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<html lang="en">
<head>
  <meta charset="utf-8">
  <title>clearInterval() — null ID</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var out = document.getElementById("out");
    var intervalId = setInterval(function () {
      out.textContent += "Running…\n";
    }, 800);
    setTimeout(function () {
      clearInterval(intervalId);
      intervalId = null;
      out.textContent += "Timer stopped and ID cleared.";
    }, 1500);
  </script>
</body>
</html>

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JavaScript clearInterval() — null ID (Try It)