Get all css style blocks matching a css selector from stylesheets

javascript-get-styles-by-selector.js

/**
 * Get all CSS style blocks matching a CSS selector from stylesheets
 * @param {string} className - class name to match
 * @param {boolean} startingWith - if true matches all items starting with selector, default = false (exact match only)
 * @example getStylesBySelector('pure-form .pure-form-html ')
 * @returns {object} key/value object containing matching styles otherwise null
 */
function getStylesBySelector(className, startingWith) {

    if (!className || className === '') throw new Error('Please provide a css class name');

    var styleSheets = window.document.styleSheets;
    var result = {};

    // go through all stylesheets in the DOM
    for (var i = 0, l = styleSheets.length; i < l; i++) {

        var classes = styleSheets[i].rules || styleSheets[i].cssRules || [];

        // go through all classes in each document
        for (var x = 0, ll = classes.length; x < ll; x++) {

            var selector = classes[x].selectorText || '';
            var content = classes[x].cssText || classes[x].style.cssText || '';

            // if the selector matches
            if ((startingWith && selector.indexOf(className) === 0) || selector === className) {

                // create an object entry with selector as key and value as content
                result[selector] = content.split(/(?:{|})/)[1].trim();
            }
        }
    }

    // only return object if we have values, otherwise null
    return Object.keys(result).length > 0 ? result : null;
}