mumbling.js

// This time no story, no theory. The examples below show you how to write function accum:

// Examples:
// accum("abcd") -> "A-Bb-Ccc-Dddd"
// accum("RqaEzty") -> "R-Qq-Aaa-Eeee-Zzzzz-Tttttt-Yyyyyyy"
// accum("cwAt") -> "C-Ww-Aaa-Tttt"

function accum(s) {
  return s.split('')
    .map((char, index) => {
      const repeat = char.repeat(index+1)
      return (
        repeat.charAt(0).toUpperCase() +
        repeat.slice(1).toLowerCase()
      )
    })
    .join('-');
}

console.log(accum("ZpglnRxqenU"), "Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnnnnnnn-Uuuuuuuuuuu");
console.log(accum("NyffsGeyylB"), "N-Yy-Fff-Ffff-Sssss-Gggggg-Eeeeeee-Yyyyyyyy-Yyyyyyyyy-Llllllllll-Bbbbbbbbbbb");
console.log(accum("MjtkuBovqrU"), "M-Jj-Ttt-Kkkk-Uuuuu-Bbbbbb-Ooooooo-Vvvvvvvv-Qqqqqqqqq-Rrrrrrrrrr-Uuuuuuuuuuu");
console.log(accum("EvidjUnokmM"), "E-Vv-Iii-Dddd-Jjjjj-Uuuuuu-Nnnnnnn-Oooooooo-Kkkkkkkkk-Mmmmmmmmmm-Mmmmmmmmmmm");
console.log(accum("HbideVbxncC"), "H-Bb-Iii-Dddd-Eeeee-Vvvvvv-Bbbbbbb-Xxxxxxxx-Nnnnnnnnn-Cccccccccc-Ccccccccccc");