I am not completely sure why there are curly brackets and plys inside but i know rest. So this regex will try with match method to match all odd OR even numbers from num(which will be always num because here we passed if/else with ternary operator) and then match is returning array of matched elements, so at the end he is passing join() method with '-' as with what to join them.

Got 'er done, anyway (with plenty of ES6, to boot).

Did MVD (Minimum Viable Debug). 10 seconds.

Nice: switch (true) so you can evaluate inequalities (even if not efficient).

Lot of lines, but hopeflly easy to follow, even for new beginners.

Was in a bit of a rush. And, what do you know, it shows! At a minimum, I would change the name of 'interim' to 'resultsArray' and 'let' to 'const'.

Refactored:

function currencyBRL(number) {
  return 'R$ ' + number.toFixed(2).replace('.', ',');
}

Nice!