Playing with passphrases

index.js

function playPass(s, n) {
    let finalStr = '';
    for (let i in s) {
      let letter = s[i];
      if (isLetter(letter)) {
        finalStr += upperOrLower(nextLetter(letter, n), i);          
      } else {
        if (isNumeric(letter)) {
          finalStr += "" + nineComplement(letter);
        } else {
          finalStr += letter;
        }
      }
    }
    
    return reverse(finalStr);
}

function nineComplement(digit) {
  return 9 - digit;
}

function isLetter(c) {
  return c.toLowerCase() != c.toUpperCase();
}

function nextLetter(char, num) {
    let alphabet = "abcdefghijklmnopqrstuvwxyz",
        isUpper = char === char.toUpperCase() ? true : false;

    char = char.toLowerCase();

    let newIndex = alphabet.indexOf(char) + num;
    if(newIndex < alphabet.length) {
      char = alphabet[newIndex];
    } else {
      let shiftedIndex = -(alphabet.length - newIndex);
      char = alphabet[shiftedIndex];
    }
    
    if (isUpper) {
      char = char.toUpperCase();
    }
      
    return char;
}

function isNumeric(n) {
  return !isNaN(parseInt(n));
}

function upperOrLower(c, position) {
  if (position%2 == 0) {
    return c.toUpperCase();
  } else {
    return c.toLowerCase();
  }
}

function reverse(s) {
  return Array.from(s).reverse().join('')
}