// create the iFrame
var iFrame = document.createElement('iframe');

// set iFrame's source
iFrame.src = "javascript:false"; // add some URL here

// inject iFrame into current (parent) doc before first script tag
var where = document.getElementsByTagName('script')[0];
where.parentNode.insertBefore(iFrame, where);

// get iFrame's window and document objects
var iFrameWindow = iFrame.contentWindow || iFrame;
var iFrameWindow = iFrame.contentDocument || iFrame.contentWindow.document;