I was under the impression that V8 and the like are so optimized that this would give marginal gains. Would love to be wrong though. Do you have any performance benchmarks?
Prepack doesn't seem to optimize for V8 in particular; rather, it just does some calculations ahead of time. V8 cannot possibly do these optimizations. How would it know that e.g. my code tries to calculate PI to 1000 decimals? That's where prepack steps in and calculates PI to 1000 decimals for you during your build-step, so that the JS you ship doesn't have to do that calculation.
> experiment with JavaScript features by tweaking a JavaScript engine written in JavaScript, all hosted just in a browser; think of it as a "Babel VM", realizing new JavaScript features that cannot just be compiled away
I've been playing with making toy languages inside of Javascript, and I believe there's lots of untapped power there. The paradigm battles don't have to be: we can run all sorts of paradigms in the same VM, with data executing as code. This means that you can decompose expressions to see where they came from (e.g. the steps in a state machine that yielded an evaluation result). If you believe (as I do) that invisibility-by-default is one of the greatest pain points in the history of computing, then these sorts of approaches are essential.
The problem with doing that is that some things will be slower than they would in "native" JS. I've been proceeding anyway and thinking that could be dealt with later. So I'm bookmarking this, because it attacks that exact problem: runtime-compilation of generated AST's.
The name is a little unfortunate, in that respect, though, especially since it will make people think it's a build tool like Webpack. Webpack is for dead fish. This is incomparably more powerful.
edit so to answer your original question, this would piggyback on the optimizations of the VM (V8 or SpiderMonkey, or whatever), taking for granted that JS which is not needlessly verbose (as generated code must sometimes be), can be run nearly optimally.
I haven't used webpack but I have used many similar tools, and I've heard a lot of praise for webpack. Can you elaborate on why you dislike webpack so much?
Babel and all sorts of other compilers can run in the browser. Whereas webpack is mainly concerned with the text that you transmit and could not "by nature" be made to do its job in the browser (because then it would be too late). That's the difference I had in mind.
No for client javascript. Compilation is a slow and intensive process. You don't have the time to optimize the mountains of code loading on every new page. It makes the page much slower to load. The returns are negative if all the code executed turned out to be a single function to flash a menu, before the user left.
Yes for server javascript. It's long running processes, web servers should take time on startup to perform JIT compilation. It's beneficial in the long run.