Mock Fetch with Jest

fetch.mock.js

const mockResponse = (status, statusText, response) => {
  return new window.Response(response, {
    status: status,
    statusText: statusText,
    headers: {
      'Content-type': 'application/json'
    }
  });
};

test('fetch something', () => {
  window.fetch = jest.fn().mockImplementation(() =>
    Promise.resolve(mockResponse(200, null, '{"foo":"bar"}')));
  
  fetch('/foo/bar')
    .then(r => r.json())
    .then(json => expect(json.foo).toEqual('bar'));
});