Promise.joinPooledTasks = (worker, poolSize) => {
  return new Promise((resolve, reject) => {
    var results = [];
    var numberOfRunningTasks = 0;
    var didEnqueAllTasks = false;
    var nextTaskIndex = 0;

    var enqueNextTaskIfPossible = () => {
      if (numberOfRunningTasks < poolSize) {
        var nextTask = worker(nextTaskIndex);
        if (nextTask === null) {
          didEnqueAllTasks = true;
        }
        else {
          results.push(0);
          enqueTask(nextTask, nextTaskIndex);
          nextTaskIndex += 1;
          if (numberOfRunningTasks < poolSize) {
            enqueNextTaskIfPossible();
          }
        }
      }
    };

    var resolveIfNeeded = () => {
      var shouldResolve = didEnqueAllTasks && numberOfRunningTasks === 0;
      if (shouldResolve) {
            resolve(results);
      }
      else if (!didEnqueAllTasks) {
        enqueNextTaskIfPossible();
      }
      return shouldResolve;
    };

    var enqueTask = (promise, index) => {
      var isPromise = typeof promise.then == 'function';
        if (isPromise) {
          numberOfRunningTasks += 1;
          promise.then(result => {
            results[index] = { result : result };
            numberOfRunningTasks -= 1;
            resolveIfNeeded();
          }, err => {
            results[index] = { error : err };
            numberOfRunningTasks -= 1;
            resolveIfNeeded();
          });
        }
        else {
          results[index] = { result : promise };
          resolveIfNeeded();
        }
    };

    enqueNextTaskIfPossible();
  });
}


//Testing code

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min;
}

var worker = index => {
  if (index > 20) return null;
  return new Promise((resolve, reject) => {
      setTimeout(() => {
        if (Math.random() > 0.5) {
          console.log(`${index} >> success`);
          resolve(index)
        }
        else {
          console.log(`${index} >> failure`);
          reject(index);
        }
    }, getRandomInt(1000, 2250));
  });
};

Promise.joinPooledTasks(worker, 3).then(results => {
  console.log(`${results.length} promises ended`);
  for (var i=0; i < results.length; i++) {
    var result = results[i];
    if (result.result) {
      console.log(`${i} >> success: ${result.result}`);
    }
    else {
      console.log(`${i} >> failure: ${result.error}`);
    }
  }
}, err => { console.log(err); } );