Test a listener is called with expected CustomEvent()

Today I realized we had an issue in a test (Jest)

We have a function that emits CustomEvents

In our Jest test, we define a listener and check that it's called with the expected event

---

I tried changing the detail (the internal object) of the custom event

... and the test was still passing 🤔

  test('events.voiceEvent() emits "CustomEvent{ lavoiceevent }" event', () => {
    const expectedEvent = new CustomEvent('lavoiceevent', {
      detail: { event: 'USER_SAID_HELLO_ERRRRROR' },
    });

    window.addEventListener('lavoiceevent', listener);
    events.voiceEvent('USER_SAID_HELLO');

    expect(listener).toHaveBeenCalledWith(expectedEvent); // ✅
    expect(listener).toHaveBeenCalledTimes(1);
  });

The problem is this line won't work as expected (its an issue related with the serialization of the Events)

  expect(listener).toHaveBeenCalledWith(expectedEvent)

In order to solve this (and make our test does check the detail of the event) we should use "mock.calls" as:

  test('events.voiceEvent() emits "CustomEvent{ lavoiceevent }" event', () => {
    const listener = jest.fn();
    window.addEventListener('lavoiceevent', listener);

    events.voiceEvent('USER_SAID_HELLO');

    expect(listener).toHaveBeenCalledTimes(1);
    expect(listener).toHaveBeenCalledWith(expect.any(CustomEvent));

    const ev = listener.mock.calls[0][0];
    expect(ev.type).toBe('lavoiceevent');
    expect(ev.detail.event).toBe('USER_SAID_HELLO'); // 🔥 will fail otherwise
  });

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thanks for reading.