basically a closure-based semaphore

async-limit.js

/**
 * Limit concurrent asynchronous execution of a function.
 *
 * Stolen from this:
 * https://dev.to/ycmjason/limit-concurrent-asynchronous-calls-5bae
 *
 * @param fn {Function} async function
 * @paranm n {Number} max execution concurrency
 */
const asyncLimit = (fn, n) => {
  const pending = new Set();
  return async (...args) => {
    while (pending.size >= n) {
      await Promise.race(pending);
    }
    const p = fn(...args);
    const r = p
      .catch(() => {})
      .finally(() => pending.delete(r)); // lit
    pending.add(r);
    return p;
  };
};