Mock Fetch with Jest

mockFetch.js

// __mocks__/mockFetch.js

const MockFetch = responseJson => (
  () => (
    new Promise(resolve => resolve({
      json: () => JSON.parse(JSON.stringify(responseJson)),
    }))
  )
);
 
export default MockFetch;

// app.test.js

/*--------------USAGE WITH JEST------------------*/
// note: for usage with other frameworks look up async testing
 
import MockFetch from '../__mocks__/mockFetch';
 
test('SomeAsyncFunc returns json parsed value from Promise', async () => {
  global.fetch = MockFetch({
    cat: 'meow',
  });
  const response = await SomeAsyncFunc();
  expect(response.cat).toBe('meow');
});

This isn't a Jest mock. This just shows how I use it with jest and the global polyfill for whatwg-fetch. You'll probably want to reset global.fetch to avoid shared state between tests (i.e. calling it with no methods will return the previous data).

Also see: facebook/create-react-app#967