rot13

rot13.coffee

input = "Guvf grkg vf rapbqrq ba ebg13"

rot13 = (input) ->
    upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    lower = "abcdefghijklmnopqrstuvwxyz"
    input.split('').map (c) ->
        [upper, lower].forEach (cs) ->
            c = cs[(i + 13) % cs.length] if ~i = cs.indexOf(c)
        c
    .join("")

console.log rot13(input)

rot13.js

var input, rot13;

input = "Guvf grkg vf rapbqrq ba ebg13";

rot13 = function(input) {
  var lower, upper;
  upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  lower = "abcdefghijklmnopqrstuvwxyz";
  return input.split('').map(function(c) {
    [upper, lower].forEach(function(cs) {
      var i;
      if (~(i = cs.indexOf(c))) return c = cs[(i + 13) % cs.length];
    });
    return c;
  }).join("");
};

console.log(rot13(input));