How Can JavaScript Efficiently Parse CSS Using CSSOM?

Parsing CSS in JavaScript / jQuery

In this article, we'll delve into the problem of parsing CSS in JavaScript, specifically focusing on the task of transforming CSS code into an object-based representation. We'll explore the limitations of a DIY implementation and introduce a potential solution using CSSOM.

DIY Implementation and its Challenges

The provided code snippet attempts to parse CSS manually, splitting rules at semicolons and braces. However, this approach falls short when encountering CSS properties with embedded semicolons, such as background URLs.

An Alternative Approach: Using CSSOM

To overcome these challenges, we can leverage the browser's built-in CSS Object Model (CSSOM). CSSOM allows us to access and manipulate CSS rules dynamically. We can utilize this capability for parsing purposes.

Implementation using CSSOM

The following JavaScript code demonstrates how to use CSSOM to parse CSS:

var rulesForCssText = function (styleContent) {
    var doc = document.implementation.createHTMLDocument(""),
        styleElement = document.createElement("style");

   styleElement.textContent = styleContent;
    // the style will only be parsed once it is added to a document
    doc.body.appendChild(styleElement);

    return styleElement.sheet.cssRules;
};

For each rule returned by rulesForCssText, we can inspect its properties using rule.style.

Example Usage

To illustrate the usage of this approach, consider the following JavaScript Fiddle (https://jsfiddle.net/v2JsZ/):

var css = "a { color: red; }";
var rules = rulesForCssText(css);

console.log(rules[0].selectorText); // "a"
console.log(rules[0].style.color); // "red"

Conclusion

While manual parsing of CSS in JavaScript is possible, it introduces potential pitfalls. By leveraging CSSOM, we can harness the browser's capabilities for reliable and efficient CSS parsing.

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