ppap.js

var plain  = "I have a pen. I have an apple. Apple-pen!";
var encoded = "I oari a fib. I oari ab affwi. Affwi-fib!";

var cpA = "a".codePointAt(); // 97
var cpZ = "z".codePointAt(); // 122
var deltaOfAandZ = cpZ - cpA; // 25

function encode(str) {
  var ret = [];
  for (var i = 0, l = str.length; i < l; ++i) {
      var char = str[i];
      var cp = char.codePointAt();
      var delta = cp - cpA;
      if (delta > 0) {
        var newCp = cp + delta > cpZ ? cpA + (cp + delta) % cpZ : cp + delta;
        ret.push(String.fromCodePoint(newCp));
      } else {
        ret.push(char);
      }
  }
  return ret.join("");
}
console.log(encode(plain) == encoded);

function decode(str) {
  var ret = [];
  for (var i = 0, l = str.length; i < l; ++i) {
      var char = str[i];
      var cp = char.codePointAt();
      var delta = cp - cpA;
      if (delta > 0) {
        var newCp = cpA + (delta % 2 == 0 ? delta / 2 : (delta + deltaOfAandZ) / 2);
        ret.push(String.fromCodePoint(newCp));
      } else {
        ret.push(char);
      }
  }
  return ret.join("");
}
console.log(decode(encoded) == plain);