BeforeUnloadEvent

The beforeunload event is fired when the window, the document and its resources are about to be unloaded.

When a non-empty string is assigned to the returnValue 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.

General info

Specification

HTML5

Interface

Event

Bubbles

No

Cancelable

Yes

Target

defaultView

Default Action

Varies (prompts the user for confirmation to leave the page).

Properties

Example

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 below example.

window.addEventListener("beforeunload", function (e) {
  var confirmationMessage = "\o/";

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

Related Events

• DOMContentLoaded
• readystatechange
• load
• beforeunload
• unload

Reference

• Unloading Documents — Prompt to unload a document