The code below was inspired by [this post][1] in Code Review.  
Here is how it was first intended by its author:

> I have the following code that converts a string that looks like :
> > aaa-bbb|ccc-ddd|eee-fff 

> to two strings that look like 
> > aaa|ccc|eee`and`bbb|ddd|fff

Somebody already proposed an answer, and I found it could be more improved using `reduce()` and `map()` methods.  
But after a few time I realized it could be made of more general use, so I hooked in the intent to have a code which both:

 - accept any main- and sub-separator (`|` and `-`) in the above example.
 - accept any number of main parts and sub-parts, the only (not checked) requirement being the number of sub-parts is the same through all parts.
 - is strictly written functional-programming style

Then I end up with pretty different requirements than in the original post I cited, so rather than an answer I prefer to post it as a question, waiting for comments about:

 - is there a simpler strategy for doing that in functional style?
 - and even is it a good idea to use functional style, since it might be actually slower than with procedural code?

<!-- begin snippet: js hide: false -->

<!-- language: lang-js -->

    function reformat(string, mainSep, subSep) {
      return string.split(mainSep).map(item => item.split(subSep)).reduce(
        (result, part) => 
          part.map((str, index) => 
            `${result[index]}${mainSep}${str}`
          )
      ).join(subSep || ' ');
    }

    console.log(reformat('HEL-CAS|MAD-STO|XXX-YYY', '|', '-')); 
    console.log(reformat('1:2:3_one:two:free_ONE:TWO:THREE_I:II:III', '_', ':'));
    console.log(reformat('abcdefgh.ABCDEFGH', '.', ''));

<!-- end snippet -->



  [1]: https://codereview.stackexchange.com/questions/119692/reformatting-string-to-two-string-values-in-javascript