为什么node express中的mongodb不能将result设置为外部变量

const express = require('express');
const MongoClient = require('mongodb').MongoClient;

const app = express();
const url = 'mongodb://localhost:27017/mydb';

app.get('/data', (req, res) => {
  MongoClient.connect(url, (err, client) => {
    if (err) throw err;

    const db = client.db('mydb');
    const collection = db.collection('mycollection');

    collection.find({}).toArray((err, result) => {
      if (err) throw err;

      // 在回调函数中处理查询结果
      console.log(result);

      // 将结果发送给客户端
      res.send(result);

      // 关闭数据库连接
      client.close();
    });
  });
});

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