TypeError：使用“useRef”時無法讀取未定義的屬性“onMonthSelect”

Question

我正在嘗試在我的反應功能組件中使用 useRef，但是當我嘗試訪問它時，我收到錯誤“TypeError：使用“useRef”時無法讀取未定義的屬性“onMonthSelect””。

這是下面的代碼

import React, { useRef, useState, useEffect } from "react";
import moment from "moment";
import "react-dates/initialize";
import "react-dates/lib/css/_datepicker.css";
import { SingleDatePicker } from "react-dates";

const SingleDatePickerComponent = () => {
  const monthController = useRef();
  const [createdAt, setCreatedAt] = useState(moment());

  const onDateChange = (createdAt) => {
    console.log(createdAt);
    setCreatedAt(createdAt);
  };

  useEffect(() => {
    console.log(monthController);
    // TODO: check if month is visible before moving
    monthController.current.onMonthSelect(
      monthController.current.month,
      createdAt.format("M")
    );
//In this useEffect i am getting the error
  }, [createdAt]);

  return (
    <div>
      <div style={{ marginLeft: "200px" }}>
      </div>
      <SingleDatePicker
        date={createdAt}
        startDateId="MyDatePicker"
        onDateChange={onDateChange}
        renderMonthElement={(...args) => {
          // console.log(args)
          monthController.current = {
            month: args[0].month,
            onMonthSelect: args[0].onMonthSelect,
          };
          // console.log(monthController)
          return args[0].month.format("MMMM");
        }}
        id="SDP"
      />
    </div>
  );
};

export default SingleDatePickerComponent;

Answer 1

ref 值不會在初始渲染時設置。 在訪問上使用保護子句或可選鏈接運算符。

useEffect(() => {
  // TODO: check if month is visible before moving
  monthController.current && monthController.current.onMonthSelect(
    monthController.current.month,
    createdAt.format("M")
  );
}, [createdAt]);

或者

useEffect(() => {
  // TODO: check if month is visible before moving
  monthController.current?.onMonthSelect(
    monthController.current.month,
    createdAt.format("M")
  );
}, [createdAt]);

它也可能有助於提供定義的初始參考值。

const monthController = useRef({
  onMonthSelect: () => {},
});