My solution works but why is this displaying "undefined" at the end?

function Quiz() {
  this.questions = [];
  this.questionIndex = 0;
  this.numberRight = 0;
  }

Quiz.prototype.add = function(Question) {
  this.questions.push(Question);
};

//Shuffle function
Quiz.prototype.shuffle = function() {
    var input = this.questions;
     
    for (var i = input.length-1; i >=0; i--) {
     
        var randomIndex = Math.floor(Math.random()*(i+1)); 
        var itemAtIndex = input[randomIndex]; 
         
        input[randomIndex] = input[i]; 
        input[i] = itemAtIndex;
    }
    return input;
}

//end Shuffle function

Quiz.prototype.next = function() {
  this.questionIndex++;

}

function Question(question, answer, fakeAnswer) {
  this.question = question;
  this.answer = answer;
  this.fakeAnswer = fakeAnswer;
}

var theQuiz = new Quiz();

var q1 = new Question("What is the capital of Arizona?", "Phoenix", "Tucson");
var q2 = new Question( 'According to Jerry, the husband has "first right of refusal" with regards to what bodily function of his wife?', "A sneeze", "An orgasm");
var q3 = new Question( 'Name a movie that plays in the SimTower movie theater', 'Waikiki Moon', 'Wheels on the Blunderbuss');
var q4 = new Question( 'The immortal Simpsons quote "The older they get, the cuter they ain\'t" was originally directed towards which Simpsons child?', 'Bart', 'Lisa');
var q5 = new Question( 'Which Woody Harrelson movie is referenced at some point during the movie "Robin Hood: Men In Tights"?', '"White Men Can\'t Jump"', 'Killer Instinct');         


theQuiz.add(q1);
theQuiz.add(q2);
theQuiz.add(q3);
theQuiz.add(q4);
theQuiz.add(q5);


//randomize the quiz question order
theQuiz.shuffle();

var theQuestion = document.getElementById("question");
var x = Math.round(Math.random());
var a1 = document.getElementById('choice0');
var a2 = document.getElementById('choice1');
var b1 = document.getElementById('guess0');
var b2 = document.getElementById('guess1');
var score = 0;
var length = theQuiz.questions.length;
var progress = document.getElementById('progress');
var theAnswer = document.getElementsByClassName("the-answer");
var wrongAnswer = document.getElementsByClassName("wrong-answer");



function nextQuestion() {
  theQuestion.innerHTML = theQuiz.questions[theQuiz.questionIndex].question;
  //shuffle the answers
  if ( x === 0) {
    a1.innerHTML = theQuiz.questions[theQuiz.questionIndex].answer;
    //add a class to the correct answer
    b1.classList.remove('the-answer');
    b1.classList.add('the-answer');
    b2.classList.remove('the-answer');
    b2.classList.add('wrong-answer');
    a2.innerHTML = theQuiz.questions[theQuiz.questionIndex].fakeAnswer;
    } else {
      b1.classList.remove('the-answer');
      a1.innerHTML = theQuiz.questions[theQuiz.questionIndex].fakeAnswer;
      a2.classList.add('wrong-answer');
      a2.innerHTML = theQuiz.questions[theQuiz.questionIndex].answer;
      b2.classList.add('the-answer');

    }

  //update the progress bar
  progress.innerHTML = "Question " + (theQuiz.questionIndex + 1) + " of " + length;
  return true;
}

nextQuestion(); 

function finishQuiz() {
  var finishHTML;
  finishHTML += '<h1>You Finished the Quiz!</h1>';
  finishHTML += '<h2>You answered ';
  finishHTML += theQuiz.numberRight;
  finishHTML += ' out of ';
  finishHTML += (theQuiz.questionIndex + 1);
  finishHTML += ' questions correctly!';
  document.getElementById('quiz').innerHTML = finishHTML;

}

theAnswer = document.getElementsByClassName("the-answer");
theAnswer[0].onclick = function (){
  console.log('right');
  theQuiz.numberRight++;
  if (theQuiz.questionIndex < length -1) {
    theQuiz.questionIndex++;
    nextQuestion();
  } else {
    finishQuiz();
  }
}

wrongAnswer = document.getElementsByClassName("wrong-answer");
wrongAnswer[0].onclick = function () {
    console.log('wrong');
    if (theQuiz.questionIndex < length -1) {
    theQuiz.questionIndex++;
    nextQuestion();
  } else {
    finishQuiz();
  }
}