wp_jsonp

jsonp.js

if (typeof wp_jsonp === 'undefined')
    var wp_jsonp = function (event, method, params, callbackFunc) {
        // data needed to send jsonp
        var data = {
            action: event,	// wp ajax action
            ajaxSSLNonce: wp_jsonp_vars.wpAJAXNonce, // nonce
            method: method, // server has switch/case for processing
            params: params // data to be processed
        };

        jQuery.ajax({
            type: "GET", // this is the essence of jsonp
            url: wp_jsonp_vars.ajaxurl, // wp ajax url
            cache: false, // to ensure proper data response
            dataType: "jsonp", // jsonp
            crossDomain: true, // enable ssl/nonssl
            data: data, // data to be sent

            success: function (response) {
                //console.log('success', response);
                // your callback function
                callbackFunc(response);
            },

            complete: function (response) {
                //console.log('complete', response);
            },

            error: function (response) {
                console.log('error', response);
            }
        });
    };

jsonp.php

<?php

/**
 * Plugin Name: WordPress JSONp Helper
 * Plugin URI: http://www.jackreichert.com/2015/07/02/how-to-jsonp-ajax-to-ssl-in-wordpress-an-easier-way/
 * Description: A paradigm for easy AJAX over SSL in WordPress using JSONP.
 * Version: 0.1
 * Author: jackreichert
 * Author URI: http://www.jackreichert.com/
 * License: GPL3
 */
class WP_AJAX_JSONp {
	// actions on instatiation
	function __construct() {
		add_action( 'wp_enqueue_scripts', array( $this, 'wp_jsonp_scripts' ) );
	}

	// enqueue scripts
	function wp_jsonp_scripts() {
		wp_enqueue_script( 'wp_jsonp_script', plugins_url( '/jsonp.js', __FILE__ ), array( 'jquery' ) );
		wp_localize_script( 'wp_jsonp_script', 'wp_jsonp_vars', array(
			'ajaxurl'     => admin_url( 'admin-ajax.php' ),
			'wpAJAXNonce' => wp_create_nonce( 'wpAJAX-nonce' )
		) );
	}
}

$WP_AJAX_JSONp = new WP_AJAX_JSONp();