Testing functions only called by useEffect

MyFunctionComponent.test.tsx

import React from 'react';
import { mount } from 'enzyme';

import formEventEmitter from '../formEventEmitter';
import MyFunctionComponent from '../MyFunctionComponent';

jest.mock('../formEventEmitter');

it('should check if preventCancel is being called', () => {
    const dialog = { confirm: jest.fn() };
    const preventCancel = jest.fn();
    const cb = jest.fn();

    // here is where the magic ✨ happens, thanks to @gabrielgene
    formEventEmitter.on.mockImplementation((_, handleCancelEvent) => handleCancelEvent(preventCancel, cb));

    mount(<MyFunctionComponent keySignal={'a'} isDirty={true} dialog={dialog} />).mount();

    expect(preventCancel).toHaveBeenCalledTimes(1);
    expect(dialog.confirm).toHaveBeenCalledTimes(1);
});

MyFunctionComponent.tsx

import React, { useEffect } from 'react';

import formEventEmitter from './formEventEmitter';
import { withConfirmDialog, ConfirmDialogProps } from './withConfirmDialog';

type Props = {
  keySignal: string;
  isDirty?: boolean | null;
} & ConfirmDialogProps;

export const MyFunctionComponent = ({ keySignal, isDirty = false, dialog }: Props) => {

  const handleCancelEvent = (preventCancel: () => void, onConfirm: () => void) => {
      if(isDirty) {
        preventCancel(); // we wanna check if this function will be called
        dialog.comfirm({
          title: 'Title',
          description: 'Description',
          onConfirm,
        });
      }
    }
  }

  useEffect(() => {
    formEventEmitter.on(keySignal, handleCancelFormEvent);

    return () => {
      formEventEmitter.removeListener(keySignal, handleCancelFormEvent);
    }
  }, []);

  return null;
};

export default withConfirmDialog(MyFunctionComponent);