Modulo Operator %%

Alex Gordon alextgordon at gmail.com
Fri Aug 16 16:33:11 UTC 2019


Code that uses % is often surprisingly buggy. For example even a simple
function such as this is broken if n is negative:

function isOdd(n) {
    return n % 2 === 1;
}

isOdd(-3) // false

The same is not true in other programming languages. The same in Python
works just fine:

def isOdd(n):
    return n % 2 == 1

isOdd(-3) // true

The advantage of an operator over a function is that it allows us to say to
people who are new to JS: "Always use %% instead of % unless you have a
good reason". Just the same as we say "Always use === instead of == unless
you have a good reason".


On Thu, Aug 15, 2019 at 10:01 PM Isiah Meadows <isiahmeadows at gmail.com>
wrote:

> BTW, I just wrote up a more precise, formalized proposal over here:
> https://github.com/isiahmeadows/proposal-divisor-dependent-modulo/,
> and I'd be more than willing to work with a TC39 champion on it. I
> personally prefer syntax (pretty strongly), but I'm not beholden to
> it.
>
> I do feel the semantics are simple enough it'd be okay to lower it to
> syntax, and it naturally just glides right in. I find it *very* odd
> that some languages use a simple operator `%` or relatively short
> function for remainder keeping the sign of the dividend but relegate
> the version keeping the sign of the divisor (the more useful and
> intuitive of them) to a much more verbose function call. Of all
> Wikipedia lists in https://en.wikipedia.org/wiki/Modulo_operation,
> here's the four that do this currently - all but one expose an
> operator for the first:
>
> - Fortran: `mod` for dividend-dependent, `modulo` for divisor-dependent
> - Julia: `%`/`rem` for dividend-dependent, `mod` for divisor-dependent
> - Java: `%` for dividend-dependent, `Math.floorMod` for divisor-dependent
> - XBase++: `%` for dividend-dependent, `Mod` for divisor-dependent
>
> And it's worth noting most other languages (including some end
> user-oriented ones) that show a syntactic preference to one or the
> other expose a simpler one where the sign matches the divisor, a more
> complicated one where the sign matches the dividend. For a variety of
> examples:
>
> - Ruby: `%`/`modulo` for divisor-dependent, `remainder` for
> dividend-dependent
> - SML: `mod` for divisor-dependent, `Int.rem` for dividend-dependent
> - Elm: `modBy` for divisor-dependent, `remainderBy` for dividend-dependent
> - Euphoria: `mod` for divisor-dependent, `remainder` for dividend-dependent
> - Python: `%` for divisor-dependent, `math.fmod` for dividend-dependent
> - Smalltalk: `\\` for divisor-dependent, `rem:` for dividend-dependent
>
> And of course, many don't even expose a type of modulo where the sign
> matches the divisor. For some examples:
>
> - APL
> - LibreOffice/Excel
> - Lua
> - Perl
> - Mathematica
> - PL/I
> - TCL
>
> There's also Dart, a relatively new language which defaults to
> non-negative always.
>
> This relatively long list of languages, *despite* C's heritage and
> semantics being inherited in much of them, makes me question using a
> function for this, and there would need to be a *lot* of FUD to get
> people to use the function more than the operator.
>
> So this is why I would prefer an operator as opposed to syntax for this.
>
> -----
>
> Isiah Meadows
> contact at isiahmeadows.com
> www.isiahmeadows.com
>
> On Thu, Aug 15, 2019 at 3:58 PM Jordan Harband <ljharb at gmail.com> wrote:
> >
> > Static functions don't have the same risk as prototype functions;
> `Math.mod` would make sense to add.
> >
> > One suggestion, though, would be to try to add the API method first, and
> look at usage for awhile before trying to add the syntax.
> >
> > On Thu, Aug 15, 2019 at 10:12 AM Andrea Giammarchi <
> andrea.giammarchi at gmail.com> wrote:
> >>
> >> To me there's no risk, as MooTools, Prototype, and Scriptacolous are
> both things of the past, and never implemented Math.mod ... so, with that
> approach, custom transpiling functions are more dangerous, as somebody
> might have implemented `%%` already for other purposes, and we break Babel
> outcome adding new syntax anyway ... the smoosh accident, is the equivalent
> of custom Babel utilities these days.
> >>
> >> Look at TypeScript and the private class fields, if you want to compare
> new syntax instead
> >>
> >> On Thu, Aug 15, 2019 at 4:50 PM Michael Haufe <tno at thenewobjective.com>
> wrote:
> >>>
> >>> Thursday, August 15, 2019 2:47 AM, Andrea Giammarchi wrote:
> >>>
> >>>
> >>>
> >>> > FWIW another disadvantage is that operators cannot be polyfilled, so
> it'll take forever for those not using transpilers to adopt these, while
> having a `Math,mod` would work right away
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> With such an approach there is risk of another ‘smooshgate’ [1][2].
> There is nothing stopping those developers from using a function anyway to
> bridge the gap if they can’t or won’t use a compiler. This is already the
> current state of affairs.
> >>>
> >>>
> >>>
> >>> [1] https://developers.google.com/web/updates/2018/03/smooshgate
> >>>
> >>> [2]
> https://adamsilver.io/articles/the-disadvantages-of-javascript-polyfills/
> >>>
> >>>
> >>>
> >>> Michael
> >>
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