HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>focus migration</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 420px; }
    label { display: block; margin-bottom: 6px; font-weight: 600; }
    input { width: 100%; padding: 10px 12px; border: 2px solid #cbd5e1; border-radius: 6px; font-size: 16px; box-sizing: border-box; margin-bottom: 14px; }
    #legacy { border-color: #f59e0b; }
    #modern { border-color: #10b981; }
    #out { padding: 10px; background: #f8fafc; border-radius: 6px; color: #334155; min-height: 1.2em; }
  </style>
</head>
<body>
  <label for="legacy">Legacy field (.focus)</label>
  <input type="text" id="legacy" placeholder="Focus into this field">
  <label for="modern">Modern field (.on focus)</label>
  <input type="text" id="modern" placeholder="Focus into this field">
  <div id="out">Focus into either field.</div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("#legacy").focus(function() {
      $("#out").text("Legacy: .focus(function(){ ... }) — deprecated since jQuery 3.3.");
    });
    $("#modern").on("focus", function() {
      $("#out").text("Modern: .on('focus', function(){ ... }) — use this for new code.");
    });
  </script>
</body>
</html>

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jQuery .focus(fn) vs .on('focus', fn) — migration (Try It)