How to Modify Global CSS Values Using JavaScript: A Comprehensive Guide

Modifying Global CSS Values Using JavaScript

Problem Overview

When manipulating the CSS styles of elements with JavaScript, it is possible to inadvertently change inline style values, overriding global CSS declarations. This can lead to inconsistent behavior and incorrect results.

Solution

To modify global CSS values, use the document.styleSheets API. This API provides access to the StyleSheetList, which contains all the CSS stylesheets in the document. By iterating through this list, you can identify the desired stylesheet and modify its rules.

Implementation

To manipulate global CSS values given an element ID, follow these steps:

1. Get the Element's Stylesheet: Use document.getElementById(id) to retrieve the HTML element and element.styleSheets to access its stylesheets.
2. Identify the Rule: Iterate through the stylesheets using for or forEach loops to find the stylesheet containing the rule for the element's style.
3. Modify the Rule: Use the cssRules or rules property (depending on the browser) to access the array of rules. Then, find the CSS rule by matching its selectorText property to the element's ID.
4. Set the New Value: Once the rule is identified, set its value property to the desired CSS value.

Example Code

The following code demonstrates how to change the background color of an element with the ID "container":

<code class="javascript">// Get the element
const element = document.getElementById('container');

// Iterate through stylesheets
for (let i = 0; i < element.styleSheets.length; i++) {
  // Access the CSS rules
  const cssRules = element.styleSheets[i].cssRules;

  // Find the rule matching the element's ID
  for (let j = 0; j < cssRules.length; j++) {
    if (cssRules[j].selectorText === '#container') {
      // Change the background color
      cssRules[j].style.backgroundColor = 'red';
      break;
    }
  }
}</code>

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