I am mocking up some function:

export const natsWrapper = {
    client: {
    publish: jest
      .fn()
      .mockImplementation(
        (subject: string, data: string, callback: () => void) => {
          console.log("Mock function is called");
          callback();
        }
      ),
     },
};

and then I am calling that in my test:

it("publishes an event when ticket is created", async () => {
    await request(app)
    .post("/api/tickets")
    .set("Cookie", await (global as any).signin())
    .send({ title: "title", price: 20 })
    .expect(201);

    expect(natsWrapper.client.publish).toHaveBeenCalled();
});

Jest logs in console “Mock function is called” but test still failing:

    expect(jest.fn()).toHaveBeenCalled()                             
                                                                     
    Expected number of calls: >= 1                                   
    Received number of calls:    0