If it's done right, it's still indexable because in the first render the content is delivered with HTML. On subsequent clicks, the browser fetches via JavaScript, but the URL in the browser still changes and if you refresh, the page is fetched containing the content again.
That's important not only for search engines, but also for screen readers, fast rendering and devises without JavaScript. I think Google is totally able to index JavaScript generated content, but pages will get a higher rank if it's done in an accessible way.