averageDemo.js

Array.prototype.average = function(propertySelector = obj => obj) {
  const intialValue = 0;
  return (
    this.reduce((sum, obj) => sum + propertySelector(obj), intialValue) /
    this.length
  );
};

function averageDemo(orders) {
  var averageQuantity = orders.average(order => order.quantity);
  var totalQuantities = orders.sum(order => order.quantity);
  var count = orders.length;
  WriteLine(
    `Average Quantity: ${totalQuantities} / ${count} = ${averageQuantity}`
  );
}