Mocking Observable.ajax.get

requests.spec.ts

jest.mock('./process-env');

import Mock = jest.Mock;
import {Observable} from 'rxjs';

import {get} from './requests';
import {getProcessEnv} from './process-env';

describe('get', () => {
  const originalAjaxGet = Observable.ajax.get;
  const API_KEY = 'some';

  beforeEach(() => {
    // because "get" method uses "API_KEY", that returned by "getProcessEnv" method
    (getProcessEnv as Mock<any>).mockImplementationOnce(() => ({
      API_KEY
    }));
  });

  afterEach(() => {
    Observable.ajax.get = originalAjaxGet;
  });

  it('Observable.ajax.get is called once', () => {
    Observable.ajax.get = jest.fn(() => Observable.of(null));

    get('http://some-url.com');

    const calls = (Observable.ajax.get as Mock<any>).mock.calls;

    expect(Array.isArray(calls)).toBeTruthy();
    expect(calls.length).toEqual(1);
  });

  it('Observable.ajax.get is called with 1-st arg which equals to a passed URL', () => {
    Observable.ajax.get = jest.fn(() => Observable.of(null));

    get('http://some-url.com');

    const firstCall = (Observable.ajax.get as Mock<any>).mock.calls[0];
    const firstArg = firstCall[0];

    expect(firstArg).toEqual('http://some-url.com');
  });

});

Thanks.
Could it possible to mock get other than null?