/* Enqueue script and Localize */
function my_scripts(){
wp_enqueue_script("custom-script", get_stylesheet_directory_uri()."/custom.js", array('jquery'));
wp_localize_script( 'custom-script', 'myajax', array('ajaxurl' => admin_url( 'admin-ajax.php' )) );
}
add_action("wp_enqueue_scripts","my_scripts");
Now you can use ajax url in js file with variable myajax.ajaxurl