Queues (FIFO) and Stacks (LIFO)

gistfile1.js

class Node {
  constructor(data) {
    this.data = data;
    this.previous = null;
  }
}

class Stack {
  constructor() {
    this.top = null;
    this.size = 0;
  }
  
  push(data) { 
    var node = new Node(data);
    
    node.previous = this.top;
    this.top = node;
    this.size++;
    return this.top;
  }
  
  pop() {
    var tmp = this.top;
    this.top = this.top.previous;
    this.size--;
    return tmp.data;
  }

  size() {
    return this.size;
  }
}

//var s1 = new Stack();
//s1.push('something');
//s1.push('something else');
//s1.pop();

class Queue {
  constructor() {
    var s1 = new Stack(),
        s2 = new Stack();
    
    this.enqueue = function(item) {
      s1.push(item);
    };
    
    this.dequeue = function() {
      if (s1.size == 0 && s2.size == 0) {
        return console.log('stacks are empty can\'t dequeue');
      }
      
      while(s1.size) {
      	s2.push(s1.pop());
      }
      
      console.log('item dequeued', s2.pop());
    };
    
      
    this.showStacks = function() {
      console.log('stack1', (s1.top) ? s1.top.data : s1.top);
      console.log('stack2', (s2.top) ? s2.top.data : s2.top);
    }
  }
}

var myQueue = new Queue();
myQueue.dequeue();
myQueue.enqueue(1);
myQueue.showStacks();

myQueue.dequeue();
myQueue.showStacks();

myQueue.enqueue(1);
myQueue.enqueue(2);
myQueue.dequeue();
myQueue.showStacks();