// Transports which don't support cross-domain communication natively ('eventsource' to name one) use an iframe trick.
// A simple page is served from the SockJS server (using its foreign domain) and is placed in an invisible iframe.
// Code run from this iframe doesn't need to worry about cross-domain issues, as it's being run from domain local to the SockJS server.
// This iframe also does need to load SockJS javascript client library, and this option lets you specify its url (if you're unsure,
// point it to the latest minified SockJS client release, this is the default). You must explicitly specify this url on the server
// side for security reasons - we don't want the possibility of running any foreign javascript within the SockJS domain (aka cross site scripting attack).
// Also, sockjs javascript library is probably already cached by the browser - it makes sense to reuse the sockjs url you're using in normally.