const stack = {

  items: [],

  push: function(arg) {

    return this.items.unshift(arg);

  },

  pop: (that) => {

     if(that.items.length) return that.items.shift();

  },

}

​

stack.push(9);

stack.push(4);

stack.push(1);

stack.push(5);

stack.push(5);

stack.push(16);

stack.push(2);

stack.pop(stack);

stack.pop(stack);

console.log(stack.items);