The unload event trigger when you unload (or refresh) a page. You can try to run the following code to learn how to implement onunload event in JavaScript.
Example
<!DOCTYPE html>
<html>
<body onunload="newFunc()">
<p>Close the page and see what happens!</p>
<p>This event may give unexpected results.</p>
<script>
function newFunc() {
alert("Thank you!");
}
</script>
</body>
</html>