Test Express.js routes which rely on a TypeORM connection

I have a very basic Express.js app which I use Jest and Supertest to test. The routes are not set up until the database is connected:

class App {
  public app: express.Application;

  public mainRoutes: Util = new Util();

  constructor() {
    this.app = express();

    AppDataSource.initialize()
      .then(() => {
        // add routes which rely on the database
        this.mainRoutes.routes(this.app);
      })
      .catch((error) => console.log(error));
  }
}

export default new App().app;

Here is my test:

describe("Util", function () {
  test("should return pong object", async () => {
    const res = await request(app).get("/ping");

    expect(res.statusCode).toEqual(200);
    expect(res.body).toEqual({ message: "pong" });
  });
});

Since I put in the promise, this has been 404ing. I can't add async to the constructor. I tried refactoring the class to separate the connection with setting up the routes, but it didn't seem to help.

This works:

test("should return pong object", async () => {
  setTimeout(async () => {
    const res = await request(app).get("/ping");

    expect(res.statusCode).toEqual(200);
    expect(res.body).toEqual({ message: "pong" });
  }, 1000);
});

But obviously I don't want to add a setTimeout. How is this usually done? I am new to testing.

Just remove the setTimeout() and await the call to the application. You should be initializing the application in the beforeAll() method, which I assume you have, to get the application up and running in the testing space. You should also mock your database connection, so you can fake the data you want back, and not have to wait for the external database to actually be available.

    // Create a mock for your database, and have it return whatever you need
    import <your-database-class> = require('database');
    jest.mock('database', () => {
        ...
    });

    describe("Util", function () {

    	beforeAll(async () => {
	    	app = await <whatever you do to launch your application>
    	});

	    test('should be defined', () => {
		    expect(app).toBeDefined();
	    });

        test("should return pong object", async () => {
            const res = await request(app).get("/ping");

            expect(res.statusCode).toEqual(200);
            expect(res.body).toEqual({ message: "pong" });
        });
    });