我目前正在使用Mongoes.js应用程序，试图使用Mongoose连接到MongoDB数据库。但是，我遇到了一个数据库连接问题，需要一些帮助来解决这个问题。以下是详细信息：
我有一个js.js应用程序，我使用Mongoose连接到MongoDB数据库。我已经按照标准步骤设置了数据库连接，但在尝试与数据库交互时出现错误。
app.js

const express = require('express');
const mongoose = require('mongoose');
const bodyParser = require('body-parser');
const cors = require('cors');
const app = express();
const PORT = process.env.PORT || 3000;

app.use(bodyParser.json());
app.use(cors({ origin: '*' }));

mongoose.connect('mongodb://127.0.0.1:27017/library_db', {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

// Other routes and middleware...

app.listen(PORT, () => {
  console.log(`Backend Server is running on port ${PORT}`);
});

  // POST /book
  app.post('/book', async (req, res) => {
    try {
      const { name, isbn, author } = req.body;
      const book = new Book({ name, isbn, author });
      await book.save();
      res.status(201).json(book);
    } catch (error) {
      res.status(500).json({ error: 'An error occurred' });
    }
  });

前端电子书. js

import React, { useState } from 'react';
import axios from 'axios';

function CreateBook() {
  const [name, setName] = useState('');
  const [isbn, setIsbn] = useState('');
  const [author, setAuthor] = useState('');
  const [message, setMessage] = useState('');

  const handleSubmit = (event) => {
    event.preventDefault();

    const newBook = {
      name,
      isbn,
      author,
    };

    axios.post('/book', newBook)
      .then(response => {
        setMessage('Book created successfully');
        setName('');
        setIsbn('');
        setAuthor('');
      })
      .catch(error => {
        console.error(error);
        setMessage('An error occurred while creating the book');
      });
  };

  return (
    <div className="container mt-4">
      <h2>Create Book</h2>
      <form onSubmit={handleSubmit}>
        <div className="mb-3">
          <label className="form-label">Name:</label>
          <input type="text" className="form-control" value={name} onChange={e => setName(e.target.value)} required />
        </div>
        <div className="mb-3">
          <label className="form-label">ISBN:</label>
          <input type="text" className="form-control" value={isbn} onChange={e => setIsbn(e.target.value)} required />
        </div>
        <div className="mb-3">
          <label className="form-label">Author:</label>
          <input type="text" className="form-control" value={author} onChange={e => setAuthor(e.target.value)} required />
        </div>
        <button type="submit" className="btn btn-primary">Create Book</button>
      </form>
      <p className="mt-3">{message}</p>
    </div>
  );
}

export default CreateBook;