index.js

const log = (() => {
  const start = Date.now();
  return (msg, ...args) => {
    const elasped = Date.now() - start;
    console.log(`%os ${msg}`, elasped / 1000, ...args);
  };
})();

async function doAsyncTask(name) {
  log("started %o", name);
  await new Promise((resolve) => setTimeout(resolve, 1000));
  log("ended %o", name);
  return name;
}

function* taskStream(total) {
  let i = 0;
  while (i < total) {
    yield doAsyncTask(`task ${++i}`);
  }
}

async function* forEachConcurrent(stream, limit) {
  let seq = 0;
  let running = new Map();
  const pushNext = (promise) => {
    const id = ++seq;
    running.set(
      id,
      promise.then((result) => [id, result])
    );
  };
  const nextDone = async () => {
    const [id, result] = await Promise.race(running.values());
    running.delete(id);
    return result;
  };
  for (const promise of stream) {
    pushNext(promise);
    if (running.size === limit) {
      yield await nextDone();
    }
  }
  while (running.size > 0) {
    yield await nextDone();
  }
}

(async () => {
  const stream = taskStream(10);
  for await (const result of forEachConcurrent(stream, 3)) {
    log("result %o", result);
  }
})();