The onbeforeunload event attribute’ fires when the document is ready to be unloaded.
Example
You can try to run the following code to implement the onbeforeunload attribute −
<!DOCTYPE html> <html> <body onbeforeunload = "return display()"> <p>Close this window or press F5.</p> <script> function display() { return "Wanna stay here or leave?"; } </script> </body> </html>