<!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>