HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>toggleAttribute return</title>
  <style>body{font-family:sans-serif;margin:1rem;} button{padding:0.5rem 1rem;}</style>
</head>
<body>
  <button type="button" id="mute">Toggle mute flag</button>
  <pre id="status">Unmuted</pre>
  <script>
    var btn = document.getElementById("mute");
    var status = document.getElementById("status");
    btn.addEventListener("click", function () {
      var muted = btn.toggleAttribute("data-muted");
      status.textContent = muted ? "Muted" : "Unmuted";
    });
  </script>
</body>
</html>

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Element.toggleAttribute — return value (Try It)