wordpress-json-server.php

<?php
/*
Plugin Name: WordPress serves JSON
Description: Serve your content as JSON or JSONP in a simple manner (based on an example by Jon Cave - http://make.wordpress.org/plugins/2012/06/07/rewrite-endpoints-api/).
Plugin URI: https://gist.github.com/lloc/8933914
Author: realloc
Author URI: http://lloc.de/
*/

namespace LLOC\realloc;

function add_rewrite_endpoint() {
	add_rewrite_endpoint( 'json', EP_PERMALINK | EP_PAGES );
}
add_action( 'init', __NAMESPACE__ . '\\add_rewrite_endpoint' );

function template_redirect() {
	global $wp_query;

	if ( ! isset( $wp_query->query_vars['json'] ) || ! is_singular() )
		return;

	header( 'Content-Type: application/json' );

	$post = get_queried_object();
	if ( get_query_var( 'callback' ) ) {
		echo get_query_var( 'callback' ) . '(' . json_encode( $post ) . ')';
	}
	else {
		echo json_encode( $post );
	}
	exit;
}
add_action( 'template_redirect', __NAMESPACE__ . '\\template_redirect' );

function add_query_var( $vars ){
	$vars[] = 'callback';
	return $vars;
}
add_filter( 'query_vars', __NAMESPACE__ . '\\add_query_var' );

function endpoints_activate() {
	add_rewrite_endpoint();
	flush_rewrite_rules();
}
register_activation_hook( __FILE__, __NAMESPACE__ . '\\endpoints_activate' );

function endpoints_deactivate() {
	flush_rewrite_rules();
}
register_deactivation_hook( __FILE__, __NAMESPACE__ . '\\endpoints_deactivate' );

xyz-json-client.js

(function($) {
   $.ajax({
      url: 'http://www.examle.com/usergroup_x/product_y/json?callback=?',
      dataType: 'jsonp',
      success: function (data) {
         $('#product').html(data);
      }
   });
})(jQuery);