Given a string `word` of arbitrary length insert character `char` into `word` every `N` characters without modifying `word`...

breakup.go

func drewboyuka__breakup(s, placeholder string, n int) string {
  var buf []byte
  for ; len(s) > n; s = s[n:] {
    buf = append(buf, s[:n]...)
    buf = append(buf, placeholder...)
  }
  buf = append(buf, s...)
  return string(buf)
}

breakup.js

/**
 * @name {USERNAME}__breakup
 *
 * @param {String} word to split by {char} every {N} characters
 * @param {String} char to insert into {word} every {N} characters
 * @param {Number} N number of characters to insert {char} after
 * @return {String} 
 */

function al_the_x__breakup (word, char, N){
  let n = 0, m = N;
  let part, parts = [ ];
  
  while ( part = word.slice(n,m) ){
    parts.push(part);
    n = m; m += N;
  }
  
  return parts.join(char);
}

function jfontanez__breakup(word, char, N){
  return word
    .split('')
    .map((val, idx) => idx && idx % N === 0 ? `${char}${val}` : val)
    .join('');
}

I wanted to put this in a Tonic notebook instead, but it looks like they've changed names to RunKit instead... and forgotten my GitHub account completely. Begin unnecessarily long username recovery process!