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<!DOCTYPE html> <html> <body> <p>This example uses the HTML DOM to assign an "onbeforeunload" event to the window object.</p> <p>Close this window, press F5 or click on the link below to invoke the onbeforeunload event.</p> <a href="../index.html">Click here to go to w3schools.com</a> <script> window.onbeforeunload = function(event) { event.returnValue = "Write something clever here.."; }; </script> </body> </html>