Restart Button for the Awesome Quiz code challenge

var QuizUI = {
  displayNext: function() {
    if(quiz.hasEnded()) {
      this.displayScore();
    } else {
      this.displayQuestion();
      this.displayChoices();
      this.displayProgress();

    }
  },
  displayQuestion: function() {
    this.populateIdWithHTML("question", quiz.getCurrentQuestion().text);
  },
  displayChoices: function() {
    var choices = quiz.getCurrentQuestion().choices;

    for(var i = 0; i < choices.length; i++) {
      this.populateIdWithHTML("choice" + i, choices[i]);
      this.guessHandler("guess" + i, choices[i]);
    }
  },
  displayScore: function(id) {
    var gameOverHTML = "<h1>Game Over</h1>";
    gameOverHTML += "<h2> Your Score is: " + quiz.score + "</h2>";
    gameOverHTML += "<p> Your got " + quiz.score + " out of " + quiz.questions.length + " questions correct." + "</h2>";
    if (quiz.score == quiz.questions.length || quiz.score >= 5) {
      gameOverHTML += "<h2> It's nice to see you've been paying attention. </h2>";
    } else if (quiz.score <= 4){
      gameOverHTML += "<h2> You don't know me very well! </h2>";
    }
    gameOverHTML += "<button id='restart' class='btn--default'> Restart Quiz. </button>";
    this.populateIdWithHTML("quiz", gameOverHTML);
    this.restartHandler();
  },
  populateIdWithHTML: function(id, text) {
    var element = document.getElementById(id);
    element.innerHTML = text;
  },
  guessHandler: function(id, guess) {
    var button = document.getElementById(id);
    button.onclick = function() {
      quiz.guess(guess);
      QuizUI.displayNext();
    }
  },
  restartHandler: function() {
    var restartButton = document.getElementById("restart");
    restartButton.onclick = function() {
      quiz.restart();
      QuizUI.displayQuestion();
    }
  },
  displayProgress: function() {
    var currenQuestionNumber = quiz.currentQuestionIndex + 1;
    this.populateIdWithHTML("progress", "Question " + currenQuestionNumber + " of " + quiz.questions.length);
  }
}

function Quiz(questions) {
  this.score = 0;
  this.questions = questions;
  this.currentQuestionIndex = 0;
}

Quiz.prototype.guess = function(answer) {
  if(this.getCurrentQuestion().isCorrectAnswer(answer)) {
    this.score++;
  }
  this.currentQuestionIndex++;
};

Quiz.prototype.getCurrentQuestion = function() {
  return this.questions[this.currentQuestionIndex];
};

Quiz.prototype.hasEnded = function() {
  return this.currentQuestionIndex >= this.questions.length;
};

Quiz.prototype.restart = function() {
  if(this.currentQuestionIndex >= this.questions.length) {
    this.currentQuestionIndex = 0;
  }
  return this.currentQuestionIndex;
}