'use strict';

class Dequeue {
  constructor() {
    this.first = null;
    this.last = null;
  }

  push(item) {
    const last = this.last;
    const element = { prev: last, next: null, item };
    if (last) last.next = element;
    else this.first = element;
    this.last = element;
  }

  pop() {
    const element = this.last;
    if (!element) return null;
    if (this.first === element) {
      this.first = null;
      this.last = null;
    } else {
      this.last = element.prev;
      this.last.next = null;
    }
    return element.item;
  }

  unshift(item) {
    const first = this.first;
    const element = { prev: null, next: first, item };
    if (first) first.prev = element;
    else this.last = element;
    this.first = element;
  }

  shift() {
    const element = this.first;
    if (!element) return null;
    if (this.last === element) {
      this.first = null;
      this.last = null;
    } else {
      this.first = element.next;
      this.first.prev = null;
    }
    return element.item;
  }
}

// Usage

const obj1 = { name: 'first' };
const obj2 = { name: 'second' };
const obj3 = { name: 'third' };

const list = new Dequeue();
list.push(obj1);
list.push(obj2);
list.unshift(obj3);

console.dir(list.pop());
console.dir(list.shift());
console.dir(list.shift());