Fixed

Some useful links: .replace(), ternary operator, comma operator, arrow functions.

1. Arrow function match => (matches++, matches % n ? match : newValue) is executed for every new RegExp(oldValue, "g") match.
2. Here we have a comma operator ( (matches++), (matches % n ? match : newValue) ), so operands are evaluated from the left to the right and the last operand is returned.

• matches++ increments our occurrences counter; now we have the current occurrence number.
• (matches % n) ? match : newValue expression uses ternary operator. If occurrence number fits with nth rule - change it to newValue, otherwise left match untouched by returning itself.

Note: using comma operator in a production code considered as a bad practice unless other is defined by team/company rules.

The way of avoiding comma operator in here is replacing matches++, matches % n ? match : newValue with ++matches % n ? match : newValue, which is also a kind of bad practice. Incrementing value should take own separate line of code.

Figured out, thank you for the interesting kata!

Numbers are not in order, but Random Test expecting true:

[ 5855, 1986, 981, 5792 ]
✘   Expected: true, instead got: false  

Thanks for noticing, the description has been updated.

Pleased to hear :)

I'm glad you liked it :)

Thanks for helping! ^_^.

I think adjusting the description a bit by kata author would be helpful for other warriors.

"In the case of duplicate alphabet letters, you have to arrange them according to the number that appeared first in the input string" - confused me.
Blocks like "22y6" not a numbers, so that sounds like subsorting by first number occurance in block is needed.

It's a bit harder than other 8 kyu katas, imho.