caesarCipher.js

const caesarCipher = (mainString, num, cb) => {
 console.log(mainString);

 let newString = '', currentIndex, newIndex;

 let myString = mainString.toLowerCase().split('');
 let alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');

 let index = 0;
 myString.map((val) => {
 if (!alphabet.includes(val)) {
 newString += val;
 index++;
 return;
 }

 currentIndex = alphabet.indexOf(val);

 newIndex = (currentIndex + num) % 26;

 if (newIndex < 0) newIndex += 26;

 if (mainString.charAt(index) == val.toUpperCase()) {
 newString += alphabet[newIndex].toUpperCase();
 } else {
 newString += alphabet[newIndex];
 }

 index ++;
 });

 cb(newString);
};

caesarCipher('Bqq Mggrgt', -55, (result) => {
 console.log('Result is: ' + result);
});