Get YouTube URL containing canonical channel ID JS function suitable for bookmarklet

yturl.js

((w) => {
  const urlPrefix = "https://www.youtube.com/";
  const href = w.location.href;
  if (!href.startsWith(urlPrefix)) {
    alert("wrong page");
    return;
  }

  fetch(href)
    .then((res) => res.text())
    .then((text) => new DOMParser().parseFromString(text, "text/html"))
    .then((doc) => doc.querySelector('meta[itemprop="channelId"]'))
    .then((attr) => attr.getAttribute("content"))
    .then((channelId) =>
      prompt("The URL is:", urlPrefix + "channel/" + channelId)
    )
    .catch((err) => console.log(err));
})(window);