In this we have to use the preventDefault() to show the confirmation dialog according to the specifications.
Let's create a demo.html file and write the below codes:
Now open this html file in the browser then write something on the input box and then try to close or refresh the tab/browser and see the result as:
Now you can see, while refreshing the page or navigating to other page, its asking for confirmation and showing an alert for Changes that you made my not be saved. as: