{{APIRef}}

The BeforeUnloadEvent interface represents the event object for the {{domxref("Window/beforeunload_event", "beforeunload")}} event, which is fired when the current window, contained document, and associated resources are about to be unloaded.

See the {{domxref("Window/beforeunload_event", "beforeunload")}} event reference for detailed guidance on using this event.

{{InheritanceDiagram}}

Inherits properties from its parent, {{DOMxRef("Event")}}.

• {{domxref("BeforeUnloadEvent.returnValue", "returnValue")}} {{Deprecated_Inline}}
  • : When set to a truthy value, triggers a browser-controlled confirmation dialog asking users to confirm if they want to leave the page when they try to close or reload it. This is a legacy feature, and best practice is to trigger the dialog by invoking event.preventDefault(), while also setting returnValue to support legacy cases.

Inherits methods from its parent, {{DOMxRef("Event")}}.

{{Specifications}}

{{Compat}}

• {{domxref("Window/beforeunload_event", "beforeunload")}} event