Deque()

deque.js

// Less optimal implementation
class Deque {
  constructor() {
    this.deque = [];
  }
  
  addToFront(item) {
    this.deque.unshift(item);
  }
  
  addToRear(item) {
    this.deque.push(item);
  }
  
  removeFromFront() {
    return this.deque.shift();
  }
  
  removeFromRear() {
    return this.deque.pop();
  }
  
  peekFront() {
    if (this.deque.length < 1) throw "Deque is empty";
    return this.deque[0];
  }
  
  peekRear() {
    if (this.deque.length < 1) throw "Deque is empty";
    return this.deque[this.deque.length - 1];
  }
  
  size() {
    return this.deque.length;
  }
  
  isEmpty() {
    return this.deque.length < 1;
  }
}

// More optimal implementation
// Let's add a method to our Queue class
Queue.prototype.pop = function() {
  return this.queue.pop();
}

class Deque {
  constructor() {
    // Separate "front" and "rear"
    this.front = new Queue();
    this.rear = new Queue();
  }
  
  addToFront(item) {
    this.front.enqueue(item);
  }
  
  addToRear(item) {
    this.rear.enqueue(item);
  }
  
  removeFromFront() {
    if (this.front.isEmpty()) {
      if (this.rear.isEmpty()) throw "Deque is empty";
      return this.rear.dequeue();
    }
    return this.front.pop();
  }
  
  removeFromRear() {
    if (this.rear.isEmpty()) {
      if (this.front.isEmpty()) throw "Deque is empty";
      return this.front.dequeue();
    }
    return this.rear.pop();
  }
  
  peekFront() {
    if (this.front.isEmpty()) {
      if (this.rear.isEmpty()) throw "Deque is empty";
      return this.rear.peek();
    }
    return this.front.queue[this.front.size() - 1];
  }
  
  peekRear() {
    if (this.rear.isEmpty()) {
      if (this.front.isEmpty()) throw "Deque is empty";
      return this.front.peek();
    }
    return this.rear.queue[this.rear.size() - 1];
  }
  
  size() {
    return this.front.size() + this.rear.size();
  }
  
  isEmpty() {
    return this.front.isEmpty() && this.rear.isEmpty();
  }
}