我收到此错误 Uncaught TypeError: notes.map is not a function in my app

Question

我也遇到这个错误

GET http://localhost:5000/api/notes/fetchallnotes

我的 noteState.js 文件中出现此错误，

import NoteContext from "./noteContext";
import { useState } from "react";




const NoteState = (props) => {
  const host = "http://localhost:5000";
  let initialNotes = []
  const [notes, setNotes] = useState(initialNotes)
  const authToken =localStorage.getItem('token');


  const getNotes = async () => {
    // API Call 
    const response = await fetch(`${host}/api/notes/fetchallnotes`, {
      method: 'GET',
      headers: {
        'Content-Type': 'application/json',
        "auth-token": authToken
      }
    });
    const json = await response.json()
    console.log(json);
    setNotes(json)

  }




  const addNote = async (title, description, tag) => {
    // TODO: API Call
    // API Call 
    const response = await fetch(`${host}/api/notes/addnote`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        "auth-token": authToken,
      },
      body: JSON.stringify({ title, description, tag })
    });

    console.log("Adding a note successfully")
    const note = await response.json();
    setNotes(notes.concat(note))
  }

  // Delete a note
  const deleteNote = async (_id) => {
    // API Call 
    const response = await fetch(`${host}/api/notes/deletenote/${_id}`, {
      method: 'DELETE',
      headers: {
        'Content-Type': 'application/json',
        "auth-token": authToken,
      },
    });
    const json = await response.json();
    console.log(json);

    console.log("Note deleted successfully" + _id)
    const newNotes = notes.filter((note) => { return note._id !== _id })
    setNotes(newNotes);
  }
  // Edit a note
  const editNote = async (id, title, description, tag) => {
    // API call
    const response = await fetch(`${host}/api/notes/updatenote/${id}`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        "auth- token": authToken,
      },
      body: JSON.stringify({ title, description, tag })
    });
    const json = await response.json();
    setNotes(json);

    // Logic to edit in client
    for (let index = 0; index < notes.length; index++) {
      const element = notes[index];
      if (element._id === id) {
        element.title = title;
        element.description = description;
        element.tag = tag;
      }

    }

  }


  return (
    <NoteContext.Provider value={{ notes, addNote, deleteNote, editNote, getNotes }}>
      {props.children}
    </NoteContext.Provider>

  )

}



export default NoteState;

一旦我将 localstorage.getItem 更改为 localstorage.getitem，就会出现此特定错误，但随后我无法获取数据库中的笔记，我在登录组件中设置了 localstorage 项目，这是我的login.js 文件

import React, { useState } from 'react';
import { useNavigate } from 'react-router-dom';

const Login = (props) => {

    const [credentials, setCredentials] = useState({ email: "", password: "" });
    const navigate = useNavigate();

    const handleSubmit = async (e) => {
        e.preventDefault();
        const response = await fetch("http://localhost:5000/api/auth/login", {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify({ email: credentials.email, password: credentials.password })
        });
        const json = await response.json();
        console.log(json);
        if (json.success) {
            // Save the auth token and redirect
            localStorage.setItem('token', json.authtoken);
            props.showAlert("Logged in successfully", "success")
            navigate('/');
        }
        else {
            props.showAlert("Please enter valid credentials", "danger")
        }

    }

    const onChange = (e) => {
        setCredentials({ ...credentials, [e.target.name]: e.target.value })
    }

    return (
        <div>
            <form onSubmit={handleSubmit}>
                <div className="form-group">
                    <label htmlFor="email">Email address</label>
                    <input type="email" className="form-control" id="email" name='email' value={credentials.email} aria-describedby="emailHelp" onChange={onChange} placeholder="Enter email" />

                </div>
                <div className="form-group my-2">
                    <label htmlFor="Password">Password</label>
                    <input type="password" className="form-control" id="Password" name='password' value={credentials.password} onChange={onChange} placeholder="Password" />
                </div>
                <button type="submit" className="btn btn-primary my-2">Submit</button>
            </form>
        </div>
    )
};

export default Login;

我已经在我的 Notes.js 文件中映射了我的笔记，我不知道为什么会出现这个错误，这是我的 Notes.js 文件

import React, { useContext, useEffect, useRef } from 'react'
import { useNavigate } from 'react-router-dom';
import noteContext from '../context/notes/noteContext';
import AddNote from './AddNote';
import Noteitem from './Noteitem';


export const Notes = (props) => {
    let navigate = useNavigate();
    const context = useContext(noteContext)
    const { notes, getNotes } = context;
    useEffect(() => {
        if (localStorage.getItem('token')) {
            getNotes();
        }
        else {
            navigate('/login')
        }
        // eslint-disable-next-line
    }, [])

    const updateNote = (props) => {
        ref.current.click()
    }
    const ref = useRef(null)

    return (
        <>
            <AddNote showAlert={props.showAlert} />
            <button type="button" ref={ref} className="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
                Launch demo modal
            </button>

            {/* <!-- Modal --> */}
            <div className="modal fade" id="exampleModal" tabIndex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
                <div className="modal-dialog" role="document">
                    <div className="modal-content">
                        <div className="modal-header">
                            <h5 className="modal-title" id="exampleModalLabel">Modal title</h5>
                            <button type="button" className="close" data-dismiss="modal" aria-label="Close">
                                <span aria-hidden="true">&times;</span>
                            </button>
                        </div>
                        <div className="modal-body">
                            ...
                        </div>
                        <div className="modal-footer">
                            <button type="button" className="btn btn-secondary" data-dismiss="modal">Close</button>
                            <button type="button" className="btn btn-primary">Save changes</button>
                        </div>
                    </div>
                </div>
            </div>
            <div className="row my-3">
                <h3 > Your Notes </h3>
                <div className="container">
                    {notes.length === 0 && 'No notes to display'}
                </div>
                {notes.map((note) => {
                    return <Noteitem key={note._id} updateNote={updateNote}   note={note} />
                })}

            </div>
        </>
    )
}

Answer 1

这里 notes 来自一个 api 调用，它是一个异步调用，这就是数据在第一次渲染时不可用的原因。所以改变你的代码：

{notes.map((note) => {
    return <Noteitem key={note._id} updateNote={updateNote}   note={note} />
})}

到

{
  ( notes && notes.length > 0 )
  ?
  notes.map((note) => {
    return <Noteitem key={note._id} updateNote={updateNote}   note={note} />
  })
  :
  null
}

或使用&&条件渲染。