why we do alerts in js?

Answer 1

ZinC is right here, of course, although in reality, alert() is used mostly for debugging. For notifications/alerts for the user, a custom html/js alert is used often, because alert() does not support CSS or multiple input fields for instance.

Answer 2

To alert the user of something.

Answer 3

The alert() method displays an alert box with a specified message and an OK button.