a throttled file downloader (Deno)

dl.deno.ts

import { Timeout, TimeoutError } from "https://deno.land/x/timeout/mod.ts";

const delay = (ms) => new Promise((res) => setTimeout(res, ms));

function newThrottler({ isBusy, lock, unlock, waitMs, size }) {
  async function throttler(cb, ...args) {
    size(1);
    await Promise.resolve();
    while (!!isBusy()) {
      await delay(waitMs()); // waits in event loop queue, until it interrupts for another attempt!
    }
    lock();
    // ... DO ALL WORK for result
    const result = await cb.call(this, ...args);
    unlock();
    size(-1);
    return result;
  }
  
  throttler.size = () => size(); // read-only
  throttler.isBusy = isBusy;
  
  return throttler;
}

const throttler = (threads: number) => newThrottler((function(){
  let size = 0;
  let semaphore = 0;
  return {
    isBusy: () => (semaphore >= threads),
    lock: () => (semaphore += 1),
    unlock: () => (semaphore -= 1),
    waitMs: () => 1000 + (1000 * Math.random()),
    size: (n) => n ? (size += n) : size,
  };
})());

export const createDownloadThrottled =
  (threads: number) => {
      const thr = throttler(threads);
      function dl(...args) { thr(download, ...args); };
      Object.entries(thr).forEach(([k, v]) => { dl[k] = v; });
      return dl;
  }
// Eg. const dl = createDownloadThrottled(4);
// dl('https://a.com/1.jpg', '~/imgs/1.jpg', myHeaders);
// console.log(dl.size());
// console.log(dl.isBusy());

export async function download(
  source: string,
  destination: string,
  headers: any = {},
): Promise<boolean> {
  try {
    const req = fetch(source, { headers });
    const response = await Timeout.race([req], headers.timeout || 9000);
    const blob = await response.blob();
    const data = new Uint8Array(await blob.arrayBuffer());
    const file = await Deno.create(destination);
    await Deno.writeAll(file, data);
    Deno.close(file.rid);
    return true;
  } catch(err) {
    if (err instanceof TimeoutError) {
      console.error(`ERROR Timed out; skipping: ${source}`);
      return false;
    }

    console.error("ERROR while dl'ing:", err.message);
    console.error("---RETRYING...");
    await delay(1000);
    return download(source, destination, headers);
  }
}