Tips for accessing elements using document.getElementsByTagName

Question

Tips for accessing elements using document.getElementsByTagName

Greetings and best wishes for the holiday season! I hope everyone is cozy and safe. I need a little help with some code here, as I am fairly new to JavaScript but not entirely new to programming. Despite finding an answer on this site, I am still encountering an issue after implementing the proposed solution. The TypeError in question relates to the usage of the 'var sections' variable, specifically when trying to append an h1 element to the current div with the 'class="section"' in the NodeList. The error message indicates that it's unable to read the property appendChild of null, suggesting that there is an issue with referencing an Element node properly to use appendChild() or the style object. I'm struggling to identify the root cause of this problem even after following the recommended approach mentioned in another thread, which involved using the 'item()' method of the NodeList object instead of directly accessing the index like so:

var sections = document.getElementsByClassName('section');
var section_header = document.createElement('h1');
sections.item(0).appendChild(section_header);

As compared to my original method...

var sections = document.getElementsByClassName('section');
var section_header = document.createElement('h1');
sections[0].appendChild(section_header);

Switching to the 'NodeList.item()' method resulted in the aforementioned TypeError. The attached image showcases Chrome Dev Tools, Sublime Text, and an empty dark background page in the text editor for reference. This coding exercise is purely for learning purposes and not tied to any specific project. Any insights or advice would be greatly appreciated, thank you!

The test code featured in the screenshot opened in Sublime Text is as follows. I even tried relocating the script tag within the HTML structure without success:

<html>
    <head>
        ...
    </head>
    <body>
        <div id="page-wrapper">
            <div class="section">

            </div>
            <div class="section">

            </div>
            <div class="section">

            </div>
        </div>
        <script type="text/javascript">
            // Code snippet omitted for brevity.
        </script>
    </body>
</html>

Screenshot link

Regarding the updates: despite changing the loop condition from <= to just <, I continue to face the same TypeError regarding 'appendChild' being null. Please refer to the updated screenshot showing the results.

Screenshot of result

Even after implementing Dan's suggestion and reviewing the results, the error persists. Please see the subsequent screenshot illustrating the outcome.

Screenshot of result

Following extensive troubleshooting where certain elements were removed from the code, the script now runs smoothly. However, the puzzling part is that while the background colors are applied successfully, the error related to reading the style property remains. Feel free to check out the updated screenshot below for comparison.

Updated screenshot

javascript getelementsbyclassname nodelist

Answer 1

Answer №1

Your loop is not set up properly. Instead of using "for(var index = 0; index <= colors.length; index++)", you should be checking if index is less than colors.length in order for it to work correctly.
Since you have 3 color items, the index should only increment as long as it's smaller than colors.length.
Here is the corrected code:

        for(var index = 0; index < colors.length; index++) {
            // The text that needs to be added to each section's header.
            var section_header_txt = 'Section #' + section_count;
            // Assign the incremented section count with the corresponding text to the sections h1 element.
            section_header.innerHTML = section_header_txt;
            // Attach the section header to its new section parent.
            sections.item(index).appendChild(section_header); // This is where the TypeError occurs. 
            // Apply styles to each new section.
            sections.item(index).style.backgroundColor = colors[index].toString();
            sections.item(index).style.width = '100%';
            sections.item(index).style.height = '100vh';
            // Increase the section title count by 1.
            section_count++;
        }

Answer 2

Your loop is not set up properly. Instead of using "for(var index = 0; index <= colors.length; index++)", you should be checking if index is less than colors.length in order for it to work correctly.
Since you have 3 color items, the index should only increment as long as it's smaller than colors.length.
Here is the corrected code:

        for(var index = 0; index < colors.length; index++) {
            // The text that needs to be added to each section's header.
            var section_header_txt = 'Section #' + section_count;
            // Assign the incremented section count with the corresponding text to the sections h1 element.
            section_header.innerHTML = section_header_txt;
            // Attach the section header to its new section parent.
            sections.item(index).appendChild(section_header); // This is where the TypeError occurs. 
            // Apply styles to each new section.
            sections.item(index).style.backgroundColor = colors[index].toString();
            sections.item(index).style.width = '100%';
            sections.item(index).style.height = '100vh';
            // Increase the section title count by 1.
            section_count++;
        }

Answer 3

Answer №2

The mistake in this code is due to an error in the for loop's condition. Using <= 4 causes the loop to run on index 4, leading to a return value of undefined for the colors array.

Additionally, there are other issues present in the provided code:

The creation of the section_header element should be moved inside the for loop to prevent overwriting the same element repeatedly and placing it only in the last section. This results in only the third section displaying the header text.
Using GetElementsByClassname returns an array of elements, so referencing them using the array index directly is sufficient without the need for .item.
You already have an index variable available, so instead of manually incrementing a separate sectionCount variable, you can simply add 1 to the existing index.

var colors = ['#455a64','#616161','#5d4037'];

// var sections is an array of  elements with 'section' as classname
var sections = document.getElementsByClassName('section');

for(var index = 0; index < colors.length; index++) {
  var section_header_txt = 'Section #' + (index + 1);
  
  // create a new section header element for each section
  var section_header = document.createElement('h1');
  section_header.innerHTML = section_header_txt;
  
  // use array index instead of .item
  sections[index].appendChild(section_header); 
  sections[index].style.backgroundColor = colors[index].toString();
  sections[index].style.height = '100vh';
}

<div id="page-wrapper">
  <div class="section"></div>
  <div class="section"></div>
  <div class="section"></div>
</div>

Answer 4

The mistake in this code is due to an error in the for loop's condition. Using <= 4 causes the loop to run on index 4, leading to a return value of undefined for the colors array.

Additionally, there are other issues present in the provided code:

The creation of the section_header element should be moved inside the for loop to prevent overwriting the same element repeatedly and placing it only in the last section. This results in only the third section displaying the header text.
Using GetElementsByClassname returns an array of elements, so referencing them using the array index directly is sufficient without the need for .item.
You already have an index variable available, so instead of manually incrementing a separate sectionCount variable, you can simply add 1 to the existing index.

var colors = ['#455a64','#616161','#5d4037'];

// var sections is an array of  elements with 'section' as classname
var sections = document.getElementsByClassName('section');

for(var index = 0; index < colors.length; index++) {
  var section_header_txt = 'Section #' + (index + 1);
  
  // create a new section header element for each section
  var section_header = document.createElement('h1');
  section_header.innerHTML = section_header_txt;
  
  // use array index instead of .item
  sections[index].appendChild(section_header); 
  sections[index].style.backgroundColor = colors[index].toString();
  sections[index].style.height = '100vh';
}

<div id="page-wrapper">
  <div class="section"></div>
  <div class="section"></div>
  <div class="section"></div>
</div>

Answer 5

Answer №3

Make a change in the for loop by replacing <= with < in the condition so that it only iterates 3 times instead of 4. The issue arises during the fourth iteration when the method appendChild is used on section number 4 (item index 4) which does not exist, resulting in an error.

// Colors assigned to each sections background.
var colors = ['#455a64','#616161','#5d4037'];

// All sections within the document.
var sections = document.getElementsByClassName('section');

// Header tag to be inserted in each section.
var section_header = document.createElement('h1');

// The count to be added to each sections title e.g. <h1>Section Title #1.</h1>
var section_count = 1;

for(var index = 0; index < colors.length; index++) {
  // Text for each section's header.
  var section_header_txt = 'Section #' + section_count;
  // Assign the text along with the incremented section count to the h1 element in each section.
  section_header.innerHTML = section_header_txt;
  // Insert the section header into its respective parent section.
  sections.item(index).appendChild(section_header); // This line triggers the TypeError.
  // Apply styles to each new section.
  sections.item(index).style.backgroundColor = colors[index].toString();
  sections.item(index).style.width = '100%';
  sections.item(index).style.height = '100vh';
  // Increase the section title count by 1.
  section_count++;
}

<div id="page-wrapper">
  <div class="section"></div>
  <div class="section"></div>
  <div class="section"></div>
</div>

Answer 6

Make a change in the for loop by replacing <= with < in the condition so that it only iterates 3 times instead of 4. The issue arises during the fourth iteration when the method appendChild is used on section number 4 (item index 4) which does not exist, resulting in an error.

// Colors assigned to each sections background.
var colors = ['#455a64','#616161','#5d4037'];

// All sections within the document.
var sections = document.getElementsByClassName('section');

// Header tag to be inserted in each section.
var section_header = document.createElement('h1');

// The count to be added to each sections title e.g. <h1>Section Title #1.</h1>
var section_count = 1;

for(var index = 0; index < colors.length; index++) {
  // Text for each section's header.
  var section_header_txt = 'Section #' + section_count;
  // Assign the text along with the incremented section count to the h1 element in each section.
  section_header.innerHTML = section_header_txt;
  // Insert the section header into its respective parent section.
  sections.item(index).appendChild(section_header); // This line triggers the TypeError.
  // Apply styles to each new section.
  sections.item(index).style.backgroundColor = colors[index].toString();
  sections.item(index).style.width = '100%';
  sections.item(index).style.height = '100vh';
  // Increase the section title count by 1.
  section_count++;
}

<div id="page-wrapper">
  <div class="section"></div>
  <div class="section"></div>
  <div class="section"></div>
</div>

Answer 7

Answer №4

Thank you to all for your help in solving this issue. It turns out that by following Dan's advice and making a few adjustments, I was able to get the code working correctly. 1) Placing

document.createElement('h1')</code> inside the for loop instead of above it made a significant difference.
2) Using <code>document.createTextNode()

rather than Element.innerHTML also contributed to resolving the problem. 3) Whether utilizing

document.getElementsByClassName('section')[0]

or

document.getElementsByClassName('section').item(0)

, both methods produced the same outcome.

Despite the code now functioning properly (as shown in the screenshot), I still encountered a TypeError regarding the inability to read the style object in the console, which seemed perplexing to me.

View the current results of my work

Check out the sources view of the error here

Answer 8

Thank you to all for your help in solving this issue. It turns out that by following Dan's advice and making a few adjustments, I was able to get the code working correctly. 1) Placing

document.createElement('h1')</code> inside the for loop instead of above it made a significant difference.
2) Using <code>document.createTextNode()

rather than Element.innerHTML also contributed to resolving the problem. 3) Whether utilizing

document.getElementsByClassName('section')[0]

or

document.getElementsByClassName('section').item(0)

, both methods produced the same outcome.

Despite the code now functioning properly (as shown in the screenshot), I still encountered a TypeError regarding the inability to read the style object in the console, which seemed perplexing to me.

View the current results of my work

Check out the sources view of the error here

Tips for accessing elements using document.getElementsByTagName

Answer №1

Answer №2

Answer №3

Answer №4

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