javascript - OO JavaScript : array not getting initialized

Question

我正在编写一个图书馆(使用 ES6 Javascript)，包含三个类:开放时间、书籍、书籍。以及“主”类库(由前面的类组成)。

我收到的控制台错误如下:Uncaught TypeError:无法在 Books.set books [as books] 处设置未定义的属性“undefined”。

错误出现在 setter 的 Books 类中。

这是代码:

'use strict';
//OO library (Classes: Libary, Book, WorkingHours)
class WorkingHours{
    constructor(open, close, lunch){
        this.open = open;
        this.close = close;
        this.lunch = lunch;
    }
}
class Book {
    constructor(title, category, author){
        this.title = title;
        this.category = category;
        this.author = author;
    }
}
class Books {
    constructor(){
        this.books = [];
        //let books = new Array();
        //let books = [];
        var bookAmount = 0;
    }
    set books(book){
        //this.books.push(book);
        this.books[this.bookAmount] = book;
        this.bookAmount++;
    }
}
class Library { 
    constructor(workingHours, booksIn){
        this.workingHours = workingHours;
        this.booksIn = booksIn;
    }

    get workingHours() {
        return this.workingHours;
    }
    get booksIn() {
        return this.booksIn;
    }
}

var workHour = new WorkingHours(900,1700,false);
var bookColl = new Books();
var newBook = new Book("Mastery", "Real Stories", "Robert Greene");
bookColl.books = newBook;
newBook = new Book("48 Laws of Power", "Slef-teaching", "Robert Greene");
bookColl.books = newBook;
var newLib = new Library(workHour, bookColl);
console.log(newLib);

Answer 1

var bookAmount = 0 不会初始化属性。 this.books = [] 导致通过 books setter 分配 books 数组，就像在类外部分配 this 一样.books[this.bookAmount] = ... 在 books 和 bookAmount 均未定义的情况下进行评估。

应该是:

class Books {
    constructor(){
        this._books = [];
        this.bookAmount = 0;
    }
    ...
}

bookAmount 值是多余的，因为它已经作为 this._books.length 提供。正确的方法是:

set books(book){
    this._books.push(book);
}

get books(){
    return this._books;
}