Chunk async tasks into batches and process in parallel v2. Demo: https://jsfiddle.net/ccnokes/jhu635r8

batch-async-tasks-v2.js

// see working demo here: https://jsfiddle.net/ccnokes/jhu635r8/73/

async function runTasks(taskFns /* Array<() => Promise<any>> */, concurrency = 3) {
  let results = [];
  taskFns = new Set(taskFns);
  let pending = new Set();
  for (let task of taskFns) {
    if (pending.size >= concurrency) {
      await Promise.race(pending);
    }
    let p = task().then((result) => {
      pending.delete(p);
      results.push(result);
      return result;
    });
    pending.add(p);
    taskFns.delete(task);
  }
  await Promise.all(pending);
  return results;
}