emulator.rs

#[wasm_bindgen]
pub struct MyEmulator {
    // Put any state that needs to persist between frames.
}

#[wasm_bindgen]
impl MyEmulator {
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        // ...   
    }

    pub fn do_one_frame(&mut self) {
        // Perform any necessary simulation for one frame.
    }
}

event_loop.js

import { MyEmulator } from './emulator';

let emulator = new MyEmulator();

function step(timestamp) {
    emulator.do_one_frame();

    window.requestAnimationFrame(step);
}

window.requestAnimationFrame(step);