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Minimize Recalculations Using the CSSOM in JavaScript

Last updated: December 12, 2024

The Document Object Model (DOM) and the Cascading Style Sheets Object Model (CSSOM) are two critical components when it comes to web development. The CSSOM represents the styles and order of applying them to elements, while the DOM represents the HTML structure. By understanding and effectively utilizing the CSSOM, JavaScript developers can significantly minimize recalculations and improve website performance.

When JavaScript makes changes to the DOM, it can force the browser to recalculate styles, reflow, and paint the entire layout, which can be expensive in terms of performance. The good news is that by learning how to manipulate the CSSOM efficiently, you can reduce these burdensome recalculations.

Understanding the CSSOM

The CSSOM is similar to the DOM, but it's dedicated to the styles applied to the elements. It is a structured representation of all rules in your stylesheets and their relationships. Browsers create the CSSOM by parsing all CSS, whether internal or external, and organizing what affects which element and how.

Accessing and Manipulating CSSOM through JavaScript

Accessing Styles

You can access and manipulate the CSSOM properties using JavaScript. To access computed styles, one can use the getComputedStyle method. This method returns an object that includes the values of all CSS properties of an element. Here's an example of how you might use it:

const element = document.querySelector('.my-element');
const style = window.getComputedStyle(element);
console.log(style.width); // Outputs the width of the element

Minimizing Layout Thrashing

Layout thrashing occurs when your code makes several measurements, followed by DOM manipulations in an alternating pattern. This frequently triggers additional layout recalculations. To minimize layout thrashing, consider the pattern of reading DOM properties first and writing afterwards.

// Avoid doing this
for (let i = 0; i < 100; i++) {
  const height = elements[i].offsetHeight;
  elements[i].style.height = (height + 10) + 'px';
}

// Do this instead
let heights = [];
for (let i = 0; i < 100; i++) {
  heights[i] = elements[i].offsetHeight;
}
for (let i = 0; i < 100; i++) {
  elements[i].style.height = (heights[i] + 10) + 'px';
}

Batching DOM Updates

Batching DOM manipulations is another effective way to reduce reflows. Instead of updating elements one by one, accumulate the updates and execute them all at once.

Using CSS Variables for Optimization

CSS Variables (also known as Custom Properties) can be powerful when combined with JavaScript, particularly regarding consistency and code maintainability. Access CSS variables using JavaScript as follows:

const root = document.documentElement;
const mainColor = getComputedStyle(root).getPropertyValue('--main-color');
console.log(mainColor);

// Updating the CSS Variable
document.documentElement.style.setProperty('--main-color', '#3498db');

By leveraging CSS Variables and JavaScript, you can dynamically update styles without extensive DOM operations.

Performance Impact of CSSOM Manipulation

Every time you modify a style property, add, or remove CSS Rules via JavaScript, the CSSOM changes. These alterations can cascade and require the browser to calculate styles again, leading to performance hits. Consider these tips when manipulating styles:

  • Where possible, prefer adding/removing class names that define styles instead of setting individual styles directly.
  • Refrain from frequent accesses to computed styles, as it demands recalculations for every property request.
  • Utilize the excellent tools provided by browser developer tools to monitor and debug performance issues related to parsing and execution.

By organizing your CSSOM manipulation wisely and following these best practices, you can ensure your applications remain responsive and performant under a variety of use cases.

Next Article: Inject and Remove Styles Dynamically with the CSSOM in JavaScript

Previous Article: Modify CSS Rules On-the-Fly via the CSSOM in JavaScript

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