beforeunload

The beforeunload event is fired when the window, the document and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point.

If a string is assigned to the returnValue Event property, a dialog box appears, asking the user for confirmation to leave the page (see example below). Some browsers display the returned string in the dialog box, others display a fixed message. If no value is provided, the event is processed silently.

Properties

Examples

window.addEventListener("beforeunload", function (event) {
  event.returnValue = "\o/";
});
// is equivalent to
window.addEventListener("beforeunload", function (event) {
  event.preventDefault();
});

WebKit-based browsers don't follow the spec for the dialog box. An almost cross-browser working example would be close to the following:

window.addEventListener("beforeunload", function (e) {
  var confirmationMessage = "\o/";
  e.returnValue = confirmationMessage;     // Gecko, Trident, Chrome 34+
  return confirmationMessage;              // Gecko, WebKit, Chrome <34
});

Notes

Since 25 May 2011, the HTML5 specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event. See the HTML5 specification for more details.

Various browsers ignore the result of the event and do not ask the user for confirmation at all. The document will always be unloaded automatically. Firefox has a switch named dom.disable_beforeunload in about:config to enable this behaviour.

Using this event handler in your page prevents Firefox from caching the page in the in-memory bfcache. See Using Firefox 1.5 caching for details.

Specifications

Browser compatibility

See also

• DOMContentLoaded
• readystatechange
• load
• unload
• Unloading Documents — Prompt to unload a document
• Remove Custom Messages in onbeforeload Dialogs after Chrome 51