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The onbeforeunload event occurs when a document is about to be unloaded.
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This event allows you to display a message in a confirmation dialog box to inform the user whether he/she wants to stay or leave the current page.
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The default message that appears in the confirmation box, is different in different browsers. However, the standard message is something like "Are you sure you want to leave this page?".
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This message cannot be removed, unless you call
preventDefault()on the event.
