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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>length — guard</title>
  <style>
    pre { font-family: ui-monospace, monospace; font-size: 13px; }
    .card { padding: 8px; margin: 4px 0; background: #f1f5f9; }
  </style>
</head>
<body>
  <div class="card">Only card</div>
  <pre id="out"></pre>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    function highlightCards(selector) {
      var $cards = jQuery(selector);
      if (!$cards.length) {
        return "No cards matched: " + selector;
      }
      $cards.css("border", "2px solid #2563eb");
      return "Highlighted " + $cards.length + " card(s)";
    }
    document.getElementById("out").textContent = [
      highlightCards(".card"),
      highlightCards(".missing")
    ].join("\n");
  </script>
</body>
</html>

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.length — Guard Before .each() (Try It)