Mocking the content of an iframe with React Testing Library

Question

I've got a React component that returns an iframe and handles sending messages to and receiving messages from the iframe. I am totally stumped as to how I can mock the iframe and simulate a message being posted in the iframe with React Testing Library. Any ideas or examples would be very helpful.

Component:

const ContextualHelpClient: React.VFC<CHCProps> = ({ onAction, data }) => {
  const [iframe, setIframe] = React.useState<HTMLIFrameElement | null>(null);

  const handleMessage = React.useCallback((event: MessageEvent<ContextualHelpAction>) => {
    onAction(event.data);
  }, [onAction])

  const contentWindow = iframe?.contentWindow;

  React.useEffect(() => {
    if (contentWindow) {
      contentWindow.addEventListener('message', handleMessage);
      return () => contentWindow.removeEventListener('message', handleMessage);
    }
  }, [handleMessage, contentWindow]);

  React.useEffect(() => {
    if (contentWindow) {
      contentWindow.postMessage({ type: CONTEXTUAL_HELP_CLIENT_DATA, data }, "/");
    }
  }, [data, contentWindow]);

  return <iframe src={IFRAME_SOURCE} width={300} height={300} ref={setIframe} title={IFRAME_TITLE} />;
};

Test:

import React from 'react';
import { render, screen, fireEvent } from '@testing-library/react';
import ContextualHelpClient from '../ContextualHelpClient';
import { IFRAME_SOURCE, IFRAME_TITLE } from '../constants';

describe('<ContextualHelpClient />', () => {
  it('should call onAction when a message is posted', () => {
    const handleAction = jest.fn();
    const content = render(<ContextualHelpClient onAction={handleAction} />);
    const iframe = content.getByTitle(IFRAME_TITLE);
    fireEvent(iframe.contentWindow, new MessageEvent('data'));
    expect(handleAction).toBeCalled(); // fails
  });
});

Answer 1

Solved!

it('should call onAction when a message is posted', async () => {
  const handleAction = jest.fn();
  const content = render(<ContextualHelpClient onAction={handleAction} />);
  const iframe = content.getByTitle(IFRAME_TITLE) as HTMLIFrameElement;
  const TEST_MESSAGE = 'TEST_MESSAGE';
  // https://github.com/facebook/jest/issues/6765
  iframe.contentWindow?.postMessage(TEST_MESSAGE, window.location.origin);
  
  await waitFor(() => expect(handleAction).toBeCalledWith(TEST_MESSAGE));
});