BeforeUnloadEvent is an interface for 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. Some implementations only show the dialog box if the frame or any embedded frame receives a user gesture or user interaction. See Browser compatibility for more information.
Note: For security reasons, only a generic string not under the control of the webpage is shown instead of the returned string.