cipher can be run with cli using node

rot13.js

// es2015
'use strict';

// run from cli as `babel rot13.js "string"`
let entry = process.argv[2];
entry => {
    const alphabet = "abcdefghijklmnopqrstuvwxyz";
    
    let outString = '';
    
    for(var i=0; i<entry.length; i++){
        let char = entry[i],
            lowercase_char = char.toLowerCase(),
            isUpperCase = ( char === lowercase_char ? false : true ),
            index = alphabet.indexOf(lowercase_char);
            
        if(index < 0){ // char is not a letter, return char
            outString += char;
        } else { // char is a letter
            
            let translate = index - 13,
                translated_index = ( translate > 0 ? translate : 26 + translate),
                translated_char = alphabet[translated_index];
                
            if(isUpperCase){
                outString += translated_char.toUpperCase();
            } else {
                outString += translated_char;
            }
        }
    }
    
    console.log(outString);
}