Add a CSS class to a dropdown menu option in HTML and JavaScript (JQuery) if it has a specified ID

Question

Add a CSS class to a dropdown menu option in HTML and JavaScript (JQuery) if it has a specified ID

I am brand new to HTML and JQuery, and I'm struggling with setting a class for my select element when the currently selected option has an ID of "answer". This is crucial for checking the correctness of a multiple choice question.

If achieving this in JQuery proves impossible, JavaScript would be an acceptable alternative. My goal is to avoid making a database query and leveraging JQuery seemed like the most efficient approach.

Below is the current HTML section I have:

<form id="ansForm" class="testClass1">
    <div id="QuestionForm" name="QuestionForm">
    <label>Question 1: This is a question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class" id="answer">Answer1</option>
        <option value="2" class="ans-class">Answer2</option>
        <option value="3" class="ans-class">Answer3</option>
        <option value="4" class="ans-class">Answer4</option>
    </select>
    <label>Question 2: This is another question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class">Another Answer</option>
        <option value="2" class="ans-class">Just some text</option>
        <option value="3" class="ans-class" id="answer">Test</option>
        <option value="4" class="ans-class">Test2</option>
    </select>
    </div>
    <button type="button" class="btn btn-primary" 
    onclick="checkAnswers()">Check</button>
</form>

A Javascript function called "checkAnswers()" is executed upon clicking the button.
This function is meant to verify if the selected option in the dropdown box has an id of "answer". Essentially, it should change the background color of the select element if the correct option (option one) is chosen.

How can I detect the currently selected dropdown option's ID? And how do I extend this functionality to multiple questions at once?
Additionally, how can I dynamically add a class to the select element using JavaScript in order to modify its background color?

Here is the JavaScript code snippet I attempted:

var s = document.getElementsByClassName("select-class");
var idSelectedOption = s[s.selectedIndex].id;
alert(idSelectedOption);

However, this resulted in an error message: "Uncaught TypeError: Cannot read property 'id' of undefined". I believe this occurs because it returns an array of classes. How would I iterate through all of them and alter the background colors of those with the correct option selected?

Thank you in advance,
Mats.

javascript jquery html css

Answer 1

Answer №1

It is advisable to use data-* attributes instead of id when dealing with multiple elements in a document sharing the same id value.

When using getElementsByClassName, it returns a nodelist, so you will need to iterate through the elements and apply conditions accordingly. In the example below, Array.prototype.forEach.call is used to iterate through elements.

Here's an example:

function checkAnswers() {
  var s = document.getElementsByClassName("select-class");
  Array.prototype.forEach.call(s, function(elem) {
    var idSelectedOption = elem[elem.selectedIndex].getAttribute('data-id');
    if (idSelectedOption == 'answer') {
      var selectedAnswer = elem[elem.selectedIndex].getAttribute('value');
      alert(selectedAnswer);
    }
  });
}

<form id="ansForm" class="testClass1">
  <div id="QuestionForm" name="QuestionForm">
    <label>Question 1: This is a question</label>
    <select class="form-control select-class">
      <option value="1" class="ans-class" data-id="answer">Answer1</option>
      <option value="2" class="ans-class">Answer2</option>
      <option value="3" class="ans-class">Answer3</option>
      <option value="4" class="ans-class">Answer4</option>
    </select>
    <label>Question 2: This is another question</label>
    <select class="form-control select-class">
      <option value="1" class="ans-class">Another Answer</option>
      <option value="2" class="ans-class">Just some text</option>
      <option value="3" class="ans-class" data-id="answer">Test</option>
      <option value="4" class="ans-class">Test2</option>
    </select>
  </div>
  <button type="button" class="btn btn-primary" onclick="checkAnswers()">Check</button>
</form>

Link to the Fiddle here

Answer 2

It is advisable to use data-* attributes instead of id when dealing with multiple elements in a document sharing the same id value.

When using getElementsByClassName, it returns a nodelist, so you will need to iterate through the elements and apply conditions accordingly. In the example below, Array.prototype.forEach.call is used to iterate through elements.

Here's an example:

function checkAnswers() {
  var s = document.getElementsByClassName("select-class");
  Array.prototype.forEach.call(s, function(elem) {
    var idSelectedOption = elem[elem.selectedIndex].getAttribute('data-id');
    if (idSelectedOption == 'answer') {
      var selectedAnswer = elem[elem.selectedIndex].getAttribute('value');
      alert(selectedAnswer);
    }
  });
}

<form id="ansForm" class="testClass1">
  <div id="QuestionForm" name="QuestionForm">
    <label>Question 1: This is a question</label>
    <select class="form-control select-class">
      <option value="1" class="ans-class" data-id="answer">Answer1</option>
      <option value="2" class="ans-class">Answer2</option>
      <option value="3" class="ans-class">Answer3</option>
      <option value="4" class="ans-class">Answer4</option>
    </select>
    <label>Question 2: This is another question</label>
    <select class="form-control select-class">
      <option value="1" class="ans-class">Another Answer</option>
      <option value="2" class="ans-class">Just some text</option>
      <option value="3" class="ans-class" data-id="answer">Test</option>
      <option value="4" class="ans-class">Test2</option>
    </select>
  </div>
  <button type="button" class="btn btn-primary" onclick="checkAnswers()">Check</button>
</form>

Link to the Fiddle here

Answer 3

Answer №2

Make sure not to have duplicate IDs for elements. Consider using custom data attributes or classes instead.

Once you've addressed that issue, the following code should work. I opted for vanilla JavaScript as you didn't specify jQuery usage.

// Lazy: Bind the event to the form.
document.getElementById('ansForm').addEventListener('change', function(event) {
  
  var selectElement = event.target;
  
  // Only respond if the clicked element is one of the selects.
  if (selectElement.classList.contains('select-class')) {
    
    // Get the option that is currently selected.
    var selectedOption = selectElement[selectElement.selectedIndex];
    // Check if this option contains the class 'answer'.
    var isAnswerSelected = selectedOption.classList.contains('answer');
    
    console.log(isAnswerSelected);
    
    // Remove the indicators. You could easily use classList.toggle, but the second
    // argument is not supported in IE.
    
    // selectElement.classList.toggle('right', isAnswerSelected);
    // selectElement.classList.toggle('wrong', !isAnswerSelected);

    // So, second best. Just remove both and re-add the class we want.
    selectElement.classList.remove('right');
    selectElement.classList.remove('wrong');
    selectElement.classList.add(isAnswerSelected?'right':'wrong');
    
  } else {
    // Ignore clicks on any other element.  
  }
});

.right {
  color: green;
}

.wrong {
  color: red; 
}

<form id="ansForm" class="testClass1">
    <div id="QuestionForm" name="QuestionForm">
    <label>Question 1: This is a question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class answer">Answer1</option>
        <option value="2" class="ans-class">Answer2</option>
        <option value="3" class="ans-class">Answer3</option>
        <option value="4" class="ans-class">Answer4</option>
    </select>
    <label>Question 2: This is another question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class">Another Answer</option>
        <option value="2" class="ans-class">Just some text</option>
        <option value="3" class="ans-class answer">Test</option>
        <option value="4" class="ans-class">Test2</option>
    </select>
    </div>
    <button type="button" class="btn btn-primary" 
    onclick="checkAnswers()">Check</button>
</form>

Answer 4

Make sure not to have duplicate IDs for elements. Consider using custom data attributes or classes instead.

Once you've addressed that issue, the following code should work. I opted for vanilla JavaScript as you didn't specify jQuery usage.

// Lazy: Bind the event to the form.
document.getElementById('ansForm').addEventListener('change', function(event) {
  
  var selectElement = event.target;
  
  // Only respond if the clicked element is one of the selects.
  if (selectElement.classList.contains('select-class')) {
    
    // Get the option that is currently selected.
    var selectedOption = selectElement[selectElement.selectedIndex];
    // Check if this option contains the class 'answer'.
    var isAnswerSelected = selectedOption.classList.contains('answer');
    
    console.log(isAnswerSelected);
    
    // Remove the indicators. You could easily use classList.toggle, but the second
    // argument is not supported in IE.
    
    // selectElement.classList.toggle('right', isAnswerSelected);
    // selectElement.classList.toggle('wrong', !isAnswerSelected);

    // So, second best. Just remove both and re-add the class we want.
    selectElement.classList.remove('right');
    selectElement.classList.remove('wrong');
    selectElement.classList.add(isAnswerSelected?'right':'wrong');
    
  } else {
    // Ignore clicks on any other element.  
  }
});

.right {
  color: green;
}

.wrong {
  color: red; 
}

<form id="ansForm" class="testClass1">
    <div id="QuestionForm" name="QuestionForm">
    <label>Question 1: This is a question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class answer">Answer1</option>
        <option value="2" class="ans-class">Answer2</option>
        <option value="3" class="ans-class">Answer3</option>
        <option value="4" class="ans-class">Answer4</option>
    </select>
    <label>Question 2: This is another question </label>
    <select class="form-control select-class">
        <option value="1" class="ans-class">Another Answer</option>
        <option value="2" class="ans-class">Just some text</option>
        <option value="3" class="ans-class answer">Test</option>
        <option value="4" class="ans-class">Test2</option>
    </select>
    </div>
    <button type="button" class="btn btn-primary" 
    onclick="checkAnswers()">Check</button>
</form>

Answer 5

Answer №3

Here is a jQuery solution you can try:

$(function(){
    // Event handler for clicking the element with id 'checkBtn'
    $('#checkBtn').on('click', function(){

        // Get all select elements with class 'select-class'.
        var $selects = $('select.select-class');

        // Iterate through each select element.
        $selects.each(function(k, v){
            // Get the option with id 'answer' for the current select element.
            var $selectAnswerOpt = $(this).children('option#answer');

            // Get the value attribute of the option element.
            var answer = $selectAnswerOpt.attr('value');

            // Get the selected value for the select element.
            var selectedValue = $(this).val();

            // Check if the selected value matches the id 'answer'
            if (selectedValue == answer)
            {
                // Change background color to green if selected value matches 'answer'
                $(this).css('background-color', 'green');
            }
            else
            {
                // Change background color to yellow if not matching 'answer'
                $(this).css('background-color', 'yellow');
            }
        });
    });
});

Check out the FIDDLE for reference.

Answer 6

Here is a jQuery solution you can try:

$(function(){
    // Event handler for clicking the element with id 'checkBtn'
    $('#checkBtn').on('click', function(){

        // Get all select elements with class 'select-class'.
        var $selects = $('select.select-class');

        // Iterate through each select element.
        $selects.each(function(k, v){
            // Get the option with id 'answer' for the current select element.
            var $selectAnswerOpt = $(this).children('option#answer');

            // Get the value attribute of the option element.
            var answer = $selectAnswerOpt.attr('value');

            // Get the selected value for the select element.
            var selectedValue = $(this).val();

            // Check if the selected value matches the id 'answer'
            if (selectedValue == answer)
            {
                // Change background color to green if selected value matches 'answer'
                $(this).css('background-color', 'green');
            }
            else
            {
                // Change background color to yellow if not matching 'answer'
                $(this).css('background-color', 'yellow');
            }
        });
    });
});

Check out the FIDDLE for reference.

Add a CSS class to a dropdown menu option in HTML and JavaScript (JQuery) if it has a specified ID

Answer №1

Answer №2

Answer №3

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