[olau]

With a JIT, you can examine what's actually running and compile on the fly a static portion and then execute that.

A downside is that you don't have as much time as time spent compiling is not spent actually doing the work. And I think typically for these dynamic languages, you still need some guard code at the edges to make sure the types you see are consistent with the compiled portion.

But there's the upside that you can observe runtime behaviour which is much more difficult to get into an ahead-of-time compiler. For instance, you might be able to JIT into a regexp engine executing a particular regexp.