Clever Geek Handbook

Onclick works but addEventListener doesn't?

Ok, so I was creating a hands-on project that I saw on another website

I use addEventListeners

as the default event handling function, however, in the code below; the quiz app I am building looks at the questions but does not update the score even if the answer was correct. I continued debugging and logging the console for almost two hours until I found out that a "very strange" problem was that I was adding a handler addEventListener

to the buttons the user clicks to select an answer instead of a function onclick

. I want to know what the problem really is ...

Please open the code panel to see the code.

The problem is in the code on line 73

.

What further adds to my confusion is that when I moved this.currentQuestionIndex++

on line 17

inside the if statement in the method Quiz.prototype.checkAnswer

, it worked fine with addEventListener

.

You need your help to understand the secret of this.

//Creating the Quiz object
function Quiz(questions) {
  this.questions = questions;
  this.score = 0;
  this.currentQuestionIndex = 0;
}

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

Quiz.prototype.checkAnswer = function(answer) {
  if(this.grabQuestion().isCorrectAnswer(answer)) {
    this.score++;
    console.log(this.score);
  }
  this.currentQuestionIndex++;
}

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

//creating the Question object.
function Question(title,choices,answer) {
  this.title = title;
  this.choices = choices;
  this.answer = answer;
}

Question.prototype.isCorrectAnswer = function(choice) {
  return this.answer === choice;
}

//Creating the Quiz Interface(Rendering into HTML)

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

  displayScore: function() {
    var scoreDisplay = 'your score is ' + quiz.score;
    this.populateIdWithHTML("quiz",scoreDisplay)
  },
  displayQuestion: function() {
    this.populateIdWithHTML("question",quiz.grabQuestion().title);
  },

  displayChoices: function() {
    var choices = quiz.grabQuestion().choices;
    for (var i = 0; i < choices.length; i++) {
      this.populateIdWithHTML("choice" + i, choices[i]);
      this.checkingHandler("guess" + i, choices[i]);
    }
  },

  displayProgress: function() {
    var currentQuestionNum = quiz.currentQuestionIndex + 1;
    this.populateIdWithHTML("progress","Question " + currentQuestionNum + " of " + quiz.questions.length)
  },

  checkingHandler: function(id,checking) {
    var btn = document.getElementById(id);

    /* The addEventListener function throws an error for some reason */

      btn.addEventListener('click',function(e){
      e.preventDefault();
      quiz.checkAnswer(checking);
      quizInterface.displayNext();
      },false)

    //The onclick function works though normally though...
    /*  btn.onclick = function() {
        quiz.checkAnswer(checking);
        quizInterface.displayNext();
      }
     */
  },

  populateIdWithHTML: function(id,theHTML) {
    var elem = document.getElementById(id);
    elem.innerHTML = theHTML;
  }
}

var questionOne = new Question('What is the whole area of Egypt?',['1MKM','2MKM'],'1MKM');
var questionTwo = new Question('What is the currency of Egypt?',['EGP','USD'],'EGP');
var questionThree = new Question('What is the capital of Egypt?',['cairo','baghdad'],'cairo');
var questionFour = new Question('What is the latest Battlefield game?',['BF4','BF1'],'BF1');
var questionFive = new Question('What is the OS of Mac laptops?',['Mac IOS','Mac OS'],'Mac OS');

var questions = [questionOne,questionTwo,questionThree,questionFour,questionFive];

var quiz = new Quiz(questions);
quizInterface.displayNext();
      

        
        
        
      

    
.grid:after {
  content: "";
  display: table;
  clear: both; }

.srt, .form__label--hidden {
  border: 0;
  clip: rect(0 0 0 0);
  height: 1px;
  margin: -1px;
  overflow: hidden;
  padding: 0;
  position: absolute;
  width: 1px; }

.panel--centered, .panel--padded--centered, [class^="btn--"] {
  text-align: center; }

[class^="progbar__"]:after, .icn--nav-toggle:before {
  display: block;
  content: '';
  position: absolute; }

.centered, .grid {
  float: none;
  margin-left: auto;
  margin-right: auto; }

html {
  font-family: sans-serif;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  padding-top: 2.5em; }

article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
main,
nav,
section,
summary {
  display: block; }

audio,
canvas,
progress,
video {
  display: inline-block;
  vertical-align: baseline; }

audio:not([controls]) {
  display: none;
  height: 0; }

[hidden],
template {
  display: none; }

a {
  background: transparent; }

a:active,
a:hover {
  outline: 0; }

abbr[title] {
  border-bottom: 1px dotted; }

b,
strong {
  font-weight: bold; }

dfn {
  font-style: italic; }

h1 {
  font-size: 2em;
  margin: 0.67em 0; }

mark {
  background: #ff0;
  color: #000; }

small {
  font-size: 80%; }

sub,
sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline; }

sup {
  top: -0.5em; }

sub {
  bottom: -0.25em; }

img {
  border: 0; }

svg:not(:root) {
  overflow: hidden; }

figure {
  margin: 1em 40px; }

hr {
  -moz-box-sizing: content-box;
  box-sizing: content-box;
  height: 0; }

pre {
  overflow: auto; }

code,
kbd,
pre,
samp {
  font-family: monospace, monospace;
  font-size: 1em; }

button,
input,
optgroup,
select,
textarea {
  color: inherit;
  font: inherit;
  margin: 0; }

button {
  overflow: visible; }

button,
select {
  text-transform: none; }

button,
html input[type="button"],
input[type="reset"],
input[type="submit"] {
  -webkit-appearance: button;
  cursor: pointer; }

button[disabled],
html input[disabled] {
  cursor: default; }

button::-moz-focus-inner,
input::-moz-focus-inner {
  border: 0;
  padding: 0; }

input {
  line-height: normal; }

*
input[type="checkbox"],
input[type="radio"] {
  box-sizing: border-box;
  padding: 0; }

input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
  height: auto; }

input[type="search"] {
  -webkit-appearance: textfield;
  -moz-box-sizing: content-box;
  -webkit-box-sizing: content-box;
  box-sizing: content-box; }

input[type="search"]::-webkit-search-cancel-button,
input[type="search"]::-webkit-search-decoration {
  -webkit-appearance: none; }

fieldset {
  border: 1px solid #c0c0c0;
  margin: 0 2px;
  padding: 0.35em 0.625em 0.75em; }

legend {
  border: 0;
  padding: 0; }

textarea {
  overflow: auto; }

optgroup {
  font-weight: bold; }

table {
  border-collapse: collapse;
  border-spacing: 0; }

td,
th {
  padding: 0; }

* {
  -moz-box-sizing: border-box;
  box-sizing: border-box; }

body {
  color: #797e83;
  font-size: 16px;
  font-family: "Lato", "Helvetica Neue", Helvetica, Arial, sans-serif;
  line-height: 1.5; }

h3 {
  font-size: 1.125em; }

h4 {
  margin-top: 1.375em;
  margin-bottom: 2.57143em;
  color: #d6d7d9;
  text-transform: uppercase;
  letter-spacing: 1px;
  font-weight: 400;
  font-size: 0.875em; }
  @media (min-width: 769px) {
    h4 {
      margin-top: 2.625em; } }

a {
  color: #656a6e;
  text-decoration: none;
  font-weight: 400; }

p {
  margin: .8em 0;
  font-weight: 300;
  font-size: 1.125em;
  line-height: 1.5; }

blockquote {
  font-weight: 300;
  font-style: italic;
  font-size: 1.25em; }
  @media (min-width: 769px) {
    blockquote {
      margin: 1.33333em 0;
      padding: 0 0 0 5%;
      border-left: 0.33333em solid #ebecec;
      font-size: 1.5em; } }

ul,
li {
  margin: 0;
  padding: 0;
  list-style-type: none; }

img {
  margin-bottom: 1.5em;
  max-width: 100%;
  height: auto; }

input,
textarea {
  display: block;
  padding: 15px;
  width: 100%;
  outline: 0;
  border: 0; }

input:focus,
textarea:focus {
  transition: 0.3s; }

button {
  outline: 0; }

footer {
  border-top: 1px solid #ebecec;
  margin-top: 1.375em; }
  footer p {
    font-size: 1em;
    margin-top: 1.375em; }

.panel, .panel--centered {
  padding-top: 1.875em; }
  @media (min-width: 769px) {
    .panel, .panel--centered {
      padding-bottom: 1.25em; } }

.panel--padded, .panel--padded--centered {
  padding-top: 2.125em; }
  @media (min-width: 769px) {
    .panel--padded, .panel--padded--centered {
      padding-top: 5em;
      padding-bottom: 2.125em; } }

.grid {
  width: 90%; }
  [class*="grid__col--"] > .grid {
    width: 100%; }
  @media (min-width: 1100px) {
    .grid {
      max-width: 1050px; } }

@media (min-width: 769px) {
  .grid__col--1 {
    width: 6.5%; }
  .grid__col--2 {
    width: 15%; }
  .grid__col--3 {
    width: 23.5%; }
  .grid__col--4 {
    width: 32%; }
  .grid__col--5 {
    width: 40.5%; }
  .grid__col--6 {
    width: 49%; }
  .grid__col--7 {
    width: 57.5%; }
  .grid__col--8 {
    width: 66%; }
  .grid__col--9 {
    width: 74.5%; }
  .grid__col--10 {
    width: 83%; }
  .grid__col--11 {
    width: 91.5%; }
  .grid__col--12 {
    width: 100%; } }

@media (min-width: 1px) and (max-width: 768px) {
  [class^="grid__col--"] {
    margin-top: 0.75em;
    margin-bottom: 0.75em; } }
@media (min-width: 769px) {
  [class^="grid__col--"] {
    float: left;
    min-height: 1px;
    padding-left: 10px;
    padding-right: 10px; }
    [class^="grid__col--"] + [class^="grid__col--"] {
      margin-left: 2%; }
    [class^="grid__col--"]:last-of-type {
      float: right; } }

.theme__poly .grid [class*="grid__col"] {
  font-weight: 100;
  margin-bottom: 1em;
  padding: 1.75%; }

@media (min-width: 769px) {
  .nav__item, .nav__item--current {
    display: inline-block;
    margin: 0 0.625em; } }

.nav__item--current a, .nav__item a {
  font-weight: 300;
  text-align: center;
  font-size: 1.125em;
  display: block;
  padding: 0.4em;
  border-bottom: 1px solid transparent; }
  @media (min-width: 1px) and (max-width: 768px) {
    .nav__item--current a, .nav__item a {
      border-bottom-color: #ebecec;
      padding-top: 0.77778em;
      padding-bottom: 0.77778em; } }

.nav__item--current a, .nav__item a:hover {
  color: #0b0b0b;
  border-color: #52bab3; }

h1, .headline-primary, .headline-primary--grouped {
  color: #525559;
  font-weight: 300;
  font-size: 2.625em;
  line-height: 1.09524;
  margin-top: 0; }

h2, .headline-secondary, .headline-secondary--grouped {
  color: #999da1;
  letter-spacing: 1px;
  font-weight: 300;
  font-size: 1.5em;
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; }

.form__btn, [class^="btn--"] {
  padding: 10px 20px;
  border: 0;
  border-radius: 0.4em;
  color: #fff;
  text-transform: uppercase;
  font-size: 0.875em;
  font-weight: 400;
  transition: opacity 0.3s;
  display: block; }
  .form__btn:hover, [class^="btn--"]:hover {
    opacity: .75; }
  .form__btn:active, [class^="btn--"]:active {
    opacity: initial; }

.menu__link, .menu__link--end {
  display: block;
  padding-top: 1em;
  padding-bottom: 1em;
  color: #fff;
  text-align: center;
  text-shadow: 0 1px rgba(11, 11, 11, 0.2);
  font-size: 1.125em; }

.icn--nav-toggle, .icn--close {
  line-height: 0;
  cursor: pointer; }

.img--wrap {
  border: 1px solid #d6d7d9;
  padding: 0.75em; }
.img--avatar {
  display: block;
  margin-left: auto;
  margin-right: auto;
  border-radius: 50%; }
  @media (min-width: 769px) {
    .img--avatar {
      margin-top: 1.5em; } }
@media (min-width: 769px) {
  .img--hero {
    margin-bottom: 2.625em; } }

.headline-primary {
  margin-bottom: 1.66667em; }
  .headline-primary--grouped {
    margin-bottom: 0; }
.headline-secondary {
  margin-bottom: 0.91667em; }
  .headline-secondary--grouped {
    margin-top: 0.41667em;
    margin-bottom: 2.25em; }

.form__label {
  display: block;
  margin-bottom: 0.625em; }
.form__input {
  font-size: 1.125em;
  margin-bottom: 1.11111em;
  border-bottom: 6px solid #d6d7d9;
  border-radius: 0.4em;
  background: #ebecec;
  color: black;
  font-weight: 300; }
  .form__input:focus {
    border-color: #52bab3; }
.form__btn {
  background: #52bab3; }

.btn--default {
  background-color: #52bab3; }
.btn--success {
  background-color: #5ece7f; }
.btn--error {
  background-color: #e67478; }
.btn--warning {
  background-color: #ff784f; }
.btn--info {
  background-color: #9279c3; }

[class^="btn--"] {
  margin-bottom: 1.42857em; }
  @media (min-width: 1px) and (max-width: 768px) {
    [class^="btn--"] {
      width: 100%; } }
  @media (min-width: 769px) {
    [class^="btn--"] {
      width: auto;
      display: inline-block; }
      [class^="btn--"] + [class^="btn--"] {
        margin-left: 20px; } }

.navbar {
  position: relative; }
  @media (min-width: 769px) {
    .navbar {
      margin-top: 3.375em;
      margin-bottom: 0; } }

.nav {
  margin-top: 1.25em;
  margin-bottom: 1.875em; }
  .nav__item a {
    color: #797e83; }

.offcanvas {
  position: relative;
  padding: 0.625em;
  letter-spacing: 1px;
  background: #39add1;
  background: linear-gradient(45deg, rgba(94, 206, 127, 0.8) 0%, #39add1 100%); }

.menu {
  margin-top: 1.25em; }
  .menu__link {
    border-bottom: solid 1px rgba(255, 255, 255, 0.3); }

.progbar {
  height: 4px;
  border-radius: 2px;
  background: #d6d7d9;
  position: relative;
  margin-bottom: 2.875em; }
  .progbar__status--default {
    background-color: #52bab3; }
    .progbar__status--default:after {
      background-color: #6fc6c0; }
  .progbar__status--success {
    background-color: #5ece7f; }
    .progbar__status--success:after {
      background-color: #7dd898; }
  .progbar__status--error {
    background-color: #e67478; }
    .progbar__status--error:after {
      background-color: #ec979a; }
  .progbar__status--warning {
    background-color: #ff784f; }
    .progbar__status--warning:after {
      background-color: #ff9778; }
  .progbar__status--info {
    background-color: #9279c3; }
    .progbar__status--info:after {
      background-color: #a995d0; }

[class^="progbar__"] {
  display: block;
  width: 50%;
  height: 100%;
  border-radius: inherit;
  position: relative; }
  [class^="progbar__"]:after {
    width: 20px;
    height: 20px;
    border-radius: 50%;
    right: -10px;
    top: -8px; }

.site-logo {
  background-image: url("../img/logo.svg");
  background-repeat: no-repeat;
  width: 115px;
  height: 45px;
  display: inline-block; }

.icn--nav-toggle {
  width: 25px;
  height: 17px;
  border-top: solid 3px #797e83;
  border-bottom: solid 3px #797e83;
  position: relative; }
  .icn--nav-toggle:before {
    width: 25px;
    height: 3px;
    background: #999da1;
    top: 4px; }
.icn--close {
  background-image: url("../img/icn-close.svg");
  background-repeat: no-repeat;
  width: 20px;
  height: 20px;
  display: block;
  position: absolute;
  right: 4%;
  top: 4%; }

@media (min-width: 1px) and (max-width: 768px) {
  .is-displayed-mobile {
    display: block; }
    .is-hidden-mobile {
      display: none; } }
@media (min-width: 769px) {
  .is-displayed-mobile {
    display: none; } }

@media (min-width: 1px) and (max-width: 768px) {
  .is-collapsed-mobile {
    visibility: collapse;
    padding: 0;
    height: 0;
    margin: 0;
    line-height: 0; } }

.theme__poly .grid__col--12 {
  background-color: #DEF4E5; }

.theme__poly .grid__col--8 {
  background-color: #DCE0F2; }

.theme__poly .grid__col--7 {
  background-color: #DCF0EF; }

.theme__poly .grid__col--6 {
  background-color: #FFE3DB; }

.theme__poly .grid__col--4 {
  background-color: #F8EDD0; }

.theme__poly .grid__col--5 {
  background-color: #EAEBEC; }

.theme__poly .grid__col--2 {
  background-color: #C5E2CE; }

.theme__poly .grid__col--3 {
  background-color: #D6EEF5; }
      

        
        
        
      

    
<head lang="en">
    <title>The Quiz</title>
</head>
<body>

<div class="grid">
    <div id="quiz" class="centered grid__col--8">
        <h1>Quiz</h1>

        <h2 id="question" class="headline-secondary--grouped"></h2>
        <h3 id="score"></h3>

        <p id="choice0"></p>
        <button id="guess0" class="btn--default">Select Answer</button>

        <p id="choice1"></p>
        <button id="guess1" class="btn--default">Select Answer</button>

        <footer>
            <p id="progress">Question x of y</p>
        </footer>
    </div>
</div>
</body>
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