如何在扩展用户模型中检查重复

const express = require('express');
const mongoose = require('mongoose');
const app = express();

mongoose.connect('mongodb://localhost/myapp', { useNewUrlParser: true, useUnifiedTopology: true });

const userSchema = new mongoose.Schema({
  username: { type: String, unique: true },
  email: { type: String, unique: true },
  password: String
});

const User = mongoose.model('User', userSchema);

app.use(express.json());

app.post('/register', async (req, res) => {
  try {
    const { username, email, password } = req.body;
    const newUser = new User({ username, email, password });
    await newUser.save();
    res.status(201).send('User registered successfully');
  } catch (error) {
    if (error.name === 'MongoError' && error.code === 11000) {
      res.status(400).send('Duplicate entry detected');
    } else {
      res.status(500).send('Internal server error');
    }
  }
});

app.listen(3000, () => console.log('Server running on port 3000'));

document.getElementById('registerForm').addEventListener('submit', function(event) {
  event.preventDefault();
  const formData = new FormData(this);
  fetch('/register', {
    method: 'POST',
    body: formData
  })
  .then(response => response.text())
  .then(result => alert(result))
  .catch(error => console.error('Error:', error));
});