Event and CustomEvent ES6 polyfill solution for IE11

eventsPolyfill.js

const eventPolyfill = () => {
  if (typeof window.Event === 'function') {
    return false;
  }

  const Event = (event) => {
    const evt = document.createEvent('Event');
    evt.initEvent(event, true, true);
    return evt;
  };

  Event.prototype = window.Event.prototype;
  window.Event = Event;
  return true;
};

const customEventPolyfill = () => {
  if (typeof window.CustomEvent === 'function') {
    return false;
  }

  const CustomEvent = (event, params) => {
    const eventParams = params || {
      bubbles: false,
      cancelable: false,
      detail: undefined,
    };
    const evt = document.createEvent('CustomEvent');
    evt.initCustomEvent(event, eventParams.bubbles, eventParams.cancelable, eventParams.detail);
    return evt;
  };

  CustomEvent.prototype = window.Event.prototype;
  window.CustomEvent = CustomEvent;
  return true;
};

export default () => {
  eventPolyfill();
  customEventPolyfill();
};

eventsPolyfill.spec.js

import eventsPolyfill from './eventsPolyfill';

describe('eventsPolyfill.js', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });

  it('should do nothing if the browser supports Events/CustomEvent', () => {
    const event = jest.fn();
    const customEvent = jest.fn();
    global.window = {
      CustomEvent: customEvent,
      Event: event,
    };

    eventsPolyfill();

    expect(global.window.Event).toBe(event);
    expect(global.window.CustomEvent).toBe(customEvent);
  });

  it('should pollyfill Event/CustomEvent if the browser (IE11) does not support them', () => {
    global.window = {
      CustomEvent: 'I am IE9 and i succ',
      Event: 'I am IE9 and i succ',
    };

    eventsPolyfill();

    expect(global.window.Event).toBeInstanceOf(Function);
    expect(global.window.CustomEvent).toBeInstanceOf(Function);
  });

  it('should assign evt object to Event', () => {
    const evt = {
      initEvent: jest.fn(),
    };
    global.window = {
      CustomEvent: 'I am IE9 and i succ',
      Event: 'I am IE9 and i succ',
    };
    global.document = {
      createEvent: jest.fn().mockReturnValue(evt),
    };

    eventsPolyfill();

    expect(global.window.Event('My shinny event')).toBe(evt);
  });

  it('should assign evt object to CustomEvent', () => {
    const evt = {
      initCustomEvent: jest.fn(),
    };
    global.window = {
      CustomEvent: 'I am IE9 and i succ',
      Event: 'I am IE9 and i succ',
    };
    global.document = {
      createEvent: jest.fn().mockReturnValue(evt),
    };

    eventsPolyfill();

    expect(global.window.CustomEvent('SomeEvent.changed', { detail: 'some detail' })).toBe(evt);
  });
});

index.js

import eventsPolyfill from './eventsPolyfill';

eventsPolyfill();

This solution is based on the Mozilla Events docs: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#Polyfill