That's the property of the Browser you're using & You can't modify. Instead You can use a custom javascript alert box.

jQuery Alert Dialog

You can't manipulate that.
Maybe you take a look at these bastards called modal popups, if youre able to use jQuery.

• http://jquery.iceburg.net/jqModal/

There are non-jquery options, too, of course.

Script order matters. You need to load jQuery before you load your script because your script relies on jQuery.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.0/jquery.min.js"></script>
<script type="text/javascript" src="movieList.js"></script>

If you were to open your javascript console in your browser (Ctrl Shift J), you'd probably see an error saying something like $ is not a function.

If there are no errors, perhaps you've suppressed alerts for the page. Try replacing

alert('something');

With

console.log('something');

And keep an eye on your javascript console to see if the message gets printed. If it shows up, you've probably disabled alerts. To fix this, close and re-open the tab. It's recommended you use console.log() over alert() for debugging since alerts and other prompts can cause weird timing issues because of how they interrupt the javascript execution.