Limit number of promises running in parallel

parallel_promises.js

// example lib https://github.com/charto/cwait
// https://krasimirtsonev.com/blog/article/implementing-an-async-queue-in-23-lines-of-code

function createQueue(tasks, maxNumOfWorkers = 4) {
  let numOfWorkers = 0;
  let taskIndex = 0;
  const resultData = [];

  return new Promise(done => {
    const handleResult = index => result => {
      resultData[index] = result;
      numOfWorkers--;
      getNextTask();
    };
    const getNextTask = () => {
      if (numOfWorkers < maxNumOfWorkers && taskIndex < tasks.length) {
        tasks[taskIndex]()
          .then(handleResult(taskIndex))
          .catch(handleResult(taskIndex));
        taskIndex++;
        numOfWorkers++;
        getNextTask();
      } else if (numOfWorkers === 0 && taskIndex === tasks.length) {
        done(resultData);
      }
    };
    getNextTask();
  });
}