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I haven't measured anything :P

Regarding your suggestion, I cannot pass an object to Javascript, because from the perspective of the browser I am writing C. An alternative would be to have each function take a pointer to the object, but it is quite cumbersome and I doubt it would have any (positive) impact on performance.

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I was thinking you can create a byte buffer of fixed size instructions that you can pass from Rust to JavaScript. With for example DataView you can fairly easily inspect that data in JavaScript.

Or to keep things even simpler, if every class of draw instructions is always performed in order (so not drawEnemy, drawText, then another drawEnemy) you could just have separate arrays of arguments for every draw type.

Anyway for your game specifically I doubt this is even close to an issue, but I'm curious just how performance scales when you jump in and out of JS a lot in WASM.

Never heard of DataView before, thanks! This is the first time I do something "low level" in Javascript, so there is probably a lot that could be improved.

BTW, here is an example passing arrays between Javascript and Rust: https://www.hellorust.com/demos/canvas/index.html

so you can only pass simple types and nothing like a framebuffer array ? great work btw!

Since WASM in its current state is meant as a compilation target for C, it is usually possible to do things "the C way". For instance, you can allocate a buffer on the Javascript side and pass a pointer to the Rust side, which then can write to it. You can also allocate it on the Rust side and return it. This is for instance what you do when you want to return a string.