Javascript class "Object has no method error"

Question

I have a class called question . I am trying to call this display method from another class by creating its correspoding object. But I get this error. Will be glad if you can help me with this.

///////////////QUESTION///////////////
function Question(id, text){
    this.id = id;
    this.text = text;
}

function QType1(id, text, choices, answers){
//this is true or false
    Question.call(this, id, text) ;
    this.choices = choices;
   this.answers = answers;
}

QType1.prototype = new Question();

//inherit Question
QType1.prototype.constructor = QType1;

QType1.Display = function(){
    console.log("Display Question");
}

///////////////QUIZ//////////////////
function quiz(){}

quiz.prototype.SetHomeScreen = function(x,y){
    var svgCanvas = CreateCanvas(x,y);
    AddText(svgCanvas,100,50, quiz_input.Settings.Layout.Text);
    console.log("Text Added");
    start_but = AddStartButton(svgCanvas, 300, 250);
    console.log("start button Added");
    start_but
    .on("click", function(d,i) {
                            startquiz();
                                });
    var startquiz = function(){
        console.log(this);
        quiz.prototype.StartQuiz();
    };

}

quiz.prototype.question_objs = [];
quiz.prototype.user_ans = [];
quiz.prototype.corr_ans = [];

quiz.prototype.LoadQuestions = function(){
    for(var i=0, l=questions.length; i<l; i++){
           this.question_objs.push(new QType1(questions[i].id, questions[i].settings.text, questions[i].settings.choices, questions[i].settings.answers));
        }
    console.log(this.question_objs);
}

quiz.prototype.DisplayQuestions = function(){
    var i = 0;
    var l = this.question_objs.length;
    while(i < l){
        console.log(this.question_objs[i] instanceof QType1);
        this.question_objs[i].Display();
    }

}
quiz.prototype.StartQuiz = function(){
        quiz.prototype.LoadQuestions();
        console.log("Starting Quiz");
        quiz.prototype.DisplayQuestions();
}

The Error Message which I get is. enter image description here Thanks in Advance

QType1.prototype.Display = function(){ , you are using "this" in method DisplayQuesitons. I do not see how "QType1" is part of "this". Just remove "this" from the call. — james emanon
– james emanon, Commented Oct 21, 2013 at 19:39
Yes the object is instantiated using the new keyword @this.question_objs.push(new QType1(questions[i].id, questions[i].settings.text, questions[i].settings.choices, questions[i].settings.answers)); . I changed the QType1.Display to QType1.prototype.Display and Still I am getting the same error. — surya
– surya, Commented Oct 21, 2013 at 19:47
for(var i=0, l=questions.length; i<l; i++){ this.question_objs[i] = new QType1(questions[i].id, questions[i].settings.text, questions[i].settings.choices, questions[i].settings.answers); } — james emanon
– james emanon, Commented Oct 21, 2013 at 20:05
Hi James. I didnt get it. I am not able to find my mistake.. — surya
– surya, Commented Oct 21, 2013 at 20:11

Joe Minichino · Accepted Answer · 2013-10-21 19:34:08Z

1

Two possible causes:

you didn't declare function Qtype1() {} and
you didn't instatiate a qtype1 object like so var q = new Qtype1()

EDIT: you didn't make Display a method of QType1. The only way you could have a QType1.Display method is if you had declared QType1 as a variable, like var QType1 = {} That way you could have a Display method directly bound to the variable. But as you declared it as a constructor you need QType1.prototype.Display = function () { console.log('Display question...'); };

Also - your inheritance is a bit messy. You're calling Question.call(this, id, text) then you're declaring QType1.prototype = new Question() and then doing the same with the constructor. You need to review your javascript prototypal inheritance theory a bit.

edited Oct 21, 2013 at 19:34

answered Oct 21, 2013 at 19:10

Joe Minichino

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4 Comments

surya Over a year ago

Hi Joe, I did both of them. I am doing this for(var i=0, l=questions.length; i<l; i++){ this.question_objs.push(new QType1(questions[i].id, questions[i].settings.text, questions[i].settings.choices, questions[i].settings.answers)); }

surya Over a year ago

And then I am calling this.question_objs[i].display();

Joe Minichino Over a year ago

and if you do a console.log(this.question_objs[i]) what's the output?

surya Over a year ago

I am getting that as QType1 objects. When I do console.log(this.question_objs[i] instanceof QType1);, I get it as true.

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