ajax - Adding add_action in functions.php causes error

one text

I want to create a new add_action in my functions.php and call it witch ajax but it triggers an erro which says "Fatal error: Uncaught Error: Call to undefined function add_action()".

I see the error in the console.log.

I haven't developped this theme myself, and it's the only thing I add in the functions.php.

mypage.js

  const ajaxUrl = "<?php echo get_template_directory_uri(); ?>/functions.php"
  $.get(ajaxUrl, {action: 'newAddAction'}, function(response) {
    console.log(response);
  })

functions.php

add_action('wp_ajax_newAddAction', 'newAddAction');
function newAddAction() { 
    echo 'hello world';
    die;
}