admin-script.php

<?php
add_action( 'admin_enqueue_scripts', 'theme_slug_admin_script' );
function theme_slug_admin_script( $hook_suffix ) {
  // Check for post or page edit page.
  if ( 'post.php' === $hook_suffix ) {
    // Include the editor functions js.
    wp_enqueue_script( 'theme_slug_editor-functions', get_theme_file_uri( 'js/backend-editor-functions.js' ), [], filemtime( get_theme_file_path( 'js/backend-editor-functions.js' ) ), true );
  }
}