Create an Object Using Literal Notation

instock.js

var inStock = {
  item: 'T-Shirt',
  available: 40,
  purchased: 25,
  checkStock: function() {
  return this.available - this.purchased;
  } 
};

var elItem = document.getElementById('shirtsInstock');
elItem.textContent = inStock.item;

var elAvailable = document.getElementById('available');
elRooms.textContent = inStock.checkStock();