class QueueItem {
  constructor(value) {
    this.value = value;
    this.next  = void 0;
  }
}

class Queue {
  constructor() {
    this.first = void 0;
    this.last  = void 0;
  }
  
  enqueue(value) {
    var wasEmpty = !this.first;
    var item = new QueueItem(value);
    if (wasEmpty) {
      this.first = item;
    } else {
      this.last.next = item;
    }
    this.last = item;
    return wasEmpty;
  }
  
  dequeue() {
    var first = this.first;
    if (first) {
      this.first = first.next;
    }
    if (first === this.last) {
      this.last = void 0;
    }
    return first.value;  
  }
  
  isEmpty() {
    return !this.first;
  }
}