app.js

// selectGame is a function that builds the data model for a game, by randomly selecting a set of books and a correct answer.
var selectGame = function () {
        // `this` within selectGame is the array `data`. `reduce` is used to flatten the set of books into a single array.
        // `_.shuffle` randomizes the list of books.
        // `slice` selects the first 4 books.
        // `books` is then 4 randomly selected books. 
        var books = _.shuffle(this.reduce(function (p, c, i) {
            return p.concat(c.books);
        }, [])).slice(0,4);

        // `answer` is the one book (from books) randomly selected to be the answer
        var answer = books[_.random(books.length-1)];

        return {
            books: books,
            // `author` is the author of the book selected to be the answer. This has to be looked up in `data`.
            author: _.find(this, function (author) {
                return author.books.some(function (title) {
                    return title === answer;
                });
            }),
            // `checkAnswer` is a function used within the game to check if the user's selection is the correct answer.
            checkAnswer: function (title) {
                return this.author.books.some(function (t) {
                    return t === title;
                });
            }
        };
    };