为什么会出现Uncaught TypeError：无法读取未定义的属性'displayQuestion'？

Question

I tried to find the answer to my problem but really non of the answers i found here helped me. I know this is commonly asked question but i couldn't find right answer. Here is my code:

    //Constructor for creating questions
function Question(question, answers, correct) {
    this.question = question;
    this.answers = answers;
    this.correct = correct;
}
//Creating question
var q1 = Question('Is JavaScript the best programming lenguage?', ['Yes', 'No'], 0);
var q2 = Question('What is the name of your teacher?', ['Mike', 'John', 'Jonas'], 2);
var q3 = Question('How would you best describe coding?', ['Boring', 'Fun', 'Tedius', 'Hard'], 1);

//Displaying question and answers
Question.prototype.displayQuestion = function(){

    console.log(this.question);

    for (var i = 0; i < this.answers.length; i++) {
        console.log(i + '. ' + this.answers[i]);
    }
}


//Choosing random question
var questions = [q1, q2, q3];
var n = Math.floor(Math.random() * questions.length);

questions[n].displayQuestion();

I keep getting Uncaught TypeError: Cannot read property 'displayQuestion' of undefined at challenge.js:27 Thanks in advance :)

Answer 1

You are calling Question(...) but you really mean to call new Question(...) .

When you call Question(...) without new , it returns undefined because it has no return value (also, this is window instead of a new object).

When you use new , this is set to a newly-created object, and if there's no return value, the new Question call returns the newly-created this object. That's certainly what you expect your code to do.

Answer 2

在Question()之前使用new关键字