code> can contain changes to the passed item. Parameter <code>item</code> typically contains fields `content`, `start`, and optionally `end`. The type of `start`
    and `end` is determined by the DataSet type configuration and is `Date` by default.  When invoked as <code>callback(null)</code>, the action will be cancelled.</li>
  </ul>

  <p>
    Example code:
  </p>

<pre class="prettyprint lang-js">var options = {
  onUpdate: function (item, callback) {
    item.content = prompt('Edit items text:', item.