如何检查nodejs中是否存在用户

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

// 连接到MongoDB数据库
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
  .then(() => console.log('Connected to MongoDB'))
  .catch(err => console.error('Failed to connect to MongoDB', err));

// 创建用户模型
const User = mongoose.model('User', new mongoose.Schema({
  username: String,
  password: String
}));

// 创建Express应用
const app = express();

// 创建路由来处理用户检查请求
app.get('/users/:username', async (req, res) => {
  const username = req.params.username;

  // 在数据库中查找用户
  const user = await User.findOne({ username });

  if (user) {
    res.send('用户存在');
  } else {
    res.send('用户不存在');
  }
});

// 启动服务器
app.listen(3000, () => console.log('Server started on port 3000'));