Registers a beforeunload event handler for a dashboard page.
This function can only be used in page code files for dashboard pages created in the Wix Editor or with Wix Blocks.
onBeforeUnload() accepts a callback that's triggered when a site builder is about to navigate away from a dashboard page or when the browsing context of an app is being unloaded. For example, this can happen when the site builder selects another page from the dashboard's sidebar.
The event object passed to the callback function contains a function called preventDefault(). If the callback calls preventDefault(), navigation away from the page or other context unloading is paused. A dialog is displayed in the dashboard warning the site builder that there may be unsaved data. The site builder can then choose to cancel and stay in the current context or proceed.
Notes:
beforeunload event does not fire when a user closes the browser or refreshes the page.beforeunload event will always fire or that the confirmation dialog will always be presented. These behaviors vary depending on the site builder's browser.callback(event: onBeforeUnloadEvent): void Callback function to call when the beforeunload event fires. The function receives an event object as an argument.