Revision 976231 of BeforeUnloadEvent

<p>{{APIRef}}</p>

<p>The <strong><code>beforeunload</code></strong> event is fired when the window, the document and its resources are about to be unloaded.</p>

<p>When a non-empty string is assigned to the <code>returnValue</code> Event property, a dialog box appears, asking the users for confirmation to leave the page (see example below). When no value is provided, the event is processed silently.</p>

<table class="properties">
 <tbody>
  <tr>
   <td>Bubbles</td>
   <td>No</td>
  </tr>
  <tr>
   <td>Cancelable</td>
   <td>Yes</td>
  </tr>
  <tr>
   <td>Target objects</td>
   <td>defaultView</td>
  </tr>
  <tr>
   <td>Interface</td>
   <td>{{domxref("Event")}}</td>
  </tr>
 </tbody>
</table>

<h2 id="Examples">Examples</h2>

<pre class="brush:js;">
window.addEventListener("beforeunload", function( event ) {
  event.returnValue = "\o/";
});

//is equivalent to
window.addEventListener("beforeunload", function( event ) {
  event.preventDefault();
});</pre>

<p>Webkit-based browsers don't follow the spec for the dialog box. An almost cross-browser working example would be close to the below example.</p>

<pre class="brush: js">
window.addEventListener("beforeunload", function (e) {
  var confirmationMessage = "\o/";

  (e || window.event).returnValue = confirmationMessage;     //Gecko + IE
  return confirmationMessage;                                //Webkit, Safari, Chrome etc.
});</pre>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{Event("DOMContentLoaded")}}</li>
 <li>{{Event("readystatechange")}}</li>
 <li>{{Event("load")}}</li>
 <li>{{Event("beforeunload")}}</li>
 <li>{{Event("unload")}}</li>
 <li><a href="https://www.whatwg.org/specs/web-apps/current-work/#prompt-to-unload-a-document" title="https://www.whatwg.org/specs/web-apps/current-work/#prompt-to-unload-a-document">Unloading Documents — Prompt to unload a document</a></li>
</ul>