SOURCE

var Stack = function(){
  this.count = 0;
  this.storage = {};
  
  this.push = function(value)
  {
    this.storage[this.count] = value;
    this.count ++;
	}

  this.pop = function(){
    if(this.count == 0)
      {
        return undefined;
      }

    this.count --;
    var result = this.storage[this.count];
    delete this.storage[this.count];
    return result;
  }
  
  this.size = function(){
    return this.count;
  }
  
  this.peek = function()
  {
    return this.storage[this.count-1];
  }
}

var myStack = new Stack();
myStack.push(1);
myStack.push(2);
console.log(myStack.peek());
console.log(myStack.pop());
console.log(myStack.peek());
myStack.push("freeCodeCamp");
console.log(myStack.size());
console.log(myStack.peek());
console.log(myStack.pop());
console.log(myStack.peek());