Problem with testing whether a function has been run

I'm new to testing. I've tried to research this, but with no luck.

Problem is, that I'm trying to test my app root file, called couples2.ts, which is the entry for my NodeJS/TypeScript app.

It looks like this:

import App from "./app";

const app = new App();
app.run()

Now I'm trying to test whether the run() function has been called, and so I wrote this couples2.test.ts file with the following code, but it says that tests failed and the reason is that it's been called 0 times.

import App from "./app";

jest.mock('./app');

describe('couples2.ts file', () => {
    it('should run the `run` function', function () {
        const app = new App();
        expect(app.run).toHaveBeenCalled();
    });
})

What am I doing wrong?