正如Ben提到的，你在每个循环中覆盖了状态。因此，在最后一次forEach迭代结束时，你的状态是最后一次预测的状态。不幸的是，提供的修正解决方案不太正确。我本来想发表评论的，但我的声誉不够高，所以我添加了一个新的答案。
我们需要在循环外初始化一个数组，否则，它将在每次迭代时被覆盖，然后我们又回到原点。例如：

const [todayForcast, setTodayForcast] = useState()
  function checkDates() {
    const forcastArray = props.forcast.list // Get array containing all forcasts
    const date = new Date(); // Get current date
    const date1 = date.getDate(); // Get just todays date
    
    // We need to initialize the array outside of the loop, otherwise it gets overwritten
    // each iteration and we're back at square one.
    const newForecasts = []; 

    // Loop over the array and get each item
    forcastArray.forEach(forcast => {
      let get_current_dt = forcast.dt_txt // Get d & t from the obj
      let split_dt = get_current_dt.split(" ") // Split d & t at the space
      let get_full_date = split_dt[0] // Get just the date
      let get_date = get_full_date.slice(8) // Remove year & month and get just day
      if( get_date ==  date1){

        // Append the current forcast to the newForescasts array defined earlier
        newForecasts.push(forcast);
      }
    })

    // After the loop is finished, set the state to the newForecasts
    setTodayForcast(newForecasts)
  }

顺便说一句，我不完全确定forcast.dt_txt是什么格式，但是您可以简化代码的其余部分：

const [todayForcast, setTodayForcast] = useState()
  function checkDates({ forcast }) {
    const forcastArray = forcast.list
    const date = new Date().getDate()
 
    const newForecasts = []

    forcastArray.forEach(f => {if (date == new Date(f.dt_txt).getDate()) newForecasts.push(f))}
    setTodayForcast(newForecasts)
  }

你现在所做的就是每次把当前的forcast设置为状态（应该是forecast，可能是打字错误，但没关系）。当你设置forcast时，你会覆盖最后一个状态，如果你想保存所有的状态，你可以做类似的事情。

// chose an array, maybe you want an object instead at the beginning
const [todayForcast, setTodayForcast] = useState([])
function checkDates() {
  const forcastArray = props.forcast.list // Get array containing all forcasts
  const date = new Date(); // Get current date
  const date1 = date.getDate(); // Get just todays date
  // Loop over the array and get each item
  let newForecasts = [];
  forcastArray.forEach(forcast => {
    let get_current_dt = forcast.dt_txt // Get d & t from the obj
    let split_dt = get_current_dt.split(" ") // Split d & t at the space
    let get_full_date = split_dt[0] // Get just the date
    let get_date = get_full_date.slice(8) // Remove year & month and get just day
    if( get_date ==  date1){
      // make a new array (or object) and add the new forcast to it
      newForecasts = [...newForecast, forcast];
    }
  })
  setTodayForcast(newForecasts);
}

根据Evan的输入，我们创建了一个新的数组newForecasts，使用spread ...操作符来扩展newForecasts中的内容，然后将当前的forcast添加到其中，也得到了一个添加了当前forcast的新数组，然后在循环结束后将其设置回todayForcast状态。

你必须使用一个伪数组来推入它，在迭代之后你可以将伪数组设置到状态变量中。希望你得到了...