Codecademy: A Closure To The Rescue

//sample solution


/* this rotates a string: "lohel".rotate(3) is "hello" */
String.prototype.rotate = function(n) { // n is an integer
    n %= this.length;                   // if n too large: modulo
    var cut = n < 0 ? -n : this.length - n; // cutting point
    return this.substr(cut) + this.substr(0,cut);
};
/*** Wrap everything below this line in a rot13_creator function! ***/

function rot13_creator(){
    // compose the dictionary
    var alph = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    var keys = (alph + alph.toLowerCase()).split("");
    var values = (alph.rotate(13) +
                  alph.rotate(13).toLowerCase()).split("");
    var rot13_dict = {}; // dict: {A:'N', ..., z:'m'}
    keys.forEach( function(key, index){
        rot13_dict[key] = values[index]; } );
    // wrap dictionary in a mapping function
    var rot13_map = function(character) {
        return rot13_dict[character] || character;
    };

    // create the function we're actually interested in
    var rot13 = function(text) {
        return text.split("").map(rot13_map).join("");
    };
 
    return rot13();
}