p = object.source
the window that sent the message.
When pages send cross-document messages with postMessage, the target window receives a reference to the sender of the message. If the two pages are hosted in the same domain, the target window has full access to the Document Object Model (DOM) of the sender. If not, the target can only respond to the message sender with postMessage; any other interaction will return "Access Denied" error.
Build date: 11/14/2012