[question] Why do I need to be an arrow function when calling a javascript function asynchronously?

[nabezokodaikon]

Why do I need to be an arrow function when calling a javascript function asynchronously?
If you define it in function normally, you will get an error that it is not defined.

Cargo.toml

[lib]
crate-type = ["cdylib"]

[dependencies]
js-sys = "0.3.51"
wasm-bindgen = "0.2.74"
wasm-bindgen-futures = "0.4.24"

[dependencies.web-sys]
version = "0.3.51"
features = [
  'console'
]

lib.rs

use js_sys::Promise;
use std::f64;
use wasm_bindgen::prelude::*;
use wasm_bindgen_futures::JsFuture;
use web_sys::console::log_1;

#[wasm_bindgen]
extern "C" {
    fn sleep(ms: f64) -> Promise;
}

#[wasm_bindgen]
pub fn console_log(s: &str) {
    #[allow(unused_unsafe)]
    unsafe {
        log_1(&JsValue::from(String::from(s)));
    }
}

#[wasm_bindgen]
pub async fn timer(count: u32) -> Result<JsValue, JsValue> {
    for i in 1..count {
        unsafe {
            JsFuture::from(sleep(1000f64)).await?;
        }
        console_log(&i.to_string());
    }

    Ok(JsValue::undefined())
}

timer.ts

import init, {
  timer,
} from "../pkg/wasm_timer.js";

// It works.
// const sleep = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms))

// An error will occur.
function sleep(ms: number) {
  return new Promise(resolve => setTimeout(resolve, ms))
}

async function run() {
  const wasm = await fetch("public/pkg/wasm_timer_bg.wasm");
  await init(wasm);
  timer(10);
}

run();

build.sh

#!/bin/bash
cargo build --target wasm32-unknown-unknown --release
wasm-bindgen target/wasm32-unknown-unknown/release/wasm_timer.wasm --target web --out-dir ./public/pkg
deno bundle public/ts/timer.ts public/js/timer.js