function sep(letters){

  if(!letters) return [];

  let arrLetters = letters.split('')

  let arrPair = []

  for(i=0;i<arrLetters.length;i+=2){

    if(!arrLetters[i+1])  arrLetters[i+1] = '_';

    alert()

    arrPair.push(arrLetters[i]+arrLetters[i+1])

  }

  return arrPair

  }

​

console.log(sep('abc'))

console.log(sep('abcdef'))