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

    var lines = [
      "Valid: " + safeGetFullYear(new Date()),
      "Invalid: " + safeGetFullYear(new Date("not-a-date"))
    ];
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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JavaScript Date getFullYear() — validate Date (Try It)