Override JavaScript alert window

Yes … just redefine it.  Example:

<script type="text/javascript">
   alert = function(msg) {
	console.log(msg);
   }

   alert('Replaced alert with console msg!');
</script>

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While you can easily replace the vanilla "alert" display mechanism, as mentioned above, keep in mind that the in-built "alert" functionality also pauses (suspends) execution of the underlying  JavaScript.

So when using any replacement you need to take the fact that execution of your underlying script will continue while the "alert" is displayed.

For instance, the JQuery plug-in above will give you a CSS styled "alert" message, but execution is continuing while the message is up.  So if you had multiple "alert" messages in your underlying script, they overlay each other.

The only alert you cannot modify for security reason are the one created/hardcoded by the browser
for example beforeunload event