A simple way to mock/unmock globals object methods or constant

mock-global-object-jest.js

/* global expect jest */

const delay = ms => fn => setTimeout(fn, ms);

const mockGlobalProperty = globalObject => key => value => {
  // save original implementation in order to unmock later
  const original = globalObject[key];
  
  // mock key on the global object
  Object.defineProperty(globalObject, key, { value, writable: true });
  
  // return an unmock function restoring original implementation
  return () => mockGlobalProperty(globalObject)(key)(original);
};

const mockWindow = mockGlobalProperty(global);
const mockSetTimeout = mockWindow('setTimeout');

describe('delay()', () => {
  it('should call seTimeout with the good params', () => {
    const setTimeoutMock = jest.fn();
    const unmockSetTimeout = mockSetTimeout(setTimeoutMock);
    const fn = jest.fn();
    
    delay(200)(fn);
    expect(setTimeoutMock).toHaveBeenCalledWith(fn, 200);
    
    // mandatory. try to comment this line and see how 
    // the 2nd test is affected by the mock
    unmockSetTimeout();
  })
  
  it('should work with no mock', done => {
    let thing = 'foo';
    delay(200)(() => thing = 'bar');
    expect(thing).toBe('foo');
    setTimeout(() => {
      expect(thing).toBe('bar');
      done();
    }, 200);
  });
});

demo: https://repl.it/ICox/4