Revised mocking, spies and unit testing with Jest today. Revisiting fundamental testing concepts via some great blogs by Kent C Dodds:

• But really, what is a JavaScript test?
• But really, what is a JavaScript mock?

jest.fn(), lets you mock the implementation of method that might be expensive or flakey to run. Usually you write a mock implementation to return some fake data like jest.fn((x)=> x) a.k.a jest.fn(<any arbitary function>)

Kent D Dodds talks about ‘monkey patching’ using jest.fn() which term which means to overide the exsiting functionality, which is useful when we need to mock a library. We can simply overide the implementation with our mock.

const originalGetWinner = utils.getWinner;
utils.getWinner = jest.fn((p1, p2) => p2);

• When using plain JS, utils.getWinner gets new properties adding in by jest such as .mock.instances, .mock.calls and mock.results which are using for a range of different asserations like what arguments were passed in, where the original instance from and what the actual result was.

• TS will complain that mock does not exist on the property, to get around this we can use a Jest spy instead. which a way to watch when the function/method was called. it looks like this:

• utils.getWinner = jest.fn((p1, p2) => p2);
  const spy = jest.spyOn(utils, "getWinner");
  
  const winner = thumbWar("alex", "jenny");
  expect(spy).toHaveBeenCalledTimes(2);
  expect(spy).toHaveBeenCalled();
  
  spy.mock.calls.forEach((args) => {
    expect(args).toEqual(["alex", "jenny"]);
  });

  it offers all the same functionality that is exposed on a .mock property.