HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>queryCommandState bold</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;}button{margin:0.5rem 0;}</style>
</head>
<body>
  <div id="editor" contenteditable="true">Select a part of this text!</div>
  <button id="btn" type="button">Test the state of the 'bold' command</button>
  <pre id="out">Select text, then click the button.</pre>
  <script>
    function makeBold() {
      var state = typeof document.queryCommandState === "function"
        ? document.queryCommandState("bold")
        : null;
      var message;
      switch (state) {
        case true:
          message = "The bold formatting will be removed from the selected text.";
          break;
        case false:
          message = "The selected text will be displayed in bold.";
          break;
        default:
          message = "The state of the 'bold' command is indeterminable.";
          break;
      }
      document.getElementById("out").textContent =
        "state=" + String(state) + "\n" + message;
      document.execCommand("bold");
    }
    document.getElementById("btn").addEventListener("click", makeBold);
  </script>
</body>
</html>

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Document.queryCommandState — bold true/false/null (Try It)