How Deep is Your DOM?

Tech Adapter 2024-08-04

Improving web performance is essential, and one critical factor is the depth of the DOM. The DOM (Document Object Model) represents the structure of an HTML document as a tree. As the depth of this DOM increases, it can negatively impact the performance of a web page. This article explores how the depth of the DOM affects performance and offers tips for optimization.

The Relationship Between DOM Depth and Performance

You may have encountered a warning like “Avoid excessive DOM size” when measuring site performance using tools like Lighthouse. This warning arises because a large DOM can increase memory usage and the cost of style calculations. As the DOM grows deeper, more computations are required to access elements, which can significantly impact user experience.

For example, consider accessing a particular element from the root of a tree. In a shallow DOM tree, you can reach the element with just two operations. However, in a deep tree, it might take six operations to reach the same element. This shows that DOM depth is crucial to page performance.

Experimenting with DOM Depth’s Impact

To investigate how DOM depth affects performance, we conducted a simple experiment. We created two HTML pages: one with a shallow element tree where all `div` elements are directly inside the `body`, and another with a deep element tree where `div` elements are nested.

Shallow Element Tree

<html>
    <body>
      <div></div>
      <div></div>
      <div></div>
      <!-- 97 more divs omitted -->
      <div>The 100th and final div.</div>
    </body>
  </html>

Deep Element Tree

<html>
    <body>
      <div>
        <div>
          <!-- 97 more divs omitted -->
          <div>The 100th and final div.</div>
        </div>
      </div>
    </body>
  </html>

The experiment showed that the shallow tree’s page load time was 51 milliseconds, while the deep tree’s load time was 53 milliseconds. Although the deep tree took slightly longer, the difference wasn’t significant. However, when the number of elements was increased to 500, the shallow tree’s load time was 56 milliseconds, whereas the deep tree’s load time increased significantly to 102 milliseconds.

DOM Depth Optimization Techniques

1. Regularly Monitor DOM Size and Depth

You can use tools like Lighthouse or PageSpeed Insights to measure your site’s DOM size and depth. Additionally, you can check the number of elements on a page by running `document.querySelectorAll(“*”).length;` in the browser console.

2. Reduce CSS Selector Scope and Complexity

Reducing the scope and complexity of CSS selectors makes it easier for the browser to target elements, speeding up style recalculations.

3. Maintain an Appropriate Tree Structure

Keep the tree structure flat and avoid unnecessary nesting. Place frequently used elements close to the root to improve access speed.

Conclusion

Modern browsers can handle many nested DOM trees quickly, but the impact of DOM size and depth on performance remains important. When combined with style recalculations, this impact can be even greater. Therefore, optimizing the size and depth of your DOM is essential for enhancing site performance.

References: frontendatscale.com, “How Deep is Your DOM?”