Automatic cancelation of derived units

[dkirkby]

When I multiply two quantities, I would expect that any unambiguous cancellations of derived units would be applied automatically, however this example using mathjs@7.2.0 returns "(J m) / (m s)" instead of the expected "J / s":

{
  let a=math.unit(1, 'J/m');
  let b=math.unit(1, 'm/s');
  return math.multiply(a,b).toJSON().unit;
}

If there are performance reasons to defer this type of automatic simplification, perhaps a method to manually trigger would be useful (and apologies if it already exists and I missed it). The toSI method is not what I am looking for since it returns "(kg m^2) / s^3" which is less useful than "J / s" in many contexts.

I noticed the skipAutomaticSimplification attribute, but setting this false on a and b before multiplying does not change the result.

[josdejong]

In most cases the units will be simplified. @ericman314 do you have an idea why toSI returns (kg m^2) / s^3 in this case?

[ericman314]

I'm not sure. If anything, I would think it should return W since that is even simpler than J / s. It doesn't really "know" that it should cancel the m's, it just looks at the entire unit to see if there is a single unit (like W) that fits.

Units are not actually simplified until they are formatted for output; this does improve the performance if you have lots of calculations in a row, and output one answer at the end. But this also means toString has side effects--ouch!

Because of these problems, I've been working on a complete overhaul of the units for a while now, and when that is ready it should address issues like this.

[josdejong]

Thanks for your feedback. Can we already try this example out in UnitMath? :)

[ericman314]

Yes--I believe it should be working there.

[dkirkby]

@ericman314 do you have an idea why toSI returns (kg m^2) / s^3 in this case?

That is the correct answer here since toSI should reduce to the unique combination of base SI units. That's not always useful for human consumption, but needed as a canonical form, e.g. to test equality of different dimensioned expressions.