Queue using 2 stacks in Javascript

queue.js

function Stack() {
  this._size = 0;
  this._storage = {};
}

Stack.prototype.push = function(data) {
  var size = ++this._size; //returns i++
  this._storage[size] = data;
  return size;
}

Stack.prototype.pop = function() {
  var size = this._size,
      deletedData = this._storage[size];

  delete this._storage[size];
  this._size--;

  return deletedData;
}

function Queue() {
  this._inbox = new Stack(),
  this._outbox = new Stack();
}

Queue.prototype.dequeue = function() {
  if (this._inbox._size == 1) {
    return this._inbox.pop();
  }
  if (this._outbox._size == 0) {
    var inboxLength = this._inbox._size;

    for (var i = 1; i <= inboxLength; i++) {
      this._outbox.push(this._inbox.pop());
    }
  }

  return this._outbox.pop();
}

Queue.prototype.enqueue = function(data) {
  this._inbox.push(data);
}

var q = new Queue();

console.log(q.enqueue('1'));
console.log(q.enqueue('2'));
console.log(q.enqueue('3'));
console.log(q._inbox);
console.log(q.dequeue());
console.log(q._inbox);
console.log(q._outbox);

console.log(q.dequeue());
console.log(q._inbox);
console.log(q._outbox);


console.log(q.dequeue());
console.log(q._inbox);
console.log(q._outbox);