如何通过javascript在两个不同的域之间进行通信

I am building an embedded chat control with javascript, ajax, and signalr. Users can add it into their websites by adding a bit of javascript that calls back to our servers and requests the javascript needed to build the chat widget. The chat widget is then embedded on their website, but it can be popped out into a new window. The new window is from our server, no longer on their site.I am building an embedded chat control with jav