TypeError: Cannot read property '_context' of undefined

Question

我正在使用 React、Next.js 和 TailwindCSS 创建一个简单的待办事项列表。由于某些原因，我收到此错误：TypeError: Cannot read property '_context' of undefined.

此错误发生在 TodoForm.js。我在index.js中使用TodoContext.Provider定义了showModal，但是为什么会出现这个错误？

TodoContext.js

import { createContext } from "react";

const TodoContext = createContext(null);

export default TodoContext;

index.js

import { useState } from "react";

import Modal from "../components/Modal";
import TodoForm from "../components/TodoForm";
import TodoList from "../components/TodoList";
import TodoContext from "./TodoContext";

export default function Home() {
  const [open, setOpen] = useState(true);
  const [alertType, setAlertType] = useState("success");
  const [alertMessage, setAlertMessage] = useState("");

  const showModal = (type, msg) => {
    setAlertType(type);
    setAlertMessage(msg);
    setOpen(true);
  };

  return (
    <TodoContext.Provider value={{ showModal }}>
      <div className="flex flex-col min-h-screen py-2 w-full items-center">
        <TodoForm />
        <Modal setOpen={setOpen} alertMessage={alertMessage} open={open} />
        <TodoList />
      </div>
    </TodoContext.Provider>
  );
}

TodoForm.js

import { addDoc, collection, serverTimestamp } from "@firebase/firestore";
import { useContext, useState } from "react";
import { db } from "../firebase";

const TodoForm = () => {
  const [todo, setTodo] = useState({ title: "", detail: "" });
  const { showModal } = useContext();

  const onSubmit = async () => {
    const collectionRef = collection(db, "todos");
    const docRef = await addDoc(collectionRef, {
      ...todo,
      timestamp: serverTimestamp(),
    });
    setTodo({ title: "", detail: "" });

    showModal(
      "bg-blue-200",
      `Todo with id ${docRef.id} is added successfully! `
    );
  };

  return (
    <div className="flex w-3/6 flex-col justify-center mt-6">
      <pre>{JSON.stringify(todo)}</pre>
      <input
        id="title"
        type="text"
        placeholder="Title"
        value={todo.title}
        onChange={(e) => setTodo({ ...todo, title: e.target.value })}
      />
      <input
        id="detail"
        type="text"
        placeholder="Detail"
        value={todo.detail}
        onChange={(e) => setTodo({ ...todo, detail: e.target.value })}
      />
      <button
        type="button"
        onClick={onSubmit}
      >
        ADD A NEW TODO
      </button>
    </div>
  );
};

export default TodoForm;

Modal.js

import { Fragment, useRef, useState } from "react";
import { Dialog, Transition } from "@headlessui/react";
import { ExclamationIcon } from "@heroicons/react/outline";

export default function Modal({ setOpenClick, alertMessage, open }) {
  const cancelButtonRef = useRef(null);

  return (
    <Transition.Root show={open} as={Fragment}>
      <Dialog
        as="div"
        className="fixed z-10 inset-0 overflow-y-auto"
        initialFocus={cancelButtonRef}
        onClose={setOpenClick}
      >
        <div className="flex items-end justify-center min-h-screen pt-4 px-4 pb-20 text-center sm:block sm:p-0">
          <Transition.Child
            as={Fragment}
            enter="ease-out duration-300"
            enterFrom="opacity-0"
            enterTo="opacity-100"
            leave="ease-in duration-200"
            leaveFrom="opacity-100"
            leaveTo="opacity-0"
          >
            <Dialog.Overlay className="fixed inset-0 bg-gray-500 bg-opacity-75 transition-opacity" />
          </Transition.Child>

          {/* This element is to trick the browser into centering the modal contents. */}
          <span
            className="hidden sm:inline-block sm:align-middle sm:h-screen"
            aria-hidden="true"
          >
            &#8203;
          </span>
          <Transition.Child
            as={Fragment}
            enter="ease-out duration-300"
            enterFrom="opacity-0 translate-y-4 sm:translate-y-0 sm:scale-95"
            enterTo="opacity-100 translate-y-0 sm:scale-100"
            leave="ease-in duration-200"
            leaveFrom="opacity-100 translate-y-0 sm:scale-100"
            leaveTo="opacity-0 translate-y-4 sm:translate-y-0 sm:scale-95"
          >
            <div className="inline-block align-bottom bg-white rounded-lg text-left overflow-hidden shadow-xl transform transition-all sm:my-8 sm:align-middle sm:max-w-lg sm:w-full">
              <div className="bg-white px-4 pt-5 pb-4 sm:p-6 sm:pb-4">
                <div className="sm:flex sm:items-start">
                  <div className="mx-auto flex-shrink-0 flex items-center justify-center h-12 w-12 rounded-full bg-red-100 sm:mx-0 sm:h-10 sm:w-10">
                    <ExclamationIcon
                      className="h-6 w-6 text-red-600"
                      aria-hidden="true"
                    />
                  </div>
                  <div className="mt-3 text-center sm:mt-0 sm:ml-4 sm:text-left">
                    <div className="mt-2">
                      <p className="text-sm text-gray-500">{alertMessage}</p>
                    </div>
                  </div>
                </div>
              </div>
              <div className="bg-gray-50 px-4 py-3 sm:px-6 sm:flex sm:flex-row-reverse">
                <button
                  type="button"
                  className="w-full inline-flex justify-center rounded-md border border-transparent shadow-sm px-4 py-2 bg-red-600 text-base font-medium text-white hover:bg-red-700 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-red-500 sm:ml-3 sm:w-auto sm:text-sm"
                  onClick={() => setOpenClick(false)}
                >
                  Deactivate
                </button>
                <button
                  type="button"
                  className="mt-3 w-full inline-flex justify-center rounded-md border border-gray-300 shadow-sm px-4 py-2 bg-white text-base font-medium text-gray-700 hover:bg-gray-50 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-500 sm:mt-0 sm:ml-3 sm:w-auto sm:text-sm"
                  onClick={() => setOpenClick(false)}
                  ref={cancelButtonRef}
                >
                  Cancel
                </button>
              </div>
            </div>
          </Transition.Child>
        </div>
      </Dialog>
    </Transition.Root>
  );
}

你能告诉我我在这里遗漏了什么吗？

这是在 TodoForm.js

上使用 const showModal = useContext(TodoContext) 时显示错误的图像

Answer 1

您正试图破坏对象的上下文

const { showModal } = useContext();

但初始值为空

const TodoContext = createContext(null);

一种解决方案是给一个初始值：

const TodoContext = createContext({showModal:()=>{}});

TypeError: Cannot read property '_context' of undefined

TypeError: Cannot read property '_context' of undefined

reactjs

use-context