将Express函数传递到另一个调用HTTPS createServer的文件中

根据Node Best Practices，建议将HTTP和Express服务器API分开。

我的问题是，createServer需要一个函数作为第二个参数，但我正在传递一个对象。

app.js

const config = require('./config');
const express = require('express');
const app = express();

app.use(express.static(__dirname + config.STATIC_DIR));
app.use(express.json());

app.get('/', function (req, res) {
  res.sendFile(__dirname + '/index.html');
});

// POST /agent used for registration requests
app.post('/agent', (req, res) => {
  console.log('Received registration request from ', req.body);
  return res.send('Received a POST HTTP method');
});

server.js

const fs = require('fs');
const config = require('./config');
const mysql = require('mysql2/promise');
const app = require('./app');

const sslConfig = {
  key: fs.readFileSync(config.SSL_KEY),
  cert: fs.readFileSync(config.SSL_CERT)
};

const server = require('https').createServer(sslConfig, app);
const io = require('socket.io')(server);

我收到以下错误：

 throw new errors.ERR_INVALID_ARG_TYPE('listener', 'Function', listener);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "listener" argument must be of type Function. Received type object
    at checkListener (events.js:55:11)
    at _addListener (events.js:214:3)
    at Server.addListener (events.js:272:10)
    at new Server (https.js:67:10)
    at Object.createServer (https.js:85:10)
    at Object.<anonymous> (/Users/Code/server.js:18:8)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)

如果要将SSL配置传递到createServer方法中，需要进行哪些更改？

我能够通过在app.js的末尾添加以下行来解决此问题：

module.exports = app;