TypeError：使用“useRef”时无法读取未定义的属性“onMonthSelect”答案

【问题标题】：TypeError: Cannot read property 'onMonthSelect' of undefined while using "useRef"TypeError：使用“useRef”时无法读取未定义的属性“onMonthSelect”
【发布时间】：2021-08-21 07:14:27
【问题描述】：

我正在尝试在我的反应功能组件中使用 useRef，但是当我尝试访问它时出现错误 “TypeError：使用“useRef”时无法读取未定义的属性“onMonthSelect””。

这是下面的代码

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;

【问题讨论】：

标签： reactjs use-ref react-dates

【解决方案1】：

在初始渲染时尚未设置 ref 值。在访问上使用保护子句或可选链接运算符。

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

或

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

提供定义的初始参考值也可能会有所帮助。

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

【讨论】：