无法从javascript中的子类访问父类变量

Question

我有两个类，一个父类和一个从父类继承的子类。初始化子类时，我使用 super() 关键字调用父类的构造函数。然后我尝试在子方法中访问父类变量。但是，当我尝试这样做时，我收到此错误：Cannot read property 'undefined'。为什么变量未定义？构造函数没有像我预期的那样工作吗？

    class Book {
      constructor(title, author, chapters) {
        this.title = title; //string
        this.author = author; //string
        this.chapters = chapters; //array of strings
      }
      getTitle() {
        return this.title;
      }
      getAuthor() {
        return this.author;
      }
    }
    class Chapter extends Book {
      constructor(title, author, chapters, numberPages, subject, time, chapterIndex) {
        super(title, author, chapters);
        this.numberPages = numberPages;
        this.subject = subject;
        this.time = time;
        this.chapterIndex = chapterIndex;
      }
      getChapterText() {
        return this.chapters[this.chapterIndex];
      }
    }

    var chapterOne = new Chapter("title", "author", ["lorem ipsum...", "lorem ipsum...", "lorem ipsum..."], 42, "about lorem ipsum", "3:01", 0); //book_object is an array of everything the chapter constructor needs
    console.log(chapterOne.getChapterText());

我也尝试过使用super.chapters 来访问父类变量，但我得到了这个错误：unexpected keyword super。

更新

也许使用${book_object} 让我的问题太混乱了。这个 javascript 作为 JSP（java 服务器页面）运行。因此，它在提供服务之前被编译。我更新了我的问题以减少混淆。

更新 2

    class Book {
      constructor(title, author, chapters) {
        this.title = title; //string
        this.author = author; //string
        this.chapters = chapters; //array of strings
      }
      getTitle() {
        return this.title;
      }
      getAuthor() {
        return this.author;
      }
    }
    class Chapter extends Book {
      constructor(title, author, chapters, numberPages, subject, time, chapterIndex) {
        super(title, author, chapters);
        this.numberPages = numberPages;
        this.subject = subject;
        this.time = time;
        this.currentChapter = this.getChapterText(); //I forgot to include this line in my original question.
        this.chapterIndex = chapterIndex;
      }
      getChapterText() {
        return this.chapters[this.chapterIndex];
      }
    }

    var chapterOne = new Chapter("title", "author", ["lorem ipsum...", "lorem ipsum...", "lorem ipsum..."], 42, "about lorem ipsum", "3:01", 0); //book_object is an array of everything the chapter constructor needs
    console.log(chapterOne.currentChapter);

我刚刚意识到在我的实际代码中（这个问题中的代码基于我的实际代码）我在我的子类构造函数中调用我的子类方法，并且在该方法中我试图访问我的父类变量.这是其中的一个sn-p。原来我的问题一直是这个。有人愿意解释为什么会这样吗？

Answer 1

var chapterOne = new Chapter(${book_object}); //book_object 是章节构造函数需要的所有内容的数组

您定义的构造函数需要一个逗号分隔的标题、作者、章节、numberPages 等序列。只要您将其传入，一切都会起作用。但是如果你像你描述的那样传入一个对象或数组，那么这只是一个参数，它将是构造函数中的“title”参数。其余参数未定义。

所以要么改变你的构造函数来期待你传入的对象，或者改变你调用构造函数的方式

const chapterOne = new Chapter('some title', 'some author', ['chapter1', 'chapter2'], 42, /*etc*/);

如果 ${book_object} 是一个实际的数组，它的参数顺序正确，您可以使用扩展运算符将其转换为参数列表：

const bookArray = [
  'some title', 
  'some author', 
  ['chapter1', 'chapter'2],
  42
  // etc
]
const chapterOne = new Chapter(...bookArray);

Answer 2

如果book_object是构造函数需要的参数数组，则需要展开。正确的语法是...variable，而不是${variable}。

var chapterOne = new Chapter(...book_object)