HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>queryCommandState toolbar</title>
  <style>
    body{font-family:system-ui,sans-serif;margin:1rem;}
    #editor{border:1px solid #ccc;padding:0.75rem;min-height:3rem;margin:0.5rem 0;}
    #boldBtn.is-pressed{outline:2px solid #2563eb;font-weight:700;}
  </style>
</head>
<body>
  <button id="boldBtn" type="button" aria-pressed="false">Bold</button>
  <div id="editor" contenteditable="true">Select me and toggle Bold</div>
  <pre id="out"></pre>
  <script>
    var btn = document.getElementById("boldBtn");
    var editor = document.getElementById("editor");
    function syncBoldButton() {
      editor.focus();
      var state =
        typeof document.queryCommandState === "function"
          ? document.queryCommandState("bold")
          : false;
      var pressed = state === true;
      btn.setAttribute("aria-pressed", String(pressed));
      btn.classList.toggle("is-pressed", pressed);
      document.getElementById("out").textContent =
        "bold pressed UI: " + pressed + " (raw state: " + String(state) + ")";
    }
    editor.addEventListener("mouseup", syncBoldButton);
    editor.addEventListener("keyup", syncBoldButton);
    btn.addEventListener("click", function () {
      document.execCommand("bold", false, null);
      syncBoldButton();
    });
    syncBoldButton();
  </script>
</body>
</html>

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Document.queryCommandState — pressed toolbar button (Try It)