我試圖渲染這段代碼，.map()但日期時(shí)出現(xiàn)錯(cuò)誤。（第 50 行）：無(wú)法讀取未定義的屬性“地圖”在我使用條件渲染后{dates && dates.map((item) => <DateItem item={item} />)} ，渲染成功。我不明白的是為什么我應(yīng)該使用日期。當(dāng)我只渲染 line47 和 line48 時(shí)，我不需要使用條件渲染。是渲染速度的問(wèn)題嗎？如果我使用條件渲染，日期狀態(tài)何時(shí)生成？import React, { useState, useEffect } from "react";const DateItem = ({ item }) => {  return (    <div>      <div>{item.day}</div>      <div>{item.date}</div>    </div>  );};const Dates = () => {  const [date, setDate] = useState();  const [day, setDay] = useState();  const [dates, setDates] = useState();  const getDateArr = () => {    let arr = [];    var weekday = new Array(7);    weekday[0] = "sunday";    weekday[1] = "monday";    weekday[2] = "tuesday";    weekday[3] = "wednesday";    weekday[4] = "thursday";    weekday[5] = "friday";    weekday[6] = "saturday";    for (let i = 0; i < 31; i++) {      let d = new Date();      d.setDate(d.getDate() - i);      let date = d.getDate();      let day = weekday[d.getDay()];      arr.push({ day, date });    }    // console.log(arr);    return arr;  };  useEffect(() => {    setDate(new Date().getFullYear());    setDay(new Date().getMonth());    setDates(getDateArr());  }, []);  return (    <>      <div className="dateSection">        <div className="head">??</div>        <div className="year">{date}</div>  // line47        <div className="month">{day}</div>        <div className="dates">          {dates.map((item) => (        // <=error line50            <DateItem item={item} />          ))}        </div>      </div>    </>  );};export default Dates;