WordPress 3.5 Compatible Heartbeat API Usage

enqueue-heartbeat.php

<?php
// enqueue the script,
// this fails gracefully if no 'heartbeat' script is registered

wp_enqueue_script('heartbeat');

heartbeat-monitor.js

(function($) {
// watch for heartbeat event,
// no event will happen if the script isn't included

$(document).on('heartbeat-connection-lost', function() {
	// do what you need to do here
	$('body').addClass('connection-lost')
}).on('heartbeat-connection-restored', function() {
	// do what you need to do here
	$('body').removeClass('connection-lost')
});

})(jQuery);