caesar.js

var caesar = {
  encode: function encode(plainText, shift) {
    if (!plainText) throw new Error('No plaintext provided');
    shift = Number(shift) || 0;
    return encryptText(plainText, shift);
  }
};

function encryptText(str, shift) {
  str = str.toUpperCase().replace(/[^\w\s]/gi, '');
  var specialCodes = [32, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57];
  var shiftedCharCodes = str.split('').map(function (s) {
    var charCode = s.charCodeAt(0);
    if (specialCodes.indexOf(charCode) !== -1) return charCode;
    var newCharCode = charCode + shift;
    if (newCharCode < 64) newCharCode += 26;
    if (newCharCode > 90) newCharCode -= 26;
    return newCharCode;
  });
  var cipher = String.fromCharCode.apply(null, shiftedCharCodes);
  return cipher;
}

console.log(caesar.encode(process.argv[2], process.argv[3]));