JavaScript snippet in footer specific page

Hello,

I trying adding Javascript in the footer.

I’m adding this code in my functions.php:

function locatie_winkel($pid){
   if (is_single()) {
       global $post;
       if($post->ID=="624"){ // only for post Id = 624
            echo "<script>alert('hello world');</script>";
       }
   }
}
add_action( 'wp_footer', 'locatie_winkel' );

And this is the id of the page:
../wp-admin/post.php?post=624&action=edit

But when i look in the frontend to this page, then i don’t see the script in the source code.

What doing wrong??

Regards,
Christophe