Bookmarklet: Finds all strings that match '<number> mpg' and suffixes them with the l/100km conversion

mpg-to-Lp100km-bookmarklet.js

javascript: (function () {
  function convert(currentElement) {
      if (!currentElement.textContent) return;
      /* 
       * Match mpg strings with negative lookahead for the l/100km conversion string
       * to avoid duplicating the conversion when run multiple times.
       * E.g. "N.N mpg" not followed by " (N.N l/100km)" 
       */
      const mpgRegex = /(\d*(\.\d+)?)(\s)*mpg(?!\s\(\d+\.\d+\sl\/100km\))/gi;
      const currentText = currentElement.textContent;
      if (mpgRegex.test(currentText)) {
          const newText = currentText.replace(
              mpgRegex,
              (match, mpgText) => {
                  const mpg = parseFloat(mpgText);
                  if (!isNaN(mpg)) {
                      const lp100 = 235.21 / mpg;
                      const newValue = `${match} (${lp100.toFixed(1)} l/100km)`;
                      return newValue;
                  }
                  return match;
              }
          );
          console.log(`Updated ${newText}`);
          currentElement.textContent = newText;
      }
  }

  function textNodes(el) {
      var n, a = [], walk = document.createTreeWalker(el, NodeFilter.SHOW_TEXT, null, false);
      while (n = walk.nextNode()) a.push(n);
      return a;
  }

  textNodes(document).forEach(n => convert(n));
})();