Great course! I have a question about the UI.js

function Quiz() {

  this.questions = [];
  this.currentQuestion = 0;
};

Quiz.prototype.add = function(question) {

 this.questions.push(question); 
};


Quiz.prototype.checkAnswer = function(inputValue) {

  if (inputValue === this.questions[this.currentQuestion].correct) {

    correctCounter += 1;
  }

  quiz.nextQuestion();
};

Quiz.prototype.nextQuestion = function() {

  this.currentQuestion++;

  if (this.currentQuestion === this.questions.length) {

   quiz.endQuiz(); 
  } else {

    quiz.renderToHtml();    
  };
};

Quiz.prototype.renderToHtml = function() {

  questionheader.innerHTML = this.questions[this.currentQuestion].question;

  choice0.innerHTML = this.questions[this.currentQuestion].option0;
  choice1.innerHTML = this.questions[this.currentQuestion].option1;
  progress.innerHTML = "Question " + (this.currentQuestion + 1) + " of " + this.questions.length
}

Quiz.prototype.endQuiz = function() {

  quizDiv.innerHTML = "";
  quizDiv.innerHTML = "<h1>Game over!</h1><h2>Your score is : " + correctCounter + "</h2>"
};

function Question(question , option0 , option1 ,correct) {

  this.question = question;
  this.option0 = option0;
  this.option1 = option1;
  this.correct = correct;
}

////element selectors
var quizDiv = document.getElementById("quiz");

var questionheader = document.getElementById("question");

var choice0 = document.getElementById("choice0");
var choice1 = document.getElementById("choice1");

var button0 = document.getElementById("guess0");
var button1 = document.getElementById("guess1");

var progress = document.getElementById("progress");

//questions list:

var question0 = new Question("What is the name of Berry Saharof's long time partner?" , "Maor Cohen", "Rami Fortis" , 1);
var question1 = new Question("Which of these is a famous song of Shalom Hanoch?" , "Hatuna Levana" , "Sa Le'at" , 0); 
var question2 = new Question("What is the name of Mercedes Band's lead singer?" , "Dan Toren" , "Gal Toren" , 1);

//new quiz

var quiz = new Quiz();

//add questions

quiz.add(question0);
quiz.add(question1);
quiz.add(question2);


//render to html call

quiz.renderToHtml();


//correct counter

var correctCounter = 0;

//buttons onclick send value to checkAnswer function

button0.onclick = function () {

    quiz.checkAnswer(0);

}

button1.onclick = function () {

    quiz.checkAnswer(1);

}