Javascriptクラスの作成オブジェクトは定義されていません

Question

JavaScriptで新たに追加されました。誰かが私になぜを呼び出すのが未定義を返すのか理解できるように助けることができますか？

構築

class Quizer { 
    constructor(quizObj) { 
     this.quiz = quizObj; 
    } 
    print() { 
     console.log(quiz.title); 
    } 
}; 
var quizObjects = { 
    title: "Quiz1" 
}; 

：

var quiz = new Quizer(quizObjects); 
quiz.print(); //undefined 

Answer 1

をあなたのコードに問題が、

class Quizer { 
    constructor(quizObj) { 
     this.quiz = quizObj; 
    } 
    print() { 
     console.log(quiz.title); 
     //You are not using the `this` context here to access the quiz 
     //you have a variable quiz outside the class declaration that points the instance of this class. 
    //That will get hoisted and will be accessed here. 

    } 
}; 

var quizObjects = { title: "Quiz1" }; 
var quiz = new Quizer(quizObjects); 
quiz.printAllQuestions(); //undefined 
//--------^^^^ printAllQuestions is not a member function of Quizer 

ソリューションです：あなたはまだクラスの構文とあまり慣れていない場合は

class Quizer { 
    constructor(quizObj) { 
     this.quiz = quizObj; 
    } 
    print() { 
     console.log(this.quiz.title); 
    } 
}; 

var quizObjects = { title: "Quiz1" }; 
var quiz = new Quizer(quizObjects); 
quiz.print(); //Quiz1 

Answer 2

、以下も同様に機能するはずです。

Quizer = function (quizObj) { 
    this.quiz = quizObj; 
}; 
Quizer.prototype = { 
    print: function() { 
     console.log(this.quiz.title); 
    } 
} 
var quizObjects = { title: "Quiz1" }; 
var quiz = new Quizer(quizObjects); 
quiz.print();