I'm pretty comfortable declaring that creating a mutable-by-default date, string, or number library is a mistake. I'm not on the make-all-the-things-immutable train but making dates mutable is a painful mistake that Java painfully worked through with JSR-310, the new date and time library.
Part of the problem is that moment would both mutate the underlying object and return the object.
const a = moment()
const b = a.add(1, 'second')
Both b and a are now the same object. I've lost a few hours to the design choice.
It’s just a matter of RTFM I guess, the a.add() method returns “a” so that you can chain methods on the same object without creating a new object on every method call.
Else: a.add(1, ‘second’).add(1, ‘day’).subtract(2, ‘minute’) would create 3 more objects that are not needed. Creating a new moment object is expensive, it happened to me to work with a lib that grew exponentially slow when using above N dates because they where constantly recreating moments instead of using them wisely so creating a new one on every operation would be quite bad.
That’s why the method .clone() exists, so you can declare that explicitly:
const a = moment()
const b = a.clone().add(1, ‘second’)
PS: don’t take RTFM as an offense, I just happen to like it a lot and we use it frequently at work, I don’t mean to be disrespectful towards you :)
I think the problem there is more one of library design than specifically a mutable object problem.
If `a` is mutated by the `.add()` method, then it should probably return void rather than returning a copy of the object.
Is the immutable DX still better in this specific example? Probably. But the above at least would help developers avoid the major footgun with mutability.
I think the Dev Tools in Chrome might have changed since (I don't use Chrome anymore), but we had issues maybe 5 years ago when hovering over momentjs watch statements in the debugger kept on mutating the underlying data. That was fun.
Part of the problem is that moment would both mutate the underlying object and return the object.
Both b and a are now the same object. I've lost a few hours to the design choice.