사용할 예제 코드
import React, { useState } from 'react';
const getAverage = (numbers) => {
console.log('평균값 계산중');
if (numbers.length === 0) return 0;
const sum = numbers.reduce((a, b) => a + b);
return sum / numbers.length;
};
const Average = () => {
const [number, setNumber] = useState('');
const [list, setList] = useState([]);
const onChange = (e) => {
setNumber(e.target.value);
};
const onInsert = (e) => {
const nextList = list.concat(parseInt(number));
//const nextList = [...list, parseInt(number)];
setList(nextList);
setNumber('');
};
return (
<div>
<input value={number} onChange={onChange} />
<button onClick={onInsert}>등록</button>
<ul>
{list.map((value, index) => {
return <li key={index}>{value}</li>;
})}
</ul>
<div>
<b>평균값 : </b> {getAverage(list)}
</div>
</div>
);
};
export default Average;
코드
const avg = useMemo(() => getAverage(list), [lsit]);
return{
...
<b>평균 값 : </b> {avg}
}
코드
const onChange = useCallback((e) => {
setNumber(e.target.value);
}, []);
const onInsert = useCallback(
(e) => {
const nextList = list.concat(parseInt(number));
//const nextList = [...list, parseInt(number)];
setList(nextList);
setNumber('');
},
[number, list]
);
const inputEl = useRef(null);
const onInsert = useCallback(e => {
...
inputEl.current.focus();
})
<input value={number} onChange={onChange} ref={inputEl} />
<button onClick={onInsert}>등록</button>
로컬 변수 : 렌더링과 상관없이 바뀔 수 있는 값
const id = useRef(1);
const setId = (n) => {
id.current = n;
}
const printId = () => {
console.log(id.current)
}