How to Use JavaScript window.getComputedStyle

This article explains how the window.getComputedStyle() method works in JavaScript to access the final rendered CSS styles of an HTML element. You will learn the difference between inline styles and computed styles, understand how the browser calculates final styling values through the CSS cascade, and discover practical techniques for reading styles, including pseudo-elements, within your scripts.

What is window.getComputedStyle?

window.getComputedStyle() is a native JavaScript method that returns a live, read-only CSSStyleDeclaration object containing the values of all CSS properties applied to a target element.

While the standard element.style property only accesses inline styles declared directly on an HTML tag via the style attribute, window.getComputedStyle() captures the total result of all styling sources: external stylesheets, <style> blocks, inline styles, browser user-agent defaults, and CSS inheritance.

Syntax

const styleDeclaration = window.getComputedStyle(element, [pseudoElement]);

How the Browser Resolves Final Rendered Styles

To understand how JavaScript retrieves rendered styles, it helps to understand the browser’s style calculation pipeline:

  1. Cascade and Specificity: The browser collects all rules targeting an element from user-agent styles, external stylesheets, and inline declarations. It resolves conflicts using CSS specificity and order of appearance.
  2. Inheritance: Any inherited properties not explicitly set on the element take their values from parent elements.
  3. Value Computation and Resolution: Relative units (such as em, rem, %, or vh) and keyword values (such as auto or named colors) are converted into absolute values. For example, dimensions are generally converted to pixel values (px), and colors are standardized into rgb() or rgba() notation.

When you call window.getComputedStyle(), the JavaScript engine requests these fully resolved values directly from the browser’s rendering engine.

Basic Usage and Code Examples

Retrieving Element Properties

To read a specific property, use either the camelCase property accessor or the .getPropertyValue() method:

const box = document.querySelector('.box');
const computed = window.getComputedStyle(box);

// Using getPropertyValue (recommended for standard CSS property names)
const backgroundColor = computed.getPropertyValue('background-color');
const width = computed.getPropertyValue('width');

// Using property accessors
const fontSize = computed.fontSize;

console.log(backgroundColor); // e.g., "rgb(34, 139, 34)"
console.log(width);           // e.g., "250px"
console.log(fontSize);        // e.g., "16px"

Accessing Pseudo-Element Styles

You can inspect generated content and styles applied via CSS pseudo-elements by passing the pseudo-element selector as the second argument:

const button = document.querySelector('.btn');
const beforeStyles = window.getComputedStyle(button, '::before');

const content = beforeStyles.getPropertyValue('content');
const color = beforeStyles.getPropertyValue('color');

console.log(content); // e.g., '"★"'
console.log(color);   // e.g., "rgb(255, 215, 0)"

Key Characteristics and Considerations