root.tsx

//...

export default function App() {  
  let location = useLocation();
  React.useLayoutEffect(() => {
    [...document.querySelectorAll<HTMLSpanElement>('pre[data-lang] span')]
      .filter(el => ['let', 'const', 'var'].includes(el.innerText))
      .forEach(el => el.innerText = ['let', 'const', 'var'][Math.floor(Math.random() * 3)]);
  }, [location.pathname]);

  //...
}

//...