I'm not commenting on any other platforms, but I maintain a few C/WebAssembly libraries and they work exactly as expected without being unreasonably large. C-in-JS has been viable for a number of years now.
Unless there are some great new tools I’m not aware of, C on WebAssembly seems about the same as C on Android. You can compile stuff just fine, and run it, but actually connecting it up to any substantial UI written in JS/Java is going to be incredibly tedious.
For a computation library with well-defined inputs and outputs, and that “just works” and doesn’t need any debugging in situ, it makes a lot of sense. But I think business logic by definition is going to need a much richer interaction with the UI, and that’s what makes it hard.
I don't see why you couldn't write your entire business logic, state management, etc. in C or Rust and treat it effectively like a client-side server, but I've never used wasm in that way so I can't speak from experience there. Either way, I can certainly see how that would require some extra effort, and of course it would only really be useful for applications that don't offload most of their work to a server.
With Emscripten, you can embed Javascript source code directly into the C code and then call the embedded JS function from the C side as well as C functions from inside Javascript.
E.g. JS embedded in C code:
https://github.com/floooh/sokol/blob/4c56a2ee15d81cac91b738b...
...called from C:
https://github.com/floooh/sokol/blob/4c56a2ee15d81cac91b738b...
...and a C function:
https://github.com/floooh/sokol/blob/4c56a2ee15d81cac91b738b...
...called from a JS function embedded in the C code:
https://github.com/floooh/sokol/blob/4c56a2ee15d81cac91b738b...
How about debugging? Is it possible to step through a mix of C and JS stack frames?
https://developers.google.com/web/updates/2020/12/webassembl...
The embedded Javascript can be debugged as usual.
In my case I usually debug the platform-agnostic code compiled natively in IDEs like Visual Studio or Xcode, and for the HTML5 specific code (which is just a few hundred lines) traditional "printf-debugging".