ceasarDecode.js

const  outerOffset = 65;                                            //ASCII code of first letter
const  innerOffset = outerOffset - 13;                              //Number that is subtracted to determine new ASCII code
const allowedChars = /[A-Z]/;                                       //Characters to be transformed
var          rot13 = (str) => str.toUpperCase()                     //Function declaration, accept lowercase
                                 .split("")                         //Basic stream transforms
                                 .map(c => (c.match(allowedChars) ? //Check if its a character to be transformed;
                                      String.fromCharCode(outerOffset + ((c.charCodeAt(0) - innerOffset) % 26)) //Transform it
                                      : c))                         //Don't transform it
                                 .join("");                         //Join it back into a string.