Is there any way to clone a Unit but with a different value?

[gwhitney]

Suppose you are given a Unit value, and you don't know what units it is in, but you just want to create a different quantity in the same units. Is there a way to do it with just mathjs library functions/in the MaJE (MAthJs Expressions) parser? Diving into the internal structure of Unit in JavaScript, I think the following JavaScript function works, but it seems to break encapsulation. The basic difficulty is that I can't seem to find a function that returns the "valueless unit-only" part of a Unit value.

// put n into the same units as u
function cloneAs(n, u) {
  const w  = u.clone()
  w.value = null
  return new math.Unit(n, w)
}

Any guidance would be welcome, or should I file an issue to add a method like u.unitPart() or u.valuelessUnit() or something like that?

[dvd101x]

Hi, this is a possibility

cloneAs(n, u) = unit(n, string(u));
A = unit(100, 'm')  # 100 m
cloneAs(2, A)       # 2 m

[dvd101x]

I agree it's "a hack".

A cleaner way might be to import a similar version of your function without breaking encapsulation and following documentation.

math.import({
  cloneAs: (n, u) => math.unit(n, u.toJSON().unit)
})

And then use it

A = 9.8 m/s^2   # 9.8 m / s^2
cloneAs(2, A)   # 2 m / s^2

[gwhitney]

Fair, but I think better would be a PR to add a method on the Unit class. Reasonable?

[dvd101x]

Yes it's reasonable, maybe to be able to build those kinds of functions in the parser a function called unitOf similar to typeOf. I don't know, let's wait to hear Jos's opinion.

[gwhitney]

Sure, sounds fine to get his take ;-) But of course, unit methods can be called from MaJE -- try entering e.g. (5000 m).format() in the REPL on mathjs.org. So if a Unit method is provided, there doesn't necessarily have to be a top-level mathjs function in addition.

[dvd101x]

Oh woow. I'm out of the loop on this one. I'll try to follow better the relevant discussions.