Trying to implement queue using two stacks. What is wrong?

Hey all,

I am trying to learn how to implement a queue with two stacks but I can’t figure out why when I enqueue into the two stack, I am getting undefined. Can someone assist and give a second perspective on my code. Am I calling it wrong or is the code wrong?

  function Stack(capacity) {
    this._capacity = capacity || Infinity;
    this._storage = {};
    this._count = 0;
  }

  Stack.prototype.push = function(value) {
    if(this._count < this._capacity) {
      this._storage[this._count++] = value;
      return this._count;
    }
    return 'Max capacity already reached. Remove element before adding a new one.'
  };

  Stack.prototype.pop = function() {
    var value = this._storage[--this._count];
    delete this._storage[this._count];
    if (this._count < 0) {
      this._count = 0;
    }
    return value;
  }

  Stack.prototype.peek = function() {
    return this._storage[this._count - 1];
  };

  Stack.prototype.count = function() {
    return this._count;
  }

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

  Queue_TwoStacks.prototype.enqueue = function(value) {
    this._inbox.push(value);
  };

  Queue_TwoStacks.prototype._transferStacks = function() {
    while (this._inbox.count() > 0) {
      this._outbox.push(this._inbox.pop());
    }
  };

  Queue_TwoStacks.prototype.dequeue = function() {
    if (this._outbox.count() === 0) this._transferStacks();
    return this._outbox.pop();
  };

  Queue_TwoStacks.prototype.count = function() {
    return this._inbox.count() + this._outbox.count();
  };

  Queue_TwoStacks.prototype.peek = function() {
    if (this._outbox.count() === 0) this._transferStacks();
    return this._outbox.peek();
  };

  var myQueue_TwoStacks = new Queue_TwoStacks();
  console.log(myQueue_TwoStacks.enqueue(1), 'should be 1');

  //outputs: `undefined 'should be 1'` instead of `1 'should be 1'`

Your Queue_TwoStacks.enqueue does not return anything. So, it is only logical that the function returns undefined.