useMemo

import React, { useState } from "react";

const getAverage = (num) => {
  if (num.length === 0) return 0;
  const sum = num.reduce((a, b) => a + b);
  return sum / num.length;
};

export const Average = () => {
  const [list, setList] = useState([]);
  const [number, setNumber] = useState("");

  const onChange = (e) => {
    setNumber(e.target.value);
  };
  const onInsert = (e) => {
    const nextList = list.concat(parseInt(number));
    setList(nextList);
    setNumber("");
  };

  return (
    <div>
      <input value={number} onChange={onChange} />
      <button onClick={onInsert}>등록</button>
      {list.map((value, index) => (
        <p key={index}>{value}</p>
      ))}
      <div>평균:{getAverage(list)}</div>
    </div>
  );
};

import React, { useMemo, useState } from "react";

const getAverage = (num) => {
  console.log("average 실행중");
  if (num.length === 0) return 0;
  const sum = num.reduce((a, b) => a + b);
  return sum / num.length;
};

export const Average = () => {
  const [list, setList] = useState([]);
  const [number, setNumber] = useState("");

  const onChange = (e) => {
    setNumber(e.target.value);
  };
  const onInsert = (e) => {
    const nextList = list.concat(parseInt(number));
    setList(nextList);
    setNumber("");
  };

  const avg = useMemo(() => getAverage(list), [list]);

  return (
    <div>
      <input value={number} onChange={onChange} />
      <button onClick={onInsert}>등록</button>
      {list.map((value, index) => (
        <p key={index}>{value}</p>
      ))}
      <div>평균:{avg}</div>
    </div>
  );
};