如何在Node的app.get函数中实现回调？

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

// 定义一个GET路由
app.get('/', (req, res) => {
  // 处理请求并发送响应
  res.send('Hello, World!');
});

// 启动服务器
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

app.get('/user/:id', async (req, res) => {
  try {
    const userId = req.params.id;
    const user = await getUserById(userId); // 假设getUserById是一个异步函数
    res.send(user);
  } catch (error) {
    res.status(500).send('Internal Server Error');
  }
});

app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  getUserById(userId, (err, user) => {
    if (err) {
      return res.status(500).send('Internal Server Error');
    }
    res.send(user);
  });
});