HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>setCookieWithExpiry helper</title>
</head>
<body>
  <h1>Expiry helper</h1>
  <button type="button" id="run">Set persistent + session</button>
  <pre id="out"></pre>
  <script>
    function setCookieWithExpiry(name, value, days) {
      var encoded = encodeURIComponent(value);
      var cookie = name + "=" + encoded + "; path=/; SameSite=Lax";
      if (typeof days === "number" && days > 0) {
        cookie += "; max-age=" + (days * 24 * 60 * 60);
      }
      document.cookie = cookie;
    }

    document.getElementById("run").addEventListener("click", function () {
      setCookieWithExpiry("username", "JohnDoe", 7);
      setCookieWithExpiry("flash", "ok");
      document.getElementById("out").textContent = document.cookie;
    });
  </script>
</body>
</html>

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JavaScript Cookies Expires — setCookieWithExpiry helper (Try It)