Edit in JSFiddle

function html_entity_decode(message) {
  return message.replace(/[<>'"]/g, function(m) {
    return '&' + {
      '\'': 'apos',
      '"': 'quot',
      '&': 'amp',
      '<': 'lt',
      '>': 'gt',
    }[m] + ';';
  });
}

document.getElementById('form_decode').onsubmit = function(e) {
  e.preventDefault();
  document.getElementById('output').innerHTML = html_entity_decode(document.getElementById('input').value);
};

<form id="form_decode">
  <textarea name="decode" id="input" cols="30" rows="10"></textarea>
  <p>
    <input type="submit">
  </p>
</form>
<pre id="output"></pre>