'app.address is not a function' when testing an express HTTPS server

Question

I am running into issues when testing my express application. All tutorials use app.listen instead of https.createServer and I don't know how to properly use testing frameworks with the latter. The code:

test_node/app.js

var express = require("express");
const app = express();

app.set('port', process.env.SERVER_PORT || 443);
app.get('/', function(req, res){
    res.status(200).send('OK');
});

module.exports = {
    app: app
}

test_node/server.js

var https = require('https');
var fs = require('fs');
const app = require('./app');

let serverOptions = {
    key: fs.readFileSync('./cert/server.key'),
    cert: fs.readFileSync('./cert/server.cert')
}
const server = https.createServer(serverOptions, app)
server.listen(app.get('port'), function () {
    console.log(`Express HTTPS server listening on port ${app.get('port')}`);
});

async function handleSignal(signal) {
    console.log(`Received ${signal}`);
    server.close();
    process.exit();
}
process.on('SIGINT', handleSignal);
process.on('SIGTERM', handleSignal);
process.on('uncaughtException', handleSignal);

test_node/test/test.js

const request = require('supertest')

describe("Test the root path", () => {
    let server;
    beforeAll(function () {
        server = require('../app');
    });
    afterAll(function () {
        server.close();
    });

    test("It should respond to the GET method", async () => {
        const response = await request(server).get("/");
        expect(response.statusCode).toBe(200);
    });
});

test_node/package.json

{
  "name": "test_node",
  "version": "1.0.0",
  "description": "",
  "main": "server.js",
  "scripts": {
    "start": "node server.js",
    "test": "jest"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "jest": "^26.6.3",
    "supertest": "^6.1.3"
  },
  "dependencies": {
    "express": "^4.17.1"
  }
}

The error I get when I test the above code with jest (jest --runInBand test.js):

TypeError: app.address is not a function
    at Test.Object.<anonymous>.Test.serverAddress (C:\code\test_node\node_modules\supertest\lib\test.js:57:18)
    at new Test (C:\code\test_node\node_modules\supertest\lib\test.js:38:12)
    at Object.get (C:\code\test_node\node_modules\supertest\index.js:27:14)
    at Object.<anonymous> (C:\code\test_node\test\test.js:13:48)
    at Object.asyncJestTest (C:\code\test_node\node_modules\jest-jasmine2\build\jasmineAsyncInstall.js:106:37)
    at C:\code\test_node\node_modules\jest-jasmine2\build\queueRunner.js:45:12
    at new Promise (<anonymous>)
    at mapper (C:\code\test_node\node_modules\jest-jasmine2\build\queueRunner.js:28:19)
    at C:\code\test_node\node_modules\jest-jasmine2\build\queueRunner.js:75:41

This is obviously a stripped down version of my code. I also create MongoDB connections at start, but I removed them for simplicity. Ideally I want to use async/await instead of promises for creating my server (and MongoDB connections) before my tests start. I also need to close those connections after tests end or fail. In your answer you can safely assume I'm JS beginner :).

Additional info

I'm using node '14.15.4' on Windows 10 x64. Webstorm IDE.

Tried various combinations of the following:

• combining app and server in one file (see below)
• creating the server in a separate function and exporting it
• using mocha/chai instead of jest
• testing a stripped down version of the app

Errors I get depending on the combinations I tried:

• TypeError: app.address is not a function
• TypeError: app.get is not a function
• EADDRINUSE: address already in use ::::443
• various other errors with timeouts, wrong arguments, etc.

Relevant topics that didn't help (or I overlooked something):

• Simple node http server unit test
• Is it possible to create an Express.js server in my Jest test suite?
• TypeError: app.get is not a function
• Mocha API Testing: getting 'TypeError: app.address is not a function'
• Getting "TypeError: "listener" argument must be a function" in Node.Js
• Jest/Supertest errors with TypeError: app.address is not a function
• https://blog.campvanilla.com/jest-expressjs-and-the-eaddrinuse-error-bac39356c33a
• https://glebbahmutov.com/blog/how-to-correctly-unit-test-express-server/
• https://rahmanfadhil.com/test-express-with-supertest/
• https://zellwk.com/blog/endpoint-testing/
• https://taylor.fausak.me/2013/02/17/testing-a-node-js-http-server-with-mocha/
• https://www.albertgao.xyz/2017/05/24/how-to-test-expressjs-with-jest-and-supertest/

Context

I included the server.js file above, although the test doesn't use it, because my server application should also be able to start normally when I run node server.js. With the above code it doesn't start and I get the following error:

events.js:112
    throw new ERR_INVALID_ARG_TYPE('listener', 'Function', listener);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "listener" argument must be of type function. Received an instance of Object
    at checkListener (events.js:112:11)
    at _addListener (events.js:348:3)
    at Server.addListener (events.js:406:10)
    at new Server (https.js:71:10)
    at Object.createServer (https.js:91:10)
    at Object.<anonymous> (C:\code\test_node\server.js:9:22)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Module.load (internal/modules/cjs/loader.js:928:32)
    at Function.Module._load (internal/modules/cjs/loader.js:769:14) {
  code: 'ERR_INVALID_ARG_TYPE'
}

However, if I put all code in the same app.js file and I run node app.js, then the app starts fine, although the test produces the same error.

test_node/app.js

var https = require('https');
var fs = require('fs');
var express = require("express");
var server;
const app = express();

app.set('port', process.env.SERVER_PORT || 443);
app.get('/', function(req, res){
    res.status(200).send('OK');
});

async function handleSignal(signal) {
    console.log(`Received ${signal}`);
    server.close();
    process.exit();
}
process.on('SIGINT', handleSignal);
process.on('SIGTERM', handleSignal);
process.on('uncaughtException', handleSignal);

let serverOptions = {
    key: fs.readFileSync('./cert/server.key'),
    cert: fs.readFileSync('./cert/server.cert')
}
server = https.createServer(serverOptions, app).listen(app.get('port'), function () {
        console.log(`Express HTTPS server listening on port ${app.get('port')}`);
    });

module.exports = {
    app: app,
    server: server
}

To clarify, my question is how to fix the error from the jest test and not about the error when I run node server.js - the latter is just for context.

Answer 1

As @jonrsharpe pointed out in the comments, I was assuming that with module.exports I export the app itself, but in fact I export an object containing the app. Therefore to fix the test error, I had to simply write in my test.js:

server = require('../app').app;

instead of server = require('../app');. Similarly, in server.js I had to write:

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

This fixes both errors I mentioned in my question (TypeError: app.address is not a function).

Credits to @jonrsharpe for helping me find the solution!