[JS] Rotational cipher

rotationalCipher.js

LOWERCASE_LETTERS = "abcdefghijklmnopqrstuvwxyz";
function isNumber(char) {
  return !isNaN(Number(char));
}

function isLowerCase(char) {
  return char === char.toLowerCase();
}

function rotationalCipher(input, rotationFactor) {
  let result = "";
  for(const char of input) {
    const charIndex = LOWERCASE_LETTERS.indexOf(char.toLowerCase());
    if(charIndex > -1) {      
      const cipheredChar = LOWERCASE_LETTERS[((charIndex + rotationFactor) % LOWERCASE_LETTERS.length)];
      if(isLowerCase(char)) result += cipheredChar;
      else result += cipheredChar.toUpperCase();
    }
    else if(isNumber(char)) {
      result += String((Number(char) + rotationFactor) % 10);
    } 
    else result += char;
  }
  
  return result;
}