dayjs().add(Duration) API

[akoskm]

Hello, I'm replacing moment.js with dayjs in a project.

I'm not sure if the APIs are not compatible at this point or I'm making a mistake here:

// the old code basically had `moment` where you see dayjs now

const date1 = dayjs('2018-04-04T16:00:00.000Z')
const date2 = dayjs();
const duration = dayjs.duration(dayjs().diff(date1));
const date3 = date2.add(duration);
console.log(date3);

the output of the above code is:

    d {
      '$L': 'en',
      '$u': undefined,
      '$d': Invalid Date,
      '$x': {},
      '$y': NaN,
      '$M': NaN,
      '$D': NaN,
      '$W': NaN,
      '$H': NaN,
      '$m': NaN,
      '$s': NaN,
      '$ms': NaN
    }

Looking at the implementation of add:

dayjs/src/index.js

Lines 220 to 221 in f95ac15

	add(number, units) {
	number = Number(number) // eslint-disable-line no-param-reassign

it always expects the first parameter to be a number so that could one reason why this isn't working.

In moment.js we could add a Duration to a Moment object: https://momentjs.com/docs/#/manipulating/add/

One workaround would be to convert the duration to milliseconds and add it to the date2:

const date1 = dayjs('2018-04-04T16:00:00.000Z')
const date2 = dayjs();
const duration = dayjs.duration(dayjs().diff(date1)).asMilliseconds();;
const date3 = date2.add(duration, 'ms');

[iamkun]

Day.js does not support adding Duration at present.

However, we could support it in the Duration plugin if needed.

[iamkun]

This will be a nice feature added to the Duration plugin.

It's much like the workaround code mentioned above.

Feel free to draft a PR if anyone is interested in it.

[iamkun]

🎉 This issue has been resolved in version 1.9.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[franky47]

Thanks for this feature !

There are issues with the calculation though:

dayjs('2020-01-01').add(1, 'month') // 2020-02-01 ✅
dayjs('2020-01-01').add(dayjs.duration(1, 'month')) // 2020-01-31 ❌

Reproduction example:
https://runkit.com/franky47/600431082af3c5001a19f25d

This is because the add method uses the duration in milliseconds, which has no notion of context (years, months, days and minutes have variable numbers of milliseconds in them).

[mi-na-bot]

@iamkun I am finding that in 1.11.9 this does not work, with the same invalid date output. It might be a regression!