Get a domain-mapping safe admin-ajax URL on WordPress.com

ajaxurl.php

<?php
/** 
* Generates a domain-mapping safe URL on WordPress.com 
* Core's ajaxurl uses admin_url() which returns *.wordpress.com which doesn't work for the front-end on domain-mapped sites. 
* This works around that and generates the correct URL based on context. 
*/ 
function my_admin_ajax_url( $path = '' ) { 
	if ( is_admin() ) 
		$url = admin_url( 'admin-ajax.php' ); 
	else 
	 	$url = home_url( 'wp-admin/admin-ajax.php' );

	$url .= ltrim( $path, '/' );

	return $url; 
}