add-book-changed.js

addBook (author, title) {
  const book = this._findExisting(author, title);
  if (book) {
    book.total += 1;
    book.available += 1;
  } else {
    this._books.push({
      info: {
        author: author,
        title: title,
      },
      total: 1,
      available: 1
    });
  }
}

library-fragment.js

const book = this._findExisting(author, title);
if (!book || !book.available) {
  return null;
}

library.js

class Library {
  constructor () {
    this._books = [];
  }
  
  addBook (author, title) {
    const book = this._findExisting(author, title);
    if (book) {
      book.total += 1;
      book.available += 1;
    } else {
      this._books.push({
        author: author,
        title: title,
        total: 1,
        available: 1
      });
    }
  }
    
  takeBook (author, title) {
    const book = this._findExisting(author, title);
    if (!book || !book.available) {
      return null;
    }
    
    book.available -= 1;
    
    return book;
  }
  
  _findExisting (author, title) { ... }
}

mock-test.js

test(
  '.takeBook should return return `null` ' + 
  'if we have the book in the library, ' +
  'but there are no available copies', () => {
    // prepare  
    const library = new Library();
    library._books = [
      { 
        author: 'King', title: 'It', 
        available: 0, total: 1 
      },
      { 
        author: 'King', title: 'Carrie', 
        available: 1, total: 1 
      },
      { 
        author: 'Orwell', title: '1984', 
        available: 1, total: 1 
      }
    ];
    
    // act 
    const book = library.takeBook('King', 'It');
    
    // verify 
    isNull(bookId)
  }
);

mock.js

library._books = [
  { 
    author: 'King', title: 'It', 
    available: 0, total: 1 // no avaiable copies
  },
  { 
    author: 'King', title: 'Carrie', 
    available: 1, total: 1 
  },
  { 
    author: 'Orwell', title: '1984', 
    available: 1, total: 1 
  }
];

normal-test.js

test(
  '.takeBook should return return `null` ' + 
  'if we have the book in the library, ' +
  'but there are no available copies', () => {
    // prepare  
    const library = new Library();
    library.addBook('King', 'It'); 
    library.addBook('King', 'Carrie');
    library.addBook('Orwell', '1984');
      // remove one available copy of "It"
    library.takeBook('King', 'It'); 
    
    // act 
    const book = library.takeBook('King', 'It');
    
    // verify 
    isNull(bookId)
  }
);

take-book-incorrect.js

takeBook (author, title) {
  const book = this._findExisting(author, title);
  if (!book) {
    return null;
  }
  
  book.available -= 1;
  
  return book;
}