Builds a list of images found in the linked style sheets

getBGImgURLsFromCSSs.js

/* Builds a list of images found in the linked style sheets
 * 
 * Adapted from stackoverflow.com/questions/2430503/list-of-all-background-images-in-dom/2453880#2453880
 * 
 * This method has the advantage of finding URLs for background images that no
 * element in the DOM uses yet.
 *
 * @return {array}
 *         List of unique image URLs as specified in the style sheets.
   ------------------------------------------------------------------------- */
function getBGImgURLsFromCSSs()
{
   if (getBGImgURLsFromCSSs.BGImgURLs) {
      return getBGImgURLsFromCSSs.BGImgURLs;
   }
   
   var sheets = document.styleSheets;
   var hash = {}, sheet, rules, rule, url, match;
   // loop the stylesheets
   for (var s = 0, sheet_count = sheets.length; s < sheet_count; ++s) {
      sheet = sheets[s];
      // ie or w3c stylee rules property?
      try {
         // Some browsers will throw security exceptions when trying to access
         // stylesheets form different domains (an example would be when using
         // TypeKit).
         rules = sheet.rules ? sheet.rules : sheet.cssRules;
      }
      catch (exception) { continue; }
      if    (!rules)      continue;
      // loop the rules
      for (var r = 0, rule_count = rules.length; r < rule_count; ++r) {
         rule = rules[r];
         if (rule.selectorText && rule.style.cssText) {
            // check if there's a style setting we're interested in..
            if (rule.style.cssText.match(/background/i)) {
               // ..and if it has a URL in it, put it in the hash
               match = /url\(['"]?([^)'"]*)['"]?\)/i.exec(rule.style.cssText);
               if (match) {
                  url = match[1]
                  // Do we need to make the URL absolute?
                  if (url.search(/^(?:http|\/)/i) !== 0) {
                     // Yes.
                     url = sheet.href.replace(/[^\/]+$/, '') + url;
                  }
                  hash[url] = true;
               }
            } // if background style
         } // if has rule
      } // for each rule
   } // for each stylesheet
   
   // return an array of unique URLs
   var urls = [];
   for (url in hash) { urls.push(url); }
   
   // Save in case this gets called later.
   getBGImgURLsFromCSSs.BGImgURLs = urls;
   
   return getBGImgURLsFromCSSs.BGImgURLs;
} // getBGImgURLsFromCSSs()