Rot 13

rot13.js

const CODE_A = 65
const ALPHABET = [...Array.from({"length": 26}).keys()].map(key => String.fromCharCode(CODE_A + key))
const getCharIndex = (letter) => letter.charCodeAt(0) - CODE_A
const findNextChar = (index) => (changeTo) =>
    ALPHABET[(index + changeTo) % 26]
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!