How to implement a queue using two stacks in JavaScript

function Queue() {
  this._inStack = [];
  this._outStack = [];
}
Queue.prototype.enqueue = function(value) {
  this._inStack.push(value);      
};
Queue.prototype.dequeue = function() {
  if (this._outStack.length) {
    return this._outStack.pop();
  }
  while (this._inStack.length) {
    this._outStack.push(this._inStack.pop());
  }
  return this._outStack.length ? this._outStack.pop() : undefined;
};
Queue.prototype.getSize = function() {
  return this._inStack.length + this._outStack.length;  
}
Queue.prototype.peek = function() {
  if (this._outStack.length) {
    return this._outStack[this._outStack.length-1];
  }
  while (this._inStack.length) {
    this._outStack.push(this._inStack.pop());
  }
  return this._outStack.length ? this._outStack[this._outStack.length - 1] : undefined;
};
Queue.prototype.isEmpty = function() {
  return (this._inStack.length + this._outStack.length) === 0;
};