Vanilla JavaScript implementation of the Caesar Cipher

Caesar-cipher.js

const CaesarCipher = (function() {

  const alphabet = 'abcdefghijklmnopqrstuvwxyz'.split("");

  const encrypt = function encrypt(msg, key) {
    if ( !typeof msg == 'string' || !Number.isInteger(key) ) {
      return false;
    }
    return Array.from(msg)
      .map(function(el) {
         el = el.toLowerCase();
         return alphabet.indexOf(el) === -1 ? el : alphabet[
           (alphabet.indexOf(el) + key) % alphabet.length
         ];
      })
      .join("");
  };

  const decrypt = function decrypt(msg, key) {
    if ( !typeof msg == 'string' || !Number.isInteger(key) ) {
      return false;
    }
    return Array.from(msg)
      .map(function(el) {
         elIndex = alphabet.indexOf(el);
         alphLength = alphabet.length;
         return elIndex === -1 ? el : alphabet[
           ((elIndex % alphLength - key % alphLength) + alphLength)
           % alphLength
         ];
      })
      .join("");
  };

  return {
    encrypt: encrypt,
    decrypt: decrypt
  };

})();


// usage:

const msg = 'Gandalf and dwarves';
const key = 2232;

console.log(CaesarCipher.encrypt(msg, key));
console.log(CaesarCipher.decrypt(CaesarCipher.encrypt(msg, key), key));