HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Invalid date guard</title>
</head>
<body>
  <h1>Validate first</h1>
  <pre id="out"></pre>
  <script>
    var date = new Date("not-a-date");
    if (Number.isNaN(date.getTime())) {
      document.getElementById("out").textContent = "Bad input";
    } else {
      document.getElementById("out").textContent = date.toDateString();
    }
  </script>
</body>
</html>

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JavaScript Date toDateString() — invalid guard (Try It)