How to Efficiently Parse CSS in JavaScript?

Parsing CSS in JavaScript: A Detailed Guide

Parsing CSS in JavaScript can be a useful technique for extracting and manipulating styles from CSS code. However, it can be a complex task to implement.

Using JavaScript and jQuery Libraries

There are dedicated JavaScript and jQuery libraries available for parsing CSS. One popular library is [jQuery CSS](https://learn.jquery.com/using-jquery-core/other-topics/advanced-selectors/#css-manipulation), which provides methods for extracting and modifying CSS styles.

Alternate Approaches

If you want to implement your own CSS parsing functionality, you can use an alternative method to split property-value pairs. For example, you can use regular expressions to capture matches based on a specific pattern. Additionally, you can consider using a parser generator such as [PEG.js](https://pegjs.org/) or [JISON](https://zaach.github.io/jison/) to automatically generate a parser based on a grammar definition.

Avoiding Splitting at Semicolons

To avoid splitting property-value pairs at semicolons within values, you can use a regular expression that skips matches that include a semicolon after a colon. For instance:

/(?:[^;|)]*:.*?;)|(?:[^;|)]*?;)|(?:[;|]{1})/g

This regular expression matches property-value pairs by capturing non-semicolon characters followed by a colon, or solo semicolons, or individual semicolons.

Example Implementation

Here's an improved implementation of your CSS parsing code using the aforementioned alternate approach:

parseCSS: function(css) {
    var rules = {};
    css = this.removeComments(css);
    var blocks = css.split('}');
    blocks.pop();
    var len = blocks.length;
    for (var i = 0; i < len; i++) {
        var pair = blocks[i].split('{');
        rules[$.trim(pair[0])] = this.parseCSSBlock(pair[1]);
    }
    return rules;
},

parseCSSBlock: function(css) {
    var rule = {};
    var declarations = css.match(/(?:[^;|)]*:.*?;)|(?:[^;|)]*?;)|(?:[;|]{1})/g);
    declarations.pop();
    var len = declarations.length;
    for (var i = 0; i < len; i++) {
        var loc = declarations[i].indexOf(':');
        var property = $.trim(declarations[i].substring(0, loc));
        var value = $.trim(declarations[i].substring(loc + 1));

        if (property != "" &amp;&amp; value != "")
            rule[property] = value;
    }
    return rule;
},

removeComments: function(css) {
    return css.replace(/\/\*(\r|\n|.)*\*\//g,"");
}

Using Browser's CSSOM

You can also leverage the browser's built-in CSS Object Model (CSSOM) to parse CSS. Here's an example:

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

   styleElement.textContent = styleContent;
    doc.body.appendChild(styleElement);

    return styleElement.sheet.cssRules;
};

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