we only wish to execute once on first load        useEffect(() => {            // has some promise that will call within a `then()`                        setKey(someVal);        }, []};    // within render    < MyComponent key={key}/>}// within child componentinterface Props {    key: string;}const MyComponent = ({key}: Props) => {    // this works, I can see the correct value `someVal`    console.log("value when rendered: " + key);    const callback = () => {        // this isn't working, key is undefined        console.log("value within callback: " + key);    }  // within render, when something calls callback(), key is undefined, why?  }我可以看到它key在調用渲染時有一個值，但key未定義。我試過使用let callback =而不是const，但沒有運氣。請問如何訪問key？