HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>safeGetUTCDay</title>
</head>
<body>
  <h1>Validate first</h1>
  <pre id="out"></pre>
  <script>
    function safeGetUTCDay(date) {
      if (!(date instanceof Date) || Number.isNaN(date.getTime())) {
        return null;
      }
      return date.getUTCDay();
    }

    document.getElementById("out").textContent =
      String(safeGetUTCDay(new Date())) + "\n" +
      String(safeGetUTCDay(new Date("not-a-date")));
  </script>
</body>
</html>

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JavaScript Date getUTCDay() — validate (Try It)