Mock using jest moment().format is returning current date instead of mocked date

Question

I am trying to mock moment library's format function using jest. I have following code in my test file.

app.spec.js:

jest.mock('moment', () => {
    const moment = () => ({
        format: () => mockedTime
    });
    moment.tz = {
        setDefault: () => {}
    };
    moment.tz.setDefault('Asia/Singapore');
    return moment;
});

app.js:

moment.tz.setDefault(TIMEZONE);
moment().format('YYYYMMDD');

it is generating following output:

 - "date": "20190825", // mocked date
 - "date": "20190827", // result value

the expected output should be:

 - "date": "20190825", // mocked date
 - "date": "20190825", // result value

Can anyone help me point out what's wrong with the code?

Thanks.

Possible duplicate of: https://stackoverflow.com/questions/55838798/mocking-moment-and-moment-format-using-jest — Tymek, Aug 27 '19 at 12:27
why do you use `doMock`in favor of `.mock`? first one does not affect code been already `import`ed — skyboyer, Aug 27 '19 at 12:32

score 4 · Accepted Answer · answered Aug 28 '19 at 07:18

Mocking 'moment-timezone' instead of 'moment fixed it.

jest.mock('moment-timezone', () => {
    const moment = () => ({
        format: () => mockedTime
    });
    moment.tz = {
        setDefault: () => {}
    };
    moment.tz.setDefault('Asia/Singapore');
    return moment;
});

score 2 · Answer 2 · answered Dec 16 '20 at 18:46

2

The available answers did not work for my case. Mocking the underlying Date.now function however did as this answer suggests: https://stackoverflow.com/a/61659370/6502003

Date.now = jest.fn(() => new Date('2020-05-13T12:33:37.000Z'));

answered Dec 16 '20 at 18:46

protoEvangelion

4,355
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score 0 · Answer 3 · answered Aug 27 '19 at 12:23

0

You call format not on moment, but on the result of moment().

    jest.doMock('moment', () => {
        const moment = () => ({
            format: () => mockedTime,
        })

        moment.tz = { // prevent 'Cannot read property of undefined'
            setDefault: () => {},
        }

        return moment
    });

Is this OK, or do you need more complicated mock in you app, including timezone?

answered Aug 27 '19 at 12:23

Tymek

3,000
1
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46

i need timezone included – asfandahmed1 Aug 28 '19 at 01:44
1

It throws a `typeError: can not read property 'split' of undefined` – asfandahmed1 Aug 28 '19 at 01:55
If you have another operation on `moment` you should also mock other methods used. Since you shared only small part and not a working snippet, it's hard to debug. – Tymek Aug 28 '19 at 08:54

Mock using jest moment().format is returning current date instead of mocked date

3 Answers3