Calling add() on a moment object causes it to be mutated

[felamaslen]

Description of the Issue and Steps to Reproduce:

When calling .add on a moment object, the object is mutated in a way which is hard to debug. For example:

const { expect } = require('chai');
const moment = require('moment');

const date1 = moment(new Date('2018-03-04'));

expect(date1.format('YYYY-MM-DD')).to.equal('2018-03-04'); // succeeds

const date2 = date1.add(3, 'days'); // this *mutates* date1 so that date1 is now 2018-03-07 (!)

expect(date2.format('YYYY-MM-DD')).to.equal('2018-03-07'); // succeeds
expect(date1.format('YYYY-MM-DD')).to.equal('2018-03-04'); // FAIL: date1 has been mutated

Environment:

Node.JS v8.9.1 on Ubuntu 17.10 (this really doesn't matter)

Output of the following:

console.log( (new Date()).toString())
console.log((new Date()).toLocaleString())
console.log( (new Date()).getTimezoneOffset())
console.log( navigator.userAgent)
console.log(moment.version)

Sun Mar 04 2018 18:59:10 GMT+0000 (GMT)
2018-3-4 18:59:10
0
undefined
2.21.0

Ensure your issue is isolated to moment. Issues involving third party tools will be closed unless submitted by the tool's author/maintainer.

[felamaslen]

The same behaviour is also exhibited when running .startOf() instead of .add().

[marwahaha]

We're aware of this. Personally, I don't love the mutability, but we haven't released 3.0 (it's been a few years - see more info here).

One of the moment contributors just released Luxon which has an immutable API. If you (or any folks reading) want to help us release an immutable version of Moment, let us know.