ROT13

HTML

<textarea id="input" cols="40" rows="10" placeholder="Input string">Guvf guvat vf abg cneg bs gur pne, be gur qevire, be nal bowrpg gung vf nggnpurq gb gur pne.</textarea><br>
<button id="convert">Convert</button><br>
<textarea id="result" cols="40" rows="10" placeholder="Back"></textarea><br>
<textarea id="result2" cols="40" rows="10" placeholder="Forward"></textarea><br>

JavaScript

$('#convert').on('click', function() {
	var input = $('#input').val().toLowerCase(),
  		valueArr,
      freqs = {},
    	res = '';
      
  res = input.split('')
  	.map(function(a) {
    	return a == ' ' ? ' ' : String.fromCharCode(a.charCodeAt(0) - 13);
    }).join('');
    
  $('#result').val(res);
  
  res = input.split('')
  	.map(function(a) {
    	return a == ' ' ? ' ' : String.fromCharCode(a.charCodeAt(0) + 13);
    }).join('');
  
  $('#result2').val(res);
});