Haskell translation

Question: why 1 <= len(commands) ? 0 would be perfectly valid, and it's actually 2 ( for a single fixed test ).

A lot of katas are trivial. By your logic we shouldnt have anything below 6 kyu

There's a difference between "simple and easy" and "trivial". Trivial is making a single stdlib call into a kata, and yes, those kata should not exist in my opinion ( other opinions may be available ). I wouldn't necessarily class this kata as trivial, but it wasn't me saying it it.

I am happy ( enough ) with the one character increase of the limit. Thank you!

A multiplication and a division are a mighty thin task to build a kata out of. Consider unpublishing this task and getting more better inspiration.

The result should not be an integer. Read documentation on why rounding is bad, and how to do it instead.

Feels like a duplicate.

Example tests:

    assert(reduce([5, 3, 6, 1], (a, b, i) => a + b * i), 23, 'hint: ')

That failure message doesn't hint very much.

[].push.call isn't push, it's call, without a context. Function.prototype.call.bind(Array.prototype.push) might work; I haven't actually checked.

Sample tests do contain mixed-case strings. Submit tests do not contain fixed cases.

I also fail so see the difference from single-case handling in the example solution. If the batch processing is a design choice, the example solution should showcase that difference.

That would make sense if the additional performance were actually required, and mapping individual modpows over the list would timeout. Is it, and does it?

Array length is a number, not a string.

My actual problem is console.log([]) printing [undefined] within my functions. This is apparently not caused by the enumerability of prototype methods, because it didn't go away when I fixed that. I don't know why [] would become [undefined], but it can't be good.

    for (const key of 'at,concat,copyWithin,entries,every,fill,filter,find,findIndex,findLast,findLastIndex,flat,flatMap,forEach,includes,indexOf,join,keys,lastIndexOf,map,pop,push,reduce,reduceRight,reverse,shift,slice,some,sort,splice,toLocaleString,toReversed,toSorted,toSpliced,unshift,with'.split(',')) {
      array_backup[key] = Array.prototype[key];
      Array.prototype[key] = () => {
        throw new Error('Whoops! Mr Array no longer has access to array.' + key + '(). You must make your own!')
      }
    }

No.

The correct way to extend prototypes is non-enumerably, probably with Object.defineProperty. Adding prototype properties enumerably gives all sorts of problems.

There is a difference between reduce(xs,fn,undefined) and reduce(xs,fn). Both are valid, so this scenario should be tested. ( See my solution how to test for missing or undefined parameter. )