Get current post id in functions.php

If you hook your localize script function to wp_enqueue_scripts, then you will have access to the global $post variable. As long as you pick a hook at or after ‘wp’ you should have access to the global $post.

<?php
add_action('wp_enqueue_scripts', 'YOUR_NAME_scripts'); 

function YOUR_NAME_scripts() {

            wp_enqueue_script('YOUR_NAME-js');

            global $post;
            $params = array(
                'site_url' => site_url(),
                'admin_ajax_url' => admin_url('admin-ajax.php'),
                'post_id' => $post->ID
            );

            wp_localize_script( 'jquery', 'YOUR_NAME', $params );

}