add() unexpectedly mutates the original object.

[ssoroka]

eg:

m = moment() # lets say it's Oct 3rd.
oct6 = m.add('days', 3)

oct6 is now oct 6th, as expected, but m has also changed, unexpectedly

oct7 = m.add('days', 4)

if add() had operated in a functional manner without mutating state, oct7 would in fact be set to october 7th, but because add() mutated the state of m, it's set to october 10th!

in fact, oct6 and oct7 are now the same object, m, and both reference october 10th.

this seems really unintuitive and can easily cause unintended consequences when moment objects are passed around in non-trivial applications.

[jgrocho]

I got bit by this as well, until I read Moment Clone.

All moments are mutable.

Thus now I assume most methods will mutate the original object. I think both approaches are valid, but mutation at least matches the original Date API which mutates objects.

You have to call moment#clone on the moment or wrap it in a call to moment like

m = moment() # let's say it's Oct 3rd.
oct6 = m.clone().add('days', 3)
oct7 = moment(m).add('days', 4)

[ichernev]

This is not a bug, its a feature! Read a discussion here: #390

It don't think this is ever going to change, at least because of compatibility issues.

[akenverus]

This is not a bug, its a feature! Read a discussion here: #390

It don't think this is ever going to change, at least because of compatibility issues.

This feature sucks.

[KristjanESPERANTO]

@akenverus As moment is no longer being actively developed, it is advisable to switch your projects to another library.

https://momentjs.com/docs/#/-project-status/