HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>change validity</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 400px; }
    label { display: block; margin-bottom: 6px; font-weight: 600; }
    input[type="text"] { width: 100%; padding: 10px 12px; border: 2px solid #cbd5e1; border-radius: 6px; font-size: 16px; box-sizing: border-box; margin-bottom: 12px; }
    input.invalid { border-color: #ef4444; background: #fef2f2; }
    #log { padding: 10px; background: #f8fafc; border-radius: 6px; font-family: monospace; font-size: 14px; color: #334155; min-height: 1.2em; }
    #hint { color: #64748b; font-size: 14px; margin-bottom: 12px; }
  </style>
</head>
<body>
  <p id="hint">Min 3 characters. Edit, then tab away — change fires only if value changed.</p>
  <label for="username">Username</label>
  <input type="text" id="username" placeholder="Try ab or abc">
  <label for="city">City</label>
  <input type="text" id="city" placeholder="Optional">
  <div id="log">Waiting for change…</div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("input[type='text']").on("change", function() {
      var value = $(this).val();
      var valid = value.length >= 3;
      $(this).toggleClass("invalid", !valid);
      $("#log").text("Changed: " + value + " | valid: " + valid);
    });
  </script>
</body>
</html>

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jQuery change event — text input validity (Try It)