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The code below was inspired by this post 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|eeeandbbb|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?

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', '.', ''));

The code below was inspired by this post 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|eeeandbbb|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 ended 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?

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', '.', ''));

The code below was inspired by this post 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?

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', '.', ''));

The code below was inspired by this post 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|eeeandbbb|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?

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', '.', ''));