I intend to insert the simple script below in a Wordpress post content through Classic Editor; but whenever I switch to visual mode or publish the post, Wordpress automatically removes the whole code (both HTML tags and script):

<p id="Test"></p>
<script>
var x = 10;
document.getElementById("Test").innerHTML= x;
</script>

To solve the issue, I followed these steps respectively:

1) The following snippet code was added to functions.php- so, now, Wordpress doesn't remove formatted HTML anymore:

function uncoverwp_tiny_mce_fix( $init )
{
    $init['extended_valid_elements'] = 'div[*]';

    return $init;
}
add_filter( 'tiny_mce_before_init', 'uncoverwp_tiny_mce_fix' );

2) By adding the following code, too, the above script is supposed to run on my post (here, for example, post id=123)

function wpb_hook_javascript() {
  if (is_page ('123')) { 
    ?>
        <script type="text/javascript">
          // My javscript code
        </script>
    <?php
  }
}
add_action('wp_head', 'wpb_hook_javascript');

Everything looks fine and logical, but this method doesn't work. I also deferred the script, but the problem remained. What am I missing here?