出现错误：无法读取未定义的属性“长度”

Question

我不知道为什么它的给定长度undefiined而另一个是setDailyData不是 function

const Charts = () => {
  const { setDailyData, dailyData } = useState([]);

  useEffect(() => {
    const fetchAPI = async () => {
      setDailyData(await fetchDailyData());
    };
    console.log(dailyData);

    fetchAPI();
  }, []);

在这里，我试图从dailyData中获取数据并使用三元操作来检查是否有任何数据，然后显示图表
如果没有，（通过检查数组的长度）它应该显示null

即使使它成为dailyData[0] ，我也给出错误无法读取'0'属性。

const lineChart =
 ( dailyData.length ?(
    <Line
      data={{
        labels: dailyData.map(({ date }) => date),
        datasets: [
          {
            data: dailyData.map(({ confirmed }) => confirmed),
            label: "Infected",
            borderColor: "#3333ff",
            fill: true,
          },
          {
            data: dailyData.map(({ deaths }) => deaths),
            label: "Deaths",
            borderColor: "#3333ff",
            backgroundColor: "rgba(255,0,0,0.5)",
            fill: true,
          },
        ],
      }}
    />
  ):null
)

API 代码用于获取图表数据

export const fetchDailyData = async ()=>{
    try {
        const {data} = await axios.get(`${url}/daily`);

        const modifiedData = data.map((dailyData) =>({
            confirmed:dailyData.confirmed.total,
            deaths:dailyData.deaths.total,
            date:dailyData.reportDate
        }));
        console.log(data);

        return modifiedData;

    } catch (error) {
        return error;
    }
}

Answer 1

useState 返回一个数组，而不是 object

从你的代码

const Charts = () => {
  // const { setDailyData, dailyData } = useState([]);
  const [dailyData, setDailyData] = useState([]) // <-- change to this

  useEffect(() => {
    const fetchAPI = async () => {
      setDailyData(await fetchDailyData());
    };
    console.log(dailyData);

    fetchAPI();
  }, []);