save_post hook - headers already sent?

Question

I'm trying to create a very simple alert for each time a post is created or updated with the below...

add_action( 'save_post', 'updatedAlert' );
function updatedAlert() {
  echo '<script>alert("post updated!");</script>';
}

The moment I drop this in I get "headers already sent" in both post.php and pluggable.php.

Am I missing something obvious? It seems like there shouldn't be anything else to it!

Any advice appreciated.

Answer 1

Saving post doesn't just happens. Either you are submitting a form or ajax request, which spins up new copy of WP core, which does the saving. Typically that process isn't meant to produce output and isn't handling it accordingly.

Depending on what you want to accomplish you will have to either use pure JavaScript for solution or store the message during saving persistently and output it at more appropriate point in the process.