Generating wasm code at runtime is pretty easy (I'd imagine easier than generating valid asm.js code). We have a little library for our tests that handles a lot of it: https://searchfox.org/firefox-main/source/js/src/jit-test/li...

There's still AssemblyScript? It might meet your requirements, unless I'm misunderstanding you or the features of it.

https://www.assemblyscript.org/

Just try the asm.js subset and see how it performs for you, I remember that even without the special asm.js support in browsers Emscripten output performance was surprisingly good

It will still work. asm.js is just regular JavaScript code, after all. It just won't parse/run as fast as custom pipeline for asm.js. My guess is that you will not notice much difference unless you have a really huge application.

If you use a library it's not harder. In Rust you would use something like this, https://crates.io/crates/wasm-encoder

While in JS you would probably use https://www.npmjs.com/package/binaryen

There are some WAT compilers that are small and fast for running in the browser.