App.working.test.js

import React from 'react';
import { render } from '@testing-library/react';
import App from './App';
import MyComponent from './MyComponent';

const MockMyComponent = () => {
  React.useEffect(() => {
    console.log('using an effect');
  });
  return (<div>Hello World</div>);
};
jest.mock('./MyComponent', () => ({
  __esModule: true,
  namedExport: jest.fn(),
  default: jest.fn()
}));

beforeAll(() => {
  MyComponent.mockImplementation(MockMyComponent);
});

test('renders', () => {
  const { container } = render(<App/>);
  expect(container.textContent)
    .toMatch('Hello World');
});