If you have:

// lib.js
export const slowFib = (n) =>
  n === 0 ? 0 : n === 1 ? 1 : slowFib(n - 1) + slowFib(n - 2);

... and you write:

// lib.rs
use ferrosaur::js;

#[js(module("lib.js"))]
struct Math;

#[js(interface)]
impl Math {
    #[js(func)]
    fn slow_fib(&self, n: serde<usize>) -> serde<usize> {}
}

... then you get:

// let rt: &mut JsRuntime;
let lib = Math::main_module_init(rt).await?;
let fib = lib.slow_fib(42, rt)?;
assert_eq!(fib, 267914296);