Javascript Array reduce() sum object array by attributes

Introduction

Use the methods map(), filter(), and reduce() with the following array of objects: 

var items = [
    {title: "NoDBA", words: 561, tags: ["nosql", "people", "orm"], type: "book"},
    {title: "Infodeck", words: 1145, tags: ["nosql", "writing"], type: "book"},
    {title: "OrmHate", words: 1718, tags: ["nosql", "orm"], type: "book"},
    {title: "ruby", words: 1313, tags: ["ruby"], type: "article"},
    {title: "DDD_Aggregate", words: 482, tags: ["nosql", "ddd"], type: "book"}
];

 P:and calculate the total amount of words for all items of type "book".

The result should be 

3906


var items = [/*from   www  .  j  a  v a 2  s .c  o  m*/
  {title: "NoDBA", words: 561, tags: ["nosql", "people", "orm"], type: "book"},
  {title: "Infodeck", words: 1145, tags: ["nosql", "writing"], type: "book"},
  {title: "OrmHate", words: 1718, tags: ["nosql", "orm"], type: "book"},
  {title: "ruby", words: 1313, tags: ["ruby"], type: "article"},
  {title: "DDD_Aggregate", words: 482, tags: ["nosql", "ddd"], type: "book"}
];
var books = items.filter(function(item) {
  return item.type === "book";
});
var wordCounts = books.map(function(book) {
  return book.words;
});
var totalCount = wordCounts.reduce(function(acc, val) {
  return acc + val;
});
console.log(totalCount);



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