In this case you are trying to access a dom wich has no jQuery enhancement. You need to first load a jquery.js into this document. If done the syntax would be.
var popup = window.open('/some/url.html');
popup.document.$('body').html('test');
But be very careful, in multi document management and communication are many bugs and inconveniences between the many different browser versions and vendors.
It would really be better if you keep communication to a absolute minimum, and just load another complete html file into the popup.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…