useMemo

用于优化性能，在组件重新渲染时，返回一个记忆化的值，以避免不必要的计算和重复工作。

index.tsx

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

function MyApp() {
  const [count, setCount] = useState(0);

  const factorial = useMemo(() => {
    let value = 0;
    for (let i = 0; i <= count; i++) {
      value += i;
    }
    return value;
  }, [count]);

  return (
    <div>
      <div onClick={() => setCount(count + 1)}>{count}</div>
      <div>{factorial}</div>
    </div>
  );
}

export default MyApp;