How do you mock just certain parts of a module with jest?

Question

I've built a calendar around moment.js and I'm working on unit tests right now.

The first problem I solved was how the date will change when the tests run, so I've been able to lock down the moment using this guidance.

Currently, I'm stuck on an error:

"TypeError: Cannot read property 'weekdaysShort' of undefined"

My code has a line: const dateHeaders = moment.weekdaysShort();

By implementing the mocked moment().format(), I've essentially lost the rest of the library.

My immediate question is how I can set up jest to let me return the array that you get from moment.weekdaysShort();

My larger question is whether I've gone down the wrong path and should come up with another strategy.

Things I've tried with unsuccessful results

1. Manually adding in the weekdayShort function:

const mockMoment = function() {
  return {format: '2016–12–09T12:34:56+00:00'}
};

mockMoment['weekdaysShort'] = () => ['Sun', 'Mon', 'Tues']; // etc etc etc

jest.mock('moment', () => mockMoment);

2. Assembling a manual mock in a __mocks__ folder. I didn't go too far down this path because it started to feel like I'd have to copy/paste the entire Moment.js library into the mock. And while it'd be cool to figure out how they do what they do, that's a project for another day.
3. jest.spyOn - doesn't work because I'm not spying on a module.

At this point, I'm considering abandoning the Moment function for an array passed in through props. And while I'm confident that'll get me past this problem, it feels like I'm gonna hit another roadblock quickly afterwards.

Thanks in advance for the help.

Answer 1

Just found out the pattern I commonly use was provided in a 2016 github thread and, in all honesty, that's probably where I found it even though I don't specifically remember it :)

jest.mock('moment', () => 
  const original = jest.requireActual('moment');
  return {
    __esModule: true,
    default: {
       ...original,
       ...just the parts you want to mock
    }
  }
);