<?php
// embed the javascript file that makes the AJAX request
wp_enqueue_script( 'my-ajax-request', plugin_dir_url( __FILE__ ) . 'js/ajax.js', array( 'jquery' ) );
 
// declare the URL to the file that handles the AJAX request (wp-admin/admin-ajax.php)
wp_localize_script( 'my-ajax-request', 'MyAjax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) );