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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Elapsed with Date.now</title>
</head>
<body>
  <h1>Elapsed ms</h1>
  <pre id="out"></pre>
  <script>
    var start = Date.now();
    var sum = 0;
    for (var i = 0; i < 100000; i++) {
      sum += i;
    }
    var elapsed = Date.now() - start;
    document.getElementById("out").textContent =
      "Elapsed: " + elapsed + " ms";
  </script>
</body>
</html>

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JavaScript Date getUTCMilliseconds() — elapsed time (Try It)