> But you could push the shim into the C++ browser code That's easier said than done because of details like that you can't build a JS string on the C++ side,

> But you could push the shim into the C++ browser code That's easier said than done because of details like that you can't build a JS string on the C++ side, the translation from string data on the WASM heap into a JS object needs to happen on the JS side. But this is how all the Emscripten web API shims work, and they do this quite efficiently, and some of those shims also split their work between the JS and C/C++ side. So to the programmer it does look like with Emscripten there's direct access to the (for instance) WebGL or WebGPU APIs, but in reality there's still quite a bit of JS code involved for each call (which isn't a problem really as long as no expensive marshalling needs to happen in such a call, since the call overhead from WASM into JS alone is really minimal).