Rot 13

rot13.js

const ALPHABET = Array.from({"length": 26} , (_ , char) => String.fromCharCode(65 + char))
const getCharIndex = (letter) => ALPHABET.indexOf(letter)
const findNextChar = (index) => (changeTo) => {
    return index >= 13 
    ? ALPHABET[index - changeTo] 
    : ALPHABET[index + changeTo]
}
const getChar = (char) => (changeTo = 0) => {   
    return getCharIndex(char) >= 0 
    ? findNextChar(getCharIndex(char))(changeTo)
    : char
}
 
const rot = (str) => (changeTo) => str.split('')
           .reduce((acc, cur) => acc + getChar(cur)(changeTo), '')


console.log(rot('ABCDEFGHIJKLMNOPQRSTUVWXYZ')(13)) //NOPQRSTUVWXYZABCDEFGHIJKLM
console.log(rot('LBH QVQ VG!')(13)) //YOU DID IT!