Memory can also be statically reserved in a binary, like the .data section in x86 asm. IIRC (tell me if I'm very wrong) Emscripten malloc works by reserving a large buffer up front and then managing pointers to (parts of) it. Otherwise, you can allocate memory buffer dynamically from JS[1] and pass the pointer to it to the WASM. Obviously, you can do this in response to the request coming from WASM, but you don't have to.

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...