Checking for custom field in admin pages

Yes, you are doing it wrong. You are checking for the posts’ custom field value which the new post page will does not have, and editing older posts might also does not have the custom field set or may be set to an empty value.

Yes, you need to check for the custom post type:

if ( is_admin() && ( $post->post_type == "customType" ) {
     wp_register_style('admin_js', get_bloginfo('template_directory') . '/admin.js');
       wp_enqueue_script('admin_js');
}

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