User blog comment:Sarah Manley/New Editor Scheduled for Sitewide Release/@comment-3562424-20110903013431/@comment-1997905-20110903023713

The preview is already ajax - in fact, attempting to do a preview in a popup without using ajax would be impractical and a major waste of processing power. Previewing JS in the popup window is not possible because you'd be duplicating the onload hook and would thus perform extra modifications on elements outside of the preview popup, which would not only cause your test to be innaccurate but would mangle your editing window. The only practical way of previewing JS is to not use an annoying popup and use MediaWiki's default preview. Since this would be "inconsistent" and Wikia "doesn't support" custom CSS or JS, I doubt they'll be adding the preview back for code pages.