JS alert/confirmation pop-up before permanently erase all content items in the WordPress trash.

functions.php

<?php

function trash_empty_confirmation() { ?>
<script type="text/javascript">
	(function($) {
		$('[id=delete_all]').click(function(e){
			return confirm( "Are you sure you want to permanently erase all content items in the Trash ?" );		
		});
	})( jQuery );
</script>
<?php }
add_action( 'admin_footer-edit.php', 'trash_empty_confirmation' );