Why does npm test return TypeError: app.use() requires a middleware function when server gets passed to it?

Question

I am attempting to write some simple tests for my backend, and I am following a tutorial where a very similar code functions, while mine does not (he gets no complain about his app.use()). Why does it throw TypeError: app.use() requires a middleware function, and what can I replace my current code with to get it to run my test?

Here is my code:

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

const api = require('../server');

// make api available
app.use(api);
//app.use(bodyParser);

describe('games', () => {
    it('should return a saved game', () => {
        // send a request
        return request(app)
            .post('/games')
            .send({title: 'test', released: '1995'})
            // verify response
            .expect(201)
            .expect(res => {
                const game = res.body;
                expect(game.title).toEqual('test');
                expect(game.released).toEqual('1995');
            });
    });
});

Here is the error from the console:

"C:\Program Files\nodejs\node.exe" C:\Users\Bertinator\Desktop\web-exam\node_modules\react-scripts\scripts\test.js --colors --setupTestFrameworkScriptFile "C:\Program Files\JetBrains\IntelliJ IDEA 2017.2.4\plugins\JavaScriptLanguage\helpers\jest-intellij\lib\jest-intellij-jasmine.js" --testPathPattern ^C:\\Users\\Bertinator\\Desktop\\web\-exam\\src\\backend\\__tests__\\game\.jest\.js$
 FAIL  src\backend\__tests__\game.jest.js
  ● Test suite failed to run

    TypeError: app.use() requires a middleware function

      at Function.use (node_modules/express/lib/application.js:210:11)
      at Object.<anonymous> (src/backend/__tests__/game.jest.js:10:5)
          at <anonymous>

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        1.761s

Answer 1

I fixed it by adding

module.exports = app;

To my server.js. Jest was not able to use it as a valid middleware, hence the error.