为了避免posh错误，如果数组没有定义，则为其分配一个空数组。
像这样修改你的代码：

"use client";

import React, { useState } from "react";

const ForecastDay = ({ data }) => {
    const { list } = data;

    let currentDate = list[0].dt_txt.split(" ")[0];

    const [fiveDayForecast, setFiveDayForecast] = useState([]);

    list.forEach((forecast) => {
        const forecastDate = forecast.dt_txt.split(" ")[0];

        if (forecastDate !== currentDate) {
            setFiveDayForecast((prevForecast) => [...prevForecast, []]);
            currentDate = forecastDate;
        }

        setFiveDayForecast((prevForecast) => {
            const updatedForecast = [...prevForecast];
            if(!updatedForecast[updatedForecast.length - 1]){
                updatedForecast[updatedForecast.length - 1] = [];
            }
            updatedForecast[updatedForecast.length - 1].push(forecast);
            return updatedForecast;
        });
    });

    return (
        <div>
            {fiveDayForecast.map((day, index) => (
                <div key={index}>
                    <h2>Günün Tarihi: {day[0].dt_txt.split(" ")[0]}</h2>
                    {day.map((forecast) => (
                        <div key={forecast.dt}>
                            <p>Saat: {forecast.dt_txt.split(" ")[1]}</p>
                            <p>Sıcaklık: {forecast.main.temp}°F</p>
                        </div>
                    ))}
                </div>
            ))}
        </div>
    );
};