"basically just massively parallel CPUs" is correct-ish, but it has the added wrinkle that you have to think of them as CPUs where large chunks of cores have to be executing the same instruction at the same time, which means that it's mostly loops where each iteration runs close to the same instructions that make sense.

Also, I think you might be drastically underestimating how utterly insane modern JavaScript engines like V8 are. JS code running as well as it does on CPUs is sort of a minor miracle in the first place. Even then, there's still a place/need for WebASM on top of it.

Transposing that unto a GPU in a transparent manner is asking a LOT. Not to mention that there's a lot of stuff you can do in JS that, if feasible at all, makes no sense whatsoever in GPU-land. Even basic stuff, e.g. string-based property lookups, just does not belong in GPU code for the most part.

If I see an API that looks like `GPU.exec(Callable)`, then I expect that any callable can be passed to that API. In JavaScript, that can mean a LOT of different things, many of which would be hell on earth to convert to GPU-compatible code.