class Stack {
 constructor() {
    this.head = null;
    this.tail = null;
    this.count = 0;
  }
  get length() {
    return this.count;
  }
  push(data) {
   //..
  }

  pop() {
  //...
  }

  peek() {
  //...
  }

  get length() {
  //..
  }

}

var stack = new Stack();
stack.push(1);
stack.push(5);
stack.push(3);
console.log(stack.peek());
stack.push(2);

stack.pop();
stack.push(7);
stack.printOut(console.log);