code: function Queue(concurrency) {
    this.paused = false;
    this.concurrency = concurrency;
    this.waitTimeout = 0;
    this.processTimeout = 0;
    this.throttleCount = 0;
    this.throttleInterval = 1000;
    this.count = 0;
    this.tasks = [];
    this.waiting = [];
    this.factors = {};
    this.fifoMode = true;
    this.roundRobinMode = false;
    this.priorityMode = false;
    this.onProcess = null;
    this.onDone = null;
    this.onSuccess = null;
    this.onTimeout = null;
    this.onFailure = null;
    this.onDrain = null;
  }
docs: Queue constructor concurrency - <number>, asynchronous concurrency