コンパイラはhttps://closure-compiler.appspot.com/home などを使うといいです　ref:https://qiita.com/TakamiChie/items/4a2e5aa484877896854c

NoteExtension Bookmarklet.js

// note用 プロパティ表示ツール
// noteの編集画面において、文字数や段落数、マークダウン記法版文章を表示します。
// ==ClosureCompiler==
// @output_file_name default.js
// @compilation_level ADVANCED_OPTIMIZATIONS
// ==/ClosureCompiler==

(function() {
  let nb = (document.getElementById("note-body") ||
    document.querySelector("note-body div"));
  if(!nb){
    alert("ノードが見つかりません。Noteのテキストページが正しく表示されているかどうか確認してください");
    return;
  }
  const ID_DLG = "noteex_detail_dialog";
  const ID_MDV = ID_DLG + "_mdview";
  const ID_LEN = ID_DLG + "_txtlen";
  const ID_PARA = ID_DLG + "_txtpara";
  var dialog;
  dialog = document.getElementById(ID_DLG);
  if(!dialog){
    /* ダイアログ作成 */
    dialog = document.createElement("dialog");
    let mdview = document.createElement("textarea");
    let lenview = document.createElement("div");
    let paraview = document.createElement("div");
    let mdlabel = document.createElement("label");
    let close = document.createElement("button");
    dialog.id = ID_DLG;
    dialog.style.width = "90%";
    dialog.style.height = "90%";
    lenview.id = ID_LEN;
    lenview.style.marginLeft = "1em";
    paraview.id = ID_PARA;
    paraview.style.marginLeft = "1em";
    mdview.id = ID_MDV;
    mdview.readonly = true;
    mdview.style.display = "block";
    mdview.style.width = "100%";
    mdview.style.minHeight = "20em";
    mdview.addEventListener("focus", () => {
      mdview.select();
    });
    mdlabel.textContent = "マークダウン書式";
    mdlabel.appendChild(mdview);
    close.textContent = "閉じる";
    close.addEventListener("click", () => { dialog.close(); });
    dialog.appendChild(lenview);
    dialog.appendChild(paraview);
    dialog.appendChild(mdlabel);
    dialog.appendChild(close);
    document.body.appendChild(dialog);
  }
  /* テキスト取得 */
  var lines = [];
  nb.childNodes.forEach((item) => {
    let text = item.innerHTML.split("<br>").join("\n");
    switch(item.nodeName){
      case "P":
        if(item.childNodes && item.childNodes[0] && item.childNodes[0].nodeName == "IMG"){
          let src = item.childNodes[0].src;
          lines.push(`![画像](${src})`);
        }else{
          text = text.replace(/<a.[^>]*href="([^"]+)"[^>]*>(.*?)<\/a>/g, "[$2]($1)");
          text = text.replace(/<b>(.*?)<\/b>/g, "**$1**");
          lines.push(text);
        }
        break;
      case "H3":
        lines.push("### " + text);
        break;
      case "BLOCKQUOTE":
        lines.push("> " + text.split("\n").join("\n> "));
        break;
      case "PRE":
        lines.push("```\n" + text + "\n```");
        break;
      case "FIGURE":
        let href = item.querySelector("a").href;
        let title = item.querySelector("a strong").textContent;
        lines.push(`[${title}](${href})`);
        break;
    }
  });
  dialog.querySelector("#" + ID_MDV).value = lines.join("\n\n");
  dialog.querySelector("#" + ID_LEN).textContent =
    "文字数：" + nb.textContent.length + "文字";
  dialog.querySelector("#" + ID_PARA).textContent =
    "段落数：" + nb.querySelectorAll("p").length + "段落";
  dialog.showModal();
})();