How can I show the correct answer in my quiz?

Question

I would like to be able to show the user what the correct answer to the question is if the one that they selected was incorrect. I would like to keep it simple, but here is what I am thinking. Once the user submits their answer and if it is incorrect, before moving onto the next question I would like for the incorrect answer to be highlighted in red, and the correct answer to be highlighted in green.

I already have coded whether or not the answer is correct or incorrect, but I haven't been able to figure out how to be able to show the correct answer if an incorrect one is chosen.

let score = 0;
    let currentQuestion = 0;
    let questions = [{
        title: "At what age was Harry Potter when he received his Hogwarts letter?",
        answers: ['7', '10', '11', '13'],
        correct: 1
      },
      {
        title: "Which is not a Hogwarts house?",
        answers: ['Dunder Mifflin', 'Ravenclaw', 'Slytherin', 'Gryffindor'],
        correct: 0
      },
      {
        title: "Who teaches Transfiguration at Hogwarts?",
        answers: ['Rubeus Hagrid', 'Albus Dumbledore', 'Severus Snape', 'Minerva McGonnagle'],
        correct: 3
      },
      {
        title: "Where is Hogwarts School for Witchcraft and Wizardry located?",
        answers: ['France', 'USA', 'UK', 'New Zealand'],
        correct: 2
      },
      {
        title: "What form does Harry Potter's patronus charm take?",
        answers: ['Stag', 'Eagle', 'Bear', 'Dragon'],
        correct: 0
      },
      {
        title: "What type of animal is Harry's pet?",
        answers: ['Dog', 'Owl', 'Cat', 'Snake'],
        correct: 1
      },
      {
        title: "Who is not a member of the Order of the Phoenix?",
        answers: ['Remus Lupin', 'Siruis Black', 'Lucious Malfoy', 'Severus Snape'],
        correct: 2
      },
      {
        title: "What is not a piece of the Deathly Hallows?",
        answers: ['Elder Wand', 'Cloak of Invisibility', 'Resurrection Stone', 'Sword of Gryffindor'],
        correct: 3
      },
      {
        title: "In which house is Harry sorted into after arriving at Hogwarts?",
        answers: ['Slytherin', 'Ravenclaw', 'Gryffindor', 'Hufflepuff'],
        correct: 2
      },
      {
        title: "What prevented Voldemort from being able to kill Harry Potter when he was a baby?",
        answers: ['Love', 'Anger', 'Friendship', 'Joy'],
        correct: 0
      },
    ];



    $(document).ready(function() {

      $('.start a').click(function(e) {
        e.preventDefault();
        $('.start').hide();
        $('.quiz').show();
        showQuestion();
      });

      $('.quiz ul').on('click', 'li', function() {
        $('.selected').removeClass('selected');
        $(this).addClass('selected');
      });

      $('.quiz a').click(function(e) {
        e.preventDefault();
        if ($('li.selected').length) {
          let guess = parseInt($('li.selected').attr('id'));
          checkAnswer(guess);
        } else {
          alert('Please select an answer');
        }
      });

      $('.summary a').click(function(e) {
        e.preventDefault();
        restartQuiz();
      });

    });

    function showQuestion() {
      let question = questions[currentQuestion];
      $('.quiz h2').text(question.title);
      $('.quiz ul').html('');
      for (var i = 0; i < question.answers.length; i++) {
        $('.quiz ul').append(`<li id="${i}">${question.answers[i]}</li>`);
      }
      showProgress();
    }

    function checkAnswer(guess) {
      let question = questions[currentQuestion];
      if (question.correct === guess) {
        score++;
        showIsCorrect(true);
      } else {
        showIsCorrect(false);
      }

      currentQuestion++;
      if (currentQuestion >= questions.length) {
        showSummary();
      } else {
        showQuestion();
      }
    }

    function showSummary() {
      $('.quiz').hide();
      $('.summary').show();
      $('.summary p').text("Thank you for taking the quiz! Here's how you scored. You answered " + score + " out of " + questions.length + " correctly! Care to try again?")
    }

    function restartQuiz() {
      $('.summary').hide();
      $('.quiz').show();
      score = 0;
      currentQuestion = 0;
      showQuestion();
    }

    function showProgress() {
      $('#currentQuestion').html(currentQuestion + " out of " + questions.length);
    }

    function showIsCorrect(isCorrect) {
      var result = "Wrong";
      if (isCorrect) {
        result = "Correct";
      }
      $('#isCorrect').html(result);
    }

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  text-decoration: none;
  text-align: center;
  background-color: #837F73;
}

h1 {
  font-family: 'Poor Story', cursive;
  background-color: #950002;
  padding: 60px;
  color: #FFAB0D;
  text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
}

h2 {
  font-family: 'Poor Story', cursive;
  font-size: 30px;
  padding: 60px;
  background-color: #950002;
  color: #FFAB0D;
  text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
}

p {
 font-family: 'Poor Story', cursive;
  background-color: #FFAB0D;
  font-size: 20px;
  font-weight: bold;

}

a {
  border: 1px solid #222F5B;
  padding: 10px;
  background-color: #222F5B;
  color: silver;
  border-radius: 5px;
  margin-top: 50px;
  display: inline-block;
}

a:hover {
  border: 1px solid #000000;
  background-color: #000000;
  color: #FCBF2B;
}

.quiz li {
  cursor: pointer;
  border: 1px solid;
  display: inline-block;
  width: 200px;
  margin: 10px;
  font-family: 'Poor Story', cursive;
  font-size: 26px;
}

#currentQuestion {
  color: #000000;
  font-size: 18px;
  font-family: 'Poor Story', cursive;
  margin-top: 10px;

}

#isCorrect {
  color: white;
  font-family: 'Poor Story', cursive;
  font-weight: bold;
  font-size: 18px;

}

.quiz, .summary {
      display: none;
    }
    
    .quiz ul {
      margin-top: 20px;
      list-style: none;
      padding: 0;
    }
    
    .selected {
      background-color: #398C3F;
    }

<!DOCTYPE html>
<html>

<head>
  <link href="https://fonts.googleapis.com/css?family=Poor+Story" rel="stylesheet">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>

<body>


  <div class="start">
    <h1>How Well Do You Know Harry Potter?</h1>
    <a href="#">Start Quiz</a>
  </div>


  <div class="quiz">
    <h2>Question Title</h2>
    <ul>
      <li>Choice</li>
      <li>Choice</li>
      <li>Choice</li>
      <li>Choice</li>
    </ul>
    <a href="#">Submit Answer</a>
    <div id="currentQuestion"></div>
    <div id="isCorrect"></div>
  </div>


  <div class="summary">
    <h2>Results</h2>
    <p>Thank you for taking the quiz! Here's how you scored. You answered x out of y correctly! Care to try again?</p>
    <a href="#">Retake Quiz</a>
  </div>

  <script src="https://code.jquery.com/jquery-3.1.0.js"></script>
  

</body>

</html>

Answer 1

Added the changes needed for score updates as well:

<!DOCTYPE html>
<html>

<head>
    <script src="https://code.jquery.com/jquery-3.1.0.js"></script>
    <style>
        * {
            padding: 0;
            margin: 0;
            box-sizing: border-box;
            text-decoration: none;
            text-align: center;
            background-color: #837F73;
        }

        h1 {
            font-family: 'Poor Story', cursive;
            background-color: #950002;
            padding: 60px;
            color: #FFAB0D;
            text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
        }

        h2 {
            font-family: 'Poor Story', cursive;
            font-size: 30px;
            padding: 60px;
            background-color: #950002;
            color: #FFAB0D;
            text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
        }

        p {
            font-family: 'Poor Story', cursive;
            background-color: #FFAB0D;
            font-size: 20px;
            font-weight: bold;

        }

        a {
            border: 1px solid #222F5B;
            padding: 10px;
            background-color: #222F5B;
            color: silver;
            border-radius: 5px;
            margin-top: 50px;
            display: inline-block;
        }

        a:hover {
            border: 1px solid #000000;
            background-color: #000000;
            color: #FCBF2B;
        }

        .quiz li {
            cursor: pointer;
            border: 1px solid;
            display: inline-block;
            width: 200px;
            margin: 10px;
            font-family: 'Poor Story', cursive;
            font-size: 26px;
        }

        #currentQuestion {
            color: #000000;
            font-size: 18px;
            font-family: 'Poor Story', cursive;
            margin-top: 10px;

        }

        #isCorrect {
            color: white;
            font-family: 'Poor Story', cursive;
            font-weight: bold;
            font-size: 18px;

        }

        .quiz,
        .summary {
            display: none;
        }

        .quiz ul {
            margin-top: 20px;
            list-style: none;
            padding: 0;
        }

        .selected {
            background-color: #398C3F;
        }

        .wrong {
            background-color: red;
        }
    </style>
    <script>
        let score = 0;
        let currentQuestion = 0;
        let questions = [{
            title: "At what age was Harry Potter when he received his Hogwarts letter?",
            answers: ['7', '10', '11', '13'],
            correct: 1
        },
        {
            title: "Which is not a Hogwarts house?",
            answers: ['Dunder Mifflin', 'Ravenclaw', 'Slytherin', 'Gryffindor'],
            correct: 0
        },
        {
            title: "Who teaches Transfiguration at Hogwarts?",
            answers: ['Rubeus Hagrid', 'Albus Dumbledore', 'Severus Snape', 'Minerva McGonnagle'],
            correct: 3
        },
        {
            title: "Where is Hogwarts School for Witchcraft and Wizardry located?",
            answers: ['France', 'USA', 'UK', 'New Zealand'],
            correct: 2
        },
        {
            title: "What form does Harry Potter's patronus charm take?",
            answers: ['Stag', 'Eagle', 'Bear', 'Dragon'],
            correct: 0
        },
        {
            title: "What type of animal is Harry's pet?",
            answers: ['Dog', 'Owl', 'Cat', 'Snake'],
            correct: 1
        },
        {
            title: "Who is not a member of the Order of the Phoenix?",
            answers: ['Remus Lupin', 'Siruis Black', 'Lucious Malfoy', 'Severus Snape'],
            correct: 2
        },
        {
            title: "What is not a piece of the Deathly Hallows?",
            answers: ['Elder Wand', 'Cloak of Invisibility', 'Resurrection Stone', 'Sword of Gryffindor'],
            correct: 3
        },
        {
            title: "In which house is Harry sorted into after arriving at Hogwarts?",
            answers: ['Slytherin', 'Ravenclaw', 'Gryffindor', 'Hufflepuff'],
            correct: 2
        },
        {
            title: "What prevented Voldemort from being able to kill Harry Potter when he was a baby?",
            answers: ['Love', 'Anger', 'Friendship', 'Joy'],
            correct: 0
        },
        ];



        $(document).ready(function () {

            $('.start a').click(function (e) {
                e.preventDefault();
                $('.start').hide();
                $('.quiz').show();
                showQuestion();
            });

            $('.quiz ul').on('click', 'li', function () {
                $('.selected').removeClass('selected');
                $(this).addClass('selected');
            });

            $('.quiz a').click(function (e) {
                e.preventDefault();
                if ($('li.selected').length) {
                    let guess = parseInt($('li.selected').attr('id'));
                    let isCorrect = $('li.selected').attr('isCorrect');
                    checkAnswer(guess, isCorrect);
                } else {
                    alert('Please select an answer');
                }
            });

            $('.summary a').click(function (e) {
                e.preventDefault();
                restartQuiz();
            });

        });

        function showQuestion() {
            let question = questions[currentQuestion];
            $('.quiz h2').text(question.title);
            $('.quiz ul').html('');
            for (var i = 0; i < question.answers.length; i++) {
                $('.quiz ul').append(`<li isCorrect="true" id="${i}">${question.answers[i]}</li>`);
            }
            showProgress();
        }

        function showIncorrectQuestion(guess) {
            let question = questions[currentQuestion];
            $('.quiz h2').text(question.title);
            $('.quiz ul').html('');
            for (var i = 0; i < question.answers.length; i++) {
                let cls = i === question.correct ? "selected" : guess === i ? "wrong" : ""
                $('.quiz ul').append(`<li id="${i}" isCorrect="false" class="${cls}">${question.answers[i]}</li>`);
            }
            showProgress();
        }

        function checkAnswer(guess, isCorrect) {

            let question = questions[currentQuestion];
            if (question.correct === guess) {
                if (isCorrect === 'true') {
                    score++;
                }
                showIsCorrect(true);
                currentQuestion++;
                if (currentQuestion >= questions.length) {
                    showSummary();
                } else {
                    showQuestion();
                }
            } else {
                showIsCorrect(false);
                showIncorrectQuestion(guess);
            }


        }

        function showSummary() {
            $('.quiz').hide();
            $('.summary').show();
            $('.summary p').text("Thank you for taking the quiz! Here's how you scored. You answered " + score + " out of " + questions.length + " correctly! Care to try again?")
        }

        function restartQuiz() {
            $('.summary').hide();
            $('.quiz').show();
            score = 0;
            currentQuestion = 0;
            showQuestion();
        }

        function showProgress() {
            $('#currentQuestion').html(currentQuestion + " out of " + questions.length);
        }

        function showIsCorrect(isCorrect) {
            var result = "Wrong";
            if (isCorrect) {
                result = "Correct";
            }
            $('#isCorrect').html(result);
        }
    </script>



    <link href="https://fonts.googleapis.com/css?family=Poor+Story" rel="stylesheet">
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <title>JS Bin</title>
</head>

<body>


    <div class="start">
        <h1>How Well Do You Know Harry Potter?</h1>
        <a href="#">Start Quiz</a>
    </div>


    <div class="quiz">
        <h2>Question Title</h2>
        <ul>
            <li>Choice</li>
            <li>Choice</li>
            <li>Choice</li>
            <li>Choice</li>
        </ul>
        <a href="#">Submit Answer</a>
        <div id="currentQuestion"></div>
        <div id="isCorrect"></div>
    </div>


    <div class="summary">
        <h2>Results</h2>
        <p>Thank you for taking the quiz! Here's how you scored. You answered x out of y correctly! Care to try again?</p>
        <a href="#">Retake Quiz</a>
    </div>




</body>

</html>

Answer 2

Just a quick heads up that under this implementation, anybody who inspects the your javascript file will be able to see the answers to your quiz, even if it is minified/uglified! I doubt that anyone taking this Harry Potter quiz is going to reverse engineer it (hopefully), but you never know! If you wanted to make the quiz more secure, you create a web service and then validate answers via HTTP requests.

Anyways, for learning purposes, you can modify your code to display the answer by changing the following. As for the coloring, see addClass in the jQuery documentation and try defining some of your own classes for right and wrong answers. You can also check out this mdn documentation for some basics on web architecture.

Also, welcome to Stack Overflow and good luck!

let score = 0;
    let currentQuestion = 0;
    let questions = [{
        title: "At what age was Harry Potter when he received his Hogwarts letter?",
        answers: ['7', '10', '11', '13'],
        correct: 1
      },
      {
        title: "Which is not a Hogwarts house?",
        answers: ['Dunder Mifflin', 'Ravenclaw', 'Slytherin', 'Gryffindor'],
        correct: 0
      },
      {
        title: "Who teaches Transfiguration at Hogwarts?",
        answers: ['Rubeus Hagrid', 'Albus Dumbledore', 'Severus Snape', 'Minerva McGonnagle'],
        correct: 3
      },
      {
        title: "Where is Hogwarts School for Witchcraft and Wizardry located?",
        answers: ['France', 'USA', 'UK', 'New Zealand'],
        correct: 2
      },
      {
        title: "What form does Harry Potter's patronus charm take?",
        answers: ['Stag', 'Eagle', 'Bear', 'Dragon'],
        correct: 0
      },
      {
        title: "What type of animal is Harry's pet?",
        answers: ['Dog', 'Owl', 'Cat', 'Snake'],
        correct: 1
      },
      {
        title: "Who is not a member of the Order of the Phoenix?",
        answers: ['Remus Lupin', 'Siruis Black', 'Lucious Malfoy', 'Severus Snape'],
        correct: 2
      },
      {
        title: "What is not a piece of the Deathly Hallows?",
        answers: ['Elder Wand', 'Cloak of Invisibility', 'Resurrection Stone', 'Sword of Gryffindor'],
        correct: 3
      },
      {
        title: "In which house is Harry sorted into after arriving at Hogwarts?",
        answers: ['Slytherin', 'Ravenclaw', 'Gryffindor', 'Hufflepuff'],
        correct: 2
      },
      {
        title: "What prevented Voldemort from being able to kill Harry Potter when he was a baby?",
        answers: ['Love', 'Anger', 'Friendship', 'Joy'],
        correct: 0
      },
    ];



    $(document).ready(function() {

      $('.start a').click(function(e) {
        e.preventDefault();
        $('.start').hide();
        $('.quiz').show();
        showQuestion();
      });

      $('.quiz ul').on('click', 'li', function() {
        $('.selected').removeClass('selected');
        $(this).addClass('selected');
      });

      $('.quiz a').click(function(e) {
        e.preventDefault();
        if ($('li.selected').length) {
          let guess = parseInt($('li.selected').attr('id'));
          checkAnswer(guess);
        } else {
          alert('Please select an answer');
        }
      });

      $('.summary a').click(function(e) {
        e.preventDefault();
        restartQuiz();
      });

    });

    function showQuestion() {
      let question = questions[currentQuestion];
      $('.quiz h2').text(question.title);
      $('.quiz ul').html('');
      for (var i = 0; i < question.answers.length; i++) {
        $('.quiz ul').append(`<li id="${i}">${question.answers[i]}</li>`);
      }
      showProgress();
    }

    function checkAnswer(guess) {
      let question = questions[currentQuestion];
      if (question.correct === guess) {
        score++;
        showIsCorrect(true, questions[question.correct]);
      } else {
        showIsCorrect(false, questions[question.correct]);
      }

      currentQuestion++;
      if (currentQuestion >= questions.length) {
        showSummary();
      } else {
        showQuestion();
      }
    }

    function showSummary() {
      $('.quiz').hide();
      $('.summary').show();
      $('.summary p').text("Thank you for taking the quiz! Here's how you scored. You answered " + score + " out of " + questions.length + " correctly! Care to try again?")
    }

    function restartQuiz() {
      $('.summary').hide();
      $('.quiz').show();
      score = 0;
      currentQuestion = 0;
      showQuestion();
    }

    function showProgress() {
      $('#currentQuestion').html(currentQuestion + " out of " + questions.length);
    }

    function showIsCorrect(isCorrect, question) {
      var result = "Wrong, the correct answer is " + 
          question.answers[question.correct];
      if (isCorrect) {
        result = "Correct";
      }
      $('#isCorrect').html(result);
    }

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  text-decoration: none;
  text-align: center;
  background-color: #837F73;
}

h1 {
  font-family: 'Poor Story', cursive;
  background-color: #950002;
  padding: 60px;
  color: #FFAB0D;
  text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
}

h2 {
  font-family: 'Poor Story', cursive;
  font-size: 30px;
  padding: 60px;
  background-color: #950002;
  color: #FFAB0D;
  text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;
}

p {
 font-family: 'Poor Story', cursive;
  background-color: #FFAB0D;
  font-size: 20px;
  font-weight: bold;

}

a {
  border: 1px solid #222F5B;
  padding: 10px;
  background-color: #222F5B;
  color: silver;
  border-radius: 5px;
  margin-top: 50px;
  display: inline-block;
}

a:hover {
  border: 1px solid #000000;
  background-color: #000000;
  color: #FCBF2B;
}

.quiz li {
  cursor: pointer;
  border: 1px solid;
  display: inline-block;
  width: 200px;
  margin: 10px;
  font-family: 'Poor Story', cursive;
  font-size: 26px;
}

#currentQuestion {
  color: #000000;
  font-size: 18px;
  font-family: 'Poor Story', cursive;
  margin-top: 10px;

}

#isCorrect {
  color: white;
  font-family: 'Poor Story', cursive;
  font-weight: bold;
  font-size: 18px;

}

.quiz, .summary {
      display: none;
    }
    
    .quiz ul {
      margin-top: 20px;
      list-style: none;
      padding: 0;
    }
    
    .selected {
      background-color: #398C3F;
    }

<!DOCTYPE html>
<html>

<head>
  <link href="https://fonts.googleapis.com/css?family=Poor+Story" rel="stylesheet">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>

<body>


  <div class="start">
    <h1>How Well Do You Know Harry Potter?</h1>
    <a href="#">Start Quiz</a>
  </div>


  <div class="quiz">
    <h2>Question Title</h2>
    <ul>
      <li>Choice</li>
      <li>Choice</li>
      <li>Choice</li>
      <li>Choice</li>
    </ul>
    <a href="#">Submit Answer</a>
    <div id="currentQuestion"></div>
    <div id="isCorrect"></div>
  </div>


  <div class="summary">
    <h2>Results</h2>
    <p>Thank you for taking the quiz! Here's how you scored. You answered x out of y correctly! Care to try again?</p>
    <a href="#">Retake Quiz</a>
  </div>

  <script src="https://code.jquery.com/jquery-3.1.0.js"></script>
  

</body>

</html>