map e reduce em JS

gistfile1.js

var products = [
  {name: "Product A"}
 ,{name: "Product B", price: 100}
 ,{name: "Product C", price: 200}
 ,{name: "Product D", offerPrice: 100, price: 150}
];

function filterPrice(product){ 
  var price = product["offerPrice"] ? product["offerPrice"] : product["price"]; 
  return price ? price : 0;
};

function sum(a, b) {
  return a + b;
};

var totalSales = products.map(filterPrice).reduce(sum,0);
// 400