"Source" doesn't get altered by JavaScript after page load, it's the document object model (DOM) generated from the source that gets altered. It is this DOM that is then translated to the GUI, and is altered with every change as long as the page is not re-loaded.
The DOM is not a string of HTML code, it is an in-memory hierarchical object representation of the page. The browser does not maintain an up-to-date, flat-file representation of the DOM as it gets altered, which is why when you "view source" you only ever see what was originally sent to the browser over HTTP.
The node-for-node representation of the page/DOM, in developer tools such as Firebug is the closest you'll get to a re-generation of the source code (AFAIK) without building some new tool yourself.