Edit in JSFiddle

$(function() {
  var $input = $('#input');
  var $pre = $('#preview');

  $input.val('[http://google.com:title]');

  var hatena = new HatenaHttp();
  var parse = function() {
    var text = $input.val();
    hatena.parse(text, function(replacedText) {
      $pre.html(replacedText);
    });
  };
  $input.keyup(function() {
    parse();
  });

  parse();
});

<div id="editor">
  <textarea id="input"></textarea>
  <div id="preview"></div>
</div>

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script src="https://rawgithub.com/henteko/hatena-httpjs/v0.1/lib/hatena-http.js"></script>

html, body, #editor {
  margin: 0;
  height: 100%;
  font-family: 'Helvetica Neue', Arial, sans-serif;
  color: #333;
}

textarea, #editor div {
  display: inline-block;
  width: 49%;
  height: 100%;
  vertical-align: top;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 20px;
}

textarea {
  border: none;
  border-right: 1px solid #ccc;
  resize: none;
  outline: none;
  background-color: #f6f6f6;
  font-size: 14px;
  font-family: 'Monaco', courier, monospace;
  padding: 20px;
}

code {
  color: #f66;
}