HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>focusin delegation</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 420px; }
    form { display: flex; flex-direction: column; gap: 12px; }
    label { font-weight: 600; }
    input { padding: 10px 12px; border: 2px solid #cbd5e1; border-radius: 6px; font-size: 16px; }
    input.active { border-color: #6366f1; background: #eef2ff; }
    #hint { color: #64748b; font-size: 14px; margin-top: 8px; }
  </style>
</head>
<body>
  <form id="signup-form">
    <div>
      <label for="first">First name</label><br>
      <input type="text" id="first" name="first">
    </div>
    <div>
      <label for="last">Last name</label><br>
      <input type="text" id="last" name="last">
    </div>
    <div>
      <label for="email">Email</label><br>
      <input type="email" id="email" name="email">
    </div>
  </form>
  <p id="hint">Tab into fields — .active moves with focusin delegation.</p>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("#signup-form").on("focusin", "input", function() {
      $("#signup-form input").removeClass("active");
      $(this).addClass("active");
    });
  </script>
</body>
</html>

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jQuery focusin event — form delegation (Try It)