How Can I Modify CSS Values Defined in an External Stylesheet Using JavaScript?

Using JavaScript to Modify CSS Values

When working with inline CSS values in JavaScript, it's easy to encounter situations where you need to change the values of styles defined in an external CSS stylesheet. However, solely modifying inline styles can lead to issues.

For instance, if you have an inline style with a width of 30% and a stylesheet rule setting the width to 50%, modifying the inline style will override the stylesheet value. Additionally, fetching the width property before setting it will return "null" due to the lack of inline styling, causing problems in scenarios where the width needs to be determined for logical operations.

To resolve this, we need a way to access and modify the CSS values defined in the external stylesheet itself. Fortunately, JavaScript provides a solution.

Retrieving Stylesheet Rules

The first step is to retrieve the stylesheet objects. This can be achieved using the "document.styleSheets" property. It returns an array of all stylesheets in the document.

To identify the specific stylesheet, check the "document.styleSheets[styleIndex].href" property. This allows you to find the stylesheet you want to modify.

Accessing Rule Objects

Once you have the stylesheet, you need to access the rule objects. These objects represent individual CSS rules within the stylesheet. The available name depends on the browser: "rules" in Internet Explorer and "cssRules" in most others.

For example, to identify a particular rule with a specific selector, you would check the "selectorText" property of each rule object.

Modifying Rule Values

Finally, you can modify the values of these rules by setting the "value" property. This allows you to change styles at the stylesheet level, affecting all elements that match the rule's selector.

The code snippet provided in the original response demonstrates how to retrieve and modify stylesheet rules using JavaScript. By following this approach, you can effectively update CSS values without the limitations of inline styling, ensuring that changes are applied consistently across multiple elements in the document.

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