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BeforeUnloadEvent
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
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The BeforeUnloadEvent interface represents the event object for the beforeunload event, which is fired when the current window, contained document, and associated resources are about to be unloaded.
See the beforeunload event reference for detailed guidance on using this event.
Inherits properties from its parent, Event.
returnValue -
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, Event.