How to Efficiently Work with the DOM Tree

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Photo by Lance Anderson on Unsplash

When you’re building websites using JavaScript, working with the DOM (Document Object Model) is super important. The DOM is like a map of your webpage, and it lets you use JavaScript to find elements, change them, or move around the page. Whether you’re making a dynamic blog, a cool online store, or just a simple interactive site, knowing how to work with the DOM helps your pages run smoother and faster.

In this guide, we’ll show you how to move through the DOM (called traversal) and how to make changes to it in a way that doesn’t slow down your site. We’ll give you examples and explain what happens when you run the code, so it’s easy to follow. Along the way, we’ll also cover some common mistakes and show how to avoid them to keep your websites efficient and user-friendly.

Understanding how the DOM works will also make it easier for you to work with libraries and frameworks like React or Vue later on. These tools still interact with the DOM behind the scenes, so mastering the basics gives you a strong foundation.

What is DOM Traversal?

DOM traversal means navigating through the elements of your HTML page. You might start at one element and look at its children, siblings, or parent elements. JavaScript gives us tools like parentNode, firstChild, lastChild, nextSibling, and previousSibling to do this. There are also newer tools like querySelectorAll that let you grab exactly what you need using CSS-like rules.

It’s important to know that the DOM is a tree structure. Each element is a node, and some nodes have child nodes (like a list containing list items). By moving up, down, or across this tree, you can access and interact with almost anything on the page.

Getting Child Elements

Here’s an example of HTML:

<ul id="myList">
  <li>First</li>
  <li>Second</li>
  <li>Third</li>
</ul>

And the JavaScript to get each item:

const list = document.getElementById('myList');
const items = list.children;

for (let i = 0; i < items.length; i++) {
  console.log(items[i].textContent);
}
/* Result in console
First
Second
Third
*/

This loop goes through the children of the list and prints out their text. The children property skips over things like text nodes and only gives you actual elements. This is helpful because sometimes line breaks or white space in your HTML can show up as text nodes when you use childNodes instead.

querySelectorAll for Fast Selection

You can also do it this way:

const items = document.querySelectorAll('#myList li');
items.forEach(item => console.log(item.textContent));

Same result! This is shorter and lets you use CSS-style selectors to find what you need. querySelectorAll is very flexible and allows you to select specific classes, attributes, or tag combinations.

You can even chain selections like this:

const specialItems = document.querySelectorAll('#myList li:nth-child(2)');

That line will get the second item on the list.

How to Modify the DOM Efficiently

Changing the DOM can slow down your page if you do it too much or in the wrong way. When you add, remove, or move elements, the browser has to re-calculate everything (called reflow or repaint), which takes time. To keep things fast, there are smart ways to do this.

Instead of making one change at a time, it’s better to group your changes or use special tools like DocumentFragment to prepare everything first.

Adding Multiple Items with DocumentFragment

Here’s how you can add several elements to a list:

<ul id="fruits"></ul>
const fragment = document.createDocumentFragment();
const fruits = ['Apple', 'Banana', 'Cherry'];
fruits.forEach(fruit => {
  const li = document.createElement('li');
  li.textContent = fruit;
  fragment.appendChild(li);
});
document.getElementById('fruits').appendChild(fragment);

This adds all the list items at once, so your browser only updates the page one time instead of three separate times. It’s much faster than adding one at a time and is especially useful if you’re adding a lot of elements.

Tip: while you’re building the fragment, it’s not part of the live DOM yet. So it won’t trigger any layout changes until it’s all ready and added.

innerHTML for Fast Changes

Another way to quickly update part of the page is with innerHTML:

<div id="container"></div>
const html = `
  <p>Hello, World!</p>
  <p>This is a batch update.</p>
`;
document.getElementById('container').innerHTML = html;

This replaces whatever was inside #container with the new paragraphs. It’s very fast but also deletes anything that was already in the element, including event listeners or user input. So be careful when using it for interactive content.

Changing Styles with Classes

It’s better to use CSS classes instead of setting lots of styles in JavaScript:

const box = document.querySelector('.box');
box.classList.add('highlighted');

This adds a class to the element, which changes its look (like color or size) based on your CSS. It keeps your code cleaner and easier to manage. You can also remove or toggle classes using classList.remove() or classList.toggle().

Tips to Keep Things Fast

Here are some best practices you should follow to keep your pages fast and smooth:

  • Don’t repeat DOM lookups: If you’re using the same element more than once, save it in a variable instead of looking it up each time.
  • Group changes together: Try to make multiple changes in one go instead of one-by-one. This reduces how often the browser has to re-calculate layouts.
  • Use efficient selectors: Tools like querySelector and querySelectorAll let you get exactly what you need without extra work.
  • Debounce or throttle events: If you’re working with events that happen a lot (like scrolling or typing), limit how often your functions run by using debounce or throttle techniques.
  • Use requestAnimationFrame: For animations or big updates, let the browser choose the best time to update the screen using requestAnimationFrame().
  • Avoid layout thrashing: Don’t read layout properties (like offsetHeight) right after writing them. Group reads and writes separately.
  • Remove unused nodes: Clean up elements you no longer need to keep your DOM light.

Learning how to work with the DOM the right way can make your websites load faster and feel smoother. Instead of making lots of small changes one after the other, group them and use smart tools like DocumentFragment and querySelectorAll. Also, think about how your changes will affect the whole page layout.

By following these tips, you’ll write better code and build websites that are easier to update, easier to read, and better for users. These skills will help you as you move forward with JavaScript and web development!

Understanding the DOM and how to use it well is a key skill for any front-end developer. Whether you’re working on personal projects or starting your career in tech, mastering these basics will set you up for success.

How to Efficiently Work with the DOM Tree was originally published in CarlosRojasDev on Medium, where people are continuing the conversation by highlighting and responding to this story.

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