Allow ability to delete default units

[Virushaan]

Describe the bug
Currently a scalar divided by a time unit such as 5/seconds will get converted into hertz 5Hz. However, for my usecase, hertz(hz) doesn't make sense to convert into. Ideally it would be possible to keep the evaluated value as 5/seconds and not simplify any further.

I have tried
math.Unit.deleteUnit('Hz')
and
delete math.Unit.UNITS['hertz'];
and neither seemed to have any impact. Open to any thoughts or ideas! Thanks in advance :)

[dvd101x]

I've found a few possibilities

5 / s to s^-1      # 5 s^-1
a = unit(5, "1/s") # 5 s^-1
a + 1/s            # 6 s^-1

Another possibility is to use createUnit with an override. So you could assign Hz to something that doesn't makes sense.
math.createUnit('Hz', '1 m^100', {override: true}) but I don't know if this one works at the level you need.

Maybe there is a trick to your use case, can you show your code that converts 5 / seconds to 5 Hz? Maybe it's possible to trick the system.

[gwhitney]

I agree with @dvd101x; the "Hz" unit is just a synonym for 1/s, it's not truly a "simplification" of it. It may be used for input/output, but it doesn't affect the underlying unit value. In what code exactly is "Hz" getting in your way?

And indeed, in JavaScript rather than MaJE (the MathJs Expressions language) we have

const oneway = math.evaluate('5/s')
const otherway = math.unit(5, '1/s')
console.log(oneway.format()) // 5 Hz
console.log(otherway.format()) // 5 s^-1
assert.deepStrictEqual(oneway, otherway) // fails, but shows that the _only_
// difference between these two Unit objects is that 
// oneway.skipAutomaticSimplification is false, whereas
// otherway.skipAutomaticSimplification is true.

So if you are handling these units in JavaScript, you could just manually set the skipAutomaticSimplification property to true. If you are handling them in MaJE, then you need to either use the in operator, the to operator, or the two-argument unit function to ensure that you get an object with skipAutomaticSimplification set to true. But in all of these cases, the internal unit calculations will be identical regardless of the value of skipAutomaticSimplification. It's just a matter of display/conversion to string, not of the underlying value.

[Virushaan]

Thanks for the suggestions, for whatever reason the overrides weren't working but I was able to solve this by deleting all the references to hertz and frequency like

delete math.Unit.BASE_UNITS['FREQUENCY'];
delete math.Unit.UNIT_SYSTEMS.si['FREQUENCY'];
delete math.Unit.UNITS['hertz'];
delete math.Unit.UNITS['Hz'];

It might be nice to expose an api to handle deleting an entire system as I believe this won't delete from the _findUnit cache.

[gwhitney]

Thanks for the suggestion. At the moment I don't think we've seen the motivation for deleting built-in units, so it's unlikely such an API will be added. Thanks for closing the discussion, given that you've gotten your situation to work, apparently.

[nycos62]

is there a flag under math.config that allow or disable usage of units ?

[gwhitney]

The full documentation of math.config can be found at https://mathjs.org/docs/core/configuration.html; it does not generally speaking have any options to turn on or off entire feature sets of mathjs. There is a number-only bundle that has no Units, but it only has the JavaScript built-in number type (no BigNumber, Fraction, Complex, Matrix, etc.). You may be able to create a bundle with the just the types you want using the information at https://mathjs.org/docs/custom_bundling.html. We hope to make customization of the types/features loaded significantly simpler in a future release of mathjs.